PART 1 PART 2docshare04.docshare.tips/files/18885/188856653.pdf · 2017-03-25 · vi Understanding...

ii Understanding the National Electrical Code Rules for Backup Power Systems

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Table of Contents

PART 1ARTICLE 90—INTRODUCTION TO THE NATIONAL ELECTRICAL CODE .............................................................................................. 190.1 Purpose of the NEC ........................................................ 190.2 Scope of the NEC ........................................................... 290.3 Code Arrangement .......................................................... 490.4 Enforcement .................................................................... 590.5 Mandatory Requirements and Explanatory Material ...... 690.6 Formal Interpretations .................................................... 790.7 Examination of Equipment for Product Safety ............... 790.9 Units of Measurement .................................................... 7

ARTICLE 110—REqUIREMENTs FOR ELECTRICAL INsTALLATIONs ............................................................................ 9

Part I. General Requirements ......................................................... 9110.1 Scope. .............................................................................. 9110.2 Approval of Conductors and Equipment ........................ 9110.3 Examination, Identification, Installation, and Use of

Equipment ...................................................................... 9110.4 Voltages ..........................................................................10110.5 Copper Conductors........................................................10110.6 Conductor Sizes .............................................................11110.7 Wiring Integrity. .............................................................11110.8 Suitable Wiring Methods ...............................................12110.9 Interrupting Protection Rating .......................................12110.10 Short-Circuit Current Rating .........................................13110.11 Deteriorating Agents ......................................................13110.12 Mechanical Execution of Work ......................................14110.13 Mounting and Cooling of Equipment ............................15110.14 Conductor Termination and Splicing .............................16110.15 High-Leg Conductor Identification ................................20110.16 Flash Protection Warning ..............................................20110.20 Enclosure Types .............................................................21110.21 Manufacturer’s Markings ...............................................21110.22 Identification of Disconnecting Means ..........................21

Part II. 600V, Nominal, or Less .......................................................22110.26 Spaces About Electrical Equipment ...............................22110.27 Guarding ........................................................................27

PART 2ARTICLE 220—BRANCH-CIRCUIT, FEEDER, AND sERvICE CALCULATIONs .............................................................................29

Part I. General ...............................................................................29220.1 Scope .............................................................................29220.3 Application of Other Article ..........................................29220.5 Calculations. ..................................................................29

Part IV. Optional Calculations for Computing Feeder and Service Loads .........................................................................30220.82 Dwelling Unit—Optional Load Calculation. .................30220.84 Multifamily—Optional Load Calculation. .....................32220.85 Optional Calculation—Two Dwelling Units ...................33220.87 Determining Existing Loads ..........................................33

ARTICLE 230—sERvICEs .............................................................34

Part I. General ...............................................................................35230.7 Service Conductors Separate from Other Conductors ....35230.8 Raceway Seals ................................................................35

Part V. Service Equipment—General .............................................36230.66 Identified as Suitable for Service Equipment ..................36

Part VI. Service Equipment—Disconnecting Means ......................36230.70 General ..........................................................................36230.71 Number of Disconnects. ................................................37230.72 Grouping of Disconnects. ..............................................38230.76 Manual or Power Operated ............................................38230.79 Rating of Disconnect .....................................................39230.82 Equipment Connected to the Supply Side

of the Service Disconnect ...............................................39

Part VII. Service Equipment Overcurrent Protection ......................40230.90 Overload Protection Required ........................................40230.95 Ground-Fault Protection of Equipment ..........................40

ARTICLE 240—OvERCURRENT PROTECTION ................................42

Part I. General ...............................................................................42240.1 Scope .............................................................................42240.2 Definitions. ....................................................................43240.3 Protection of Equipment................................................44240.4 Protection of Conductors. ..............................................44240.6 Standard Ampere Ratings. .............................................44240.15 Ungrounded Conductors. ...............................................47

Part II. Location .............................................................................48240.21 Overcurrent Protection Location in Circuit ....................48

iv Understanding the National Electrical Code Rules for Backup Power Systems

Table of Contents

ARTICLE 310—CONDUCTORs FOR GENERAL WIRING ................105310.1 Scope ...........................................................................105310.2 Conductors ..................................................................105310.3 Stranded Conductors ...................................................105310.4 Conductors in Parallel. ................................................106310.5 Minimum Size Conductors ..........................................108310.8 Location. ......................................................................108310.9 Corrosive Conditions ...................................................108310.10 Insulation Temperature Limitation ..............................108310.12 Conductor Identification. .............................................109310.13 Conductor Construction ..............................................109310.15 Conductor Ampacity. ...................................................110

ARTICLE 312—CABINETs, CUTOUT BOxEs, AND METER sOCkET ENCLOsUREs ....................................................118312.1 Scope ................................................................... 118

Part I. Installation ........................................................................118312.2 Damp, Wet, or Hazardous (Classified)

Locations. ....................................................................118312.3 Installed in Walls ..........................................................119312.4 Repairing Gaps Around Plaster, Drywall, or

Plasterboard Edges ......................................................119312.5 Enclosures. ...................................................................119312.8 Used for Raceway and Splices. ....................................120

ARTICLE 408—sWITCHBOARDs AND PANELBOARDs ................122

Part I. General .............................................................................122408.1 Scope. ...........................................................................122408.4 Circuit Directory or Circuit Identification. ..................122408.5 Clearance for Conductor Entering Bus

Enclosures ....................................................................122408.7 Unused Openings. .......................................................123

Part III. Panelboards ....................................................................123408.40 Grounding (Bonding) of Panelboards ..........................123408.41 Grounded (Neutral) Conductor Terminations .............124

ARTICLE 700—EMERGENCy POWER sysTEMs ..........................126

Part I. General .............................................................................126700.8 Signs ............................................................................126

Part II. Circuit Wiring ...................................................................126700.9 Wiring ..........................................................................126

Part III. Sources of Power ............................................................126700.12 General Requirements ..................................................127

Part IV. Emergency System Circuits for Lighting and Power ...................................................................................127700.15 Loads on Emergency Branch Circuits ..........................127700.16 Emergency Illumination ...............................................127

Part VI. Overcurrent Protection ....................................................127700.25 Accessibility .................................................................127700.26 Ground-Fault Protection of Equipment.......................127700.27 Coordination ................................................................127

PART 3ARTICLE 250—GROUNDING AND BONDING ..................................53

Part I. General ...............................................................................53250.1 Scope .............................................................................53250.2 Definitions. .....................................................................53250.4 General Requirements for Grounding and

Bonding .........................................................................55250.6 Objectionable Current ....................................................61250.8 Termination of Grounding and Bonding

Cond uctors ....................................................................65250.10 Protection of Fittings .....................................................65250.12 Clean Surface .................................................................65

Part II. System Grounding and Bonding .........................................65250.20 Systems Required to be Grounded and

Bonded ...........................................................................65250.30 Grounding and Bonding of Separately

Derived AC Systems.......................................................66

Part III. Grounding Electrode System and Grounding Electrode Conductor .....................................................................................72250.50 Grounding Electrode System .........................................72250.52 Grounding (Earthing) Electrodes ...................................72250.53 Installation of Grounding Electrode System ..................75250.54 Supplementary Electrodes ..............................................77250.56 Resistance of Ground Rod Electrode .............................78250.58 Common Grounding (Earthing) Electrode .....................80250.60 Lightning Protection System Grounding

(Earthing) Electrode .......................................................80250.62 Grounding Electrode Conductor—Material ..................81250.64 Grounding Electrode Conductor Installation. ................81250.66 Grounding Electrode Conductor—Size ..........................83250.68 Grounding Electrode Conductor Termination ...............84250.70 Grounding Electrode Conductor Termination

Fitting ............................................................................85

PART 4ARTICLE 300—WIRING METHODs ................................................87

Part I. General Requirements ........................................................87300.1 Scope .............................................................................87300.3 Conductors. ...................................................................88300.4 Protection Against Physical Damage..............................89300.5 Underground Installations .............................................92300.6 Protection Against Corrosion and

Deteriora tion .................................................................96300.7 Raceways Exposed to Different Temperatures. ...............97300.8 Not Permitted in Raceways ............................................98300.10 Electrical Continuity. .....................................................98300.11 Securing and Supporting. ...............................................99300.12 Mechanical Continuity. ...............................................100300.14 Length of Free Conductors ..........................................101300.17 Raceway Sizing ............................................................101300.20 Induced Currents in Metal Parts. .................................103

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Table of Contents

ARTICLE 702—OPTIONAL sTANDBy POWER sysTEMs .............134

Part I. General .............................................................................134702.2 Definition. ....................................................................134702.3 Application of Other Articles .......................................134702.4 Equipment Approval ....................................................134702.5 Capacity and Rating ....................................................134702.6 Transfer Equipment .....................................................135702.8 Signs ............................................................................135

Part II. Circuit Wiring ...................................................................135702.9 Wiring ..........................................................................135

Part III. Grounding and Bonding ...................................................135702.10 Portable Generator Grounding and Bonding ...............135

Part IV. Sources of Power ............................................................135702.11 Outdoor Generator Sets ...............................................135

ARTICLE 701—LEGALLy REqUIRED sTANDBy POWER sysTEMs .......................................................................128

Part I. General .............................................................................128701.1 Scope ...........................................................................128701.2 Definitions. ...................................................................128701.3 Application of Other Articles .......................................128701.4 Equipment Approval ....................................................128701.5 Tests and Maintenance .................................................128701.6 Capacity and Rating ....................................................129701.7 Transfer Equipment. ....................................................129701.9 Signs ............................................................................129

Part II. Circuit Wiring ...................................................................129701.10 Wiring..............................................................................129

Part III. Sources of Power ............................................................129701.11 Legally Required Standby Systems...............................129

Part IV. Overcurrent Protection ....................................................133701.15 Accessibility .................................................................133701.17 Ground-Fault Protection of Equipment.......................133701.18 Coordination ................................................................133

vi Understanding the National Electrical Code Rules for Backup Power Systems

Introduction

This PDF is a free resource from Mike Holt Enterprises, Inc. It is always our pleasure to give back to the industry as much as we can, whenever we can. For this reason we’ve created this free Backup Power Systems PDF from Mike’s Understanding the NEC, Volume 1 & 2, 2008 Edition textbooks.

Emergency and standby power systems are those that supply power to certain loads when the normal supply of electric-ity (typically a utility service) to a building or other structure fails. The National Electrical Code has three primary articles providing requirements for these backup power systems:

Article 700—Emergency Systems covers backup power systems that are essential for life safety and legally required by federal, state, or municipal government regulations. When normal power is lost, emergency systems must be able to supply backup power within 10 seconds. Essential life safety systems supplied by emergency power include exit lighting, fire alarms, and fire pumps.

Article 701—Legally Required Standby Systems covers backup power systems that are legally required by federal, state, or municipal government regulations but not classified as “emergency.” When normal power is lost, legally required standby systems must be able to supply backup power within 60 seconds. Battery-powered individual unit equipment for standby lighting (often called “lunchboxes” in the field) falls within the scope of this article.

Article 702—Optional Standby Systems covers backup power systems that aren’t not legally required and don’t support life safety functions. Optional standby systems are often installed by process industries such as glass or steel making plants, where an unplanned shutdown due to a power outage would have expensive consequences. Backup generators used at residences and farms are also classified as optional standby systems.

In addition to these three specialized articles, many other NEC requirements also apply to the installation of emer-gency and standby power systems—rules in other articles dealing with conductors and raceways, branch circuits and feeders, grounding, enclosures, working space around electri-cal equipment, and much, much more. This free PDF down-

load from Mike Holt Enterprises gives you the “big picture” overview of everything you need to know about installing Code-compliant emergency and standby power systems.

About the Author

Mike Holt worked his way up through the electrical trade from an apprentice electrician to become one of the most recognized experts in the world as it relates to electri-cal power installation. He was a Journeyman Electrician, Master Electrician, and Electrical Contractor. Mike came from the real world, and his dedication to electrical training is the result of his own struggles as an electrician looking for a program that would help him succeed in this challenging industry.

It is for reasons like this that Mike continues to help the industry by providing free resources such as this 101 rules document. It is the goal of Mike Holt and everyone on the Mike Holt Team to do everything in our power to aid in your pursuit of excellence.

For more great Free resources from Mike Holt visit

www.NECcode.com

About This Free PDF

These articles were extracted from Mike’s Understanding the NEC, Volume 1, 2008 Edition textbook. To understand the entire National Electrical Code, you need to study Mike’s comprehesive textbooks: Understanding the NEC, Volume 1 and Volume 2.

Introduction

Mike Holt Enterprises, Inc. • www.MikeHolt.com • 888.NEC.CODE (888.632.2633) vii

Introduction

Cross-References

This PDF contains thousands of NEC cross-references to other related Code requirements to help you develop a better understanding of how the NEC rules relate to one another. These cross-references are identified by a Code Section number in brackets, such as “90.4,” which would look like “[90.4].”

Author’s Comments

This PDF contains hundreds of “Author’s Comments.” These sections were written by Mike to help you better understand the NEC material, and to bring to your attention things he believes you should be aware of. To help you find them more easily, they are printed differently than the rest of the material.

Textbook Format

This textbook follows the NEC format, but it doesn’t cover every Code requirement. For example, it doesn’t include every article, section, subsection, exception, or fine print note. So don’t be concerned if you see the textbook contains Exception No. 1 and Exception No. 3, but not Exception No. 2.

Graphics with red borders are graphics that contain a 2008 change; graphics without a red border are graphics that sup-port the concept being discussed, but nothing in the graphic was affected by a 2008 Code change.

Special Sections and Examples. Additional information to better help you understand a concept is identified with light green shading. In addition, examples are highlighted with a yellow background.

Volume 1 covers general installation requirements, branch circuits, feeders, services and overcurrent protection, ground-ing versus bonding, conductors, cables and raceways, boxes, panels, motors and transformers, and more, in Articles 90 through 460 (NEC Chapters 1 through 4).

Volume 2 covers requirements for wiring in special occu-pancies, special equipment, under special conditions, as well as communications systems requirements in Articles 500 through 830 (NEC Chapters 5-8)

NEC Library

The NEC Library includes the Understanding the NEC Volumes 1 & 2 textbooks, the NEC Practice Questions book and ten videos or DVDs (a total of 41.5 hours). This option allows you to learn at the most cost effective price.

How to Use This PDF

Not an NEC Replacement

This PDF is to be used with the NEC, not as a replacement for the NEC, it is intended to explain the requirements of the NEC. Be sure to have a copy of the 2008 National Electrical Code handy, and always compare Mike’s explanation, com-ments, and graphics to the actual language contained in the NEC.

You’ll sometimes notice that the titles of a few Articles and Sections are different than they appear in the actual Code. This only occurs when Mike feels it’s easier to understand the con-tent of the rule, so please keep this in mind when comparing the two documents.

Compare what Mike has explained in the text to your Code book, and discuss those topics that you find difficult to understand with others. As you read through this PDF, be sure to take the time to review the text with the outstanding graphics and examples.

viii Understanding the National Electrical Code Rules for Backup Power Systems

Introduction

PDF Format

This PDF follows the NEC format, but each rule doesn’t always cover the entire Code subsection. So don’t be con-cerned if you see that the Rule contains Exception No. 1 and Exception No. 3, but not Exception No. 2. In addition, at times, the title of an Article, Section, or Subsection might be rephrased differently.

Difficult Concepts

As you progress through this PDF, you might find that you don’t understand every explanation, example, calculation, or comment. Don’t get frustrated, and don’t get down on yourself. Remember, this is the National Electrical Code and sometimes the best attempt to explain a concept isn’t enough to make it perfectly clear. When this happens to you, just make it a point to highlight the section that is causing you difficulty. If you can, take this PDF to someone you feel can provide additional insight, possibly your boss, the electrical inspector, a co-worker, your instructor, etc.

PDF Errors and Corrections

Humans develop the text, graphics, and layout of this PDF, and since currently none of us is perfect, there may be a few errors. This could occur because the NEC is dramati-cally changed each Code cycle; new Articles are added, some deleted, some relocated, and many renumbered. In addition, this PDF must be written within a very narrow window of opportunity; after the NEC has been published (September), yet before it’s enforceable (January).

You can be sure we work a tremendous number of hours and use all of our available resources to produce the finest product with the fewest errors. We take great care in research-ing the Code requirements to ensure this textbook is correct. If you feel there’s an error of any type in this textbook (typo, grammar, or technical), no matter how insignificant, please let us know.

Any errors found after printing are listed on our Website, so if you find an error, first check to see if it has already been corrected. Go to www.MikeHolt.com, click on the “Books” link, and then the “Corrections” link (www.MikeHolt.com/bookcorrec-tions.htm).

If you do not find the error listed on the Website, contact us by E-mailing [email protected], calling 888.NEC.CODE (888.632.2633), or faxing 954.720.7944. Be sure to include the book title, page number, and any other pertinent information.

Internet

Today as never before, you can get your technical questions answered by posting them to Mike Holt’s Code Forum. Just visit www.MikeHolt.com and click on the “Code Forum” link.

Different Interpretations

Some electricians, contractors, instructors, inspectors, engi-neers, and others enjoy the challenge of discussing the Code requirements, hopefully in a positive and a productive manner. This action of challenging each other is important to the process of better understanding the NEC’s require-ments and its intended application. However, if you’re going to get into an NEC discussion, please do not spout out what you think without having the actual Code in your hand. The professional way of discussing an NEC requirement is by referring to a specific section, rather than by talking in vague generalities.

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TitleHow to Use the National Electrical Code

applications. If you don’t understand a term used in a Code rule, it will be impossible to properly apply the NEC require-ment. Be sure you understand that Article 100 defines the terms that apply to two or more articles. For example, the term “Dwelling Unit” applies to many articles. If you don’t know what a dwelling unit is, how can you apply the Code requirements for it?

In addition, many articles have terms unique for that specific article. This means that the definitions of those terms are only applicable for that given article. For example, Section 250.2 contains the definitions of terms that only apply to Article 250, Grounding and Bonding.

Small Words, Grammar, and Punctuation

It’s not only the technical words that require close attention, because even the simplest of words can make a big differ-ence to the intent of a rule. The word “or” can imply alter-nate choices for equipment wiring methods, while “and” can mean an additional requirement. Let’s not forget about gram-mar and punctuation. The location of a comma “,” can dra-matically change the requirement of a rule.

Slang Terms or Technical Jargon

Electricians, engineers, and other trade-related professionals use slang terms or technical jargon that isn’t shared by all. This makes it very difficult to communicate because not everybody understands the intent or application of those slang terms. So where possible, be sure you use the proper word, and don’t use a word if you don’t understand its definition and applica-tion. For example, lots of electricians use the term “pigtail” when describing the short conductor for the connection of a receptacle, switch, luminaire, or equipment. Although they may understand this, not everyone does.

NEC Style and Layout

Before we get into the details of the NEC, we need to take a few moments to understand its style and layout. Understanding the structure and writing style of the Code is very important before it can be used effectively. If you think about it, how

This textbook is to be used with the NEC, not as a replacement for the NEC, so be sure to have a copy of the 2008 National Electrical Code handy. Compare what Mike explains in the text to your Code book, and discuss those topics that you find difficult to understand with others. As you read through this textbook, be sure to take the time to review the text with the outstanding graphics and examples.

The National Electrical Code is written for persons who under-stand electrical terms, theory, safety procedures, and electrical trade practices. These individuals include electricians, elec-trical contractors, electrical inspectors, electrical engineers, designers, and other qualified persons. The Code is not writ-ten to serve as an instructive or teaching manual for untrained individuals [90.1(C)].

Learning to use the NEC is somewhat like learning to play the game of chess; it’s a great game if you enjoy mental warfare. When learning to play chess, you must first learn the names of the game pieces, how the pieces are placed on the board, and how each piece moves.

Once you understand the fundamentals of the game of chess, you’re ready to start playing the game. Unfortunately, at this point all you can do is make crude moves, because you really don’t understand how all the information works together. To play chess well, you’ll need to learn how to use your knowl-edge by working on subtle strategies before you can work your way up to the more intriguing and complicated moves.

Not a Game

Electrical work isn’t a game, and it must be taken very seri-ously. Learning the basics of electricity, important terms and concepts, as well as the basic layout of the NEC gives you just enough knowledge to be dangerous. There are thousands of specific and unique applications of electrical installations, and the Code doesn’t cover every one of them. To safely apply the NEC, you must understand the purpose of a rule and how it affects the safety aspects of the installation.

NEC Terms and Concepts

The NEC contains many technical terms, so it’s cru-cial for Code users to understand their meanings and their

x Understanding the National Electrical Code Rules for Backup Power Systems


• Article 110 General Requirements• Article 250 Grounding and Bonding• Article 300 Wiring Methods• Article 430 Motors and Motor Controllers• Article 500 Hazardous (Classified) Locations• Article 680 Swimming Pools, Fountains, and

Similar Installations• Article 725 Remote-Control, Signaling, and

Power-Limited Circuits• Article 800 Communications Systems

5. Parts. Larger articles are subdivided into parts.

Author’s Comment: Because the parts of a Code article aren’t included in the section numbers, we have a tendency to forget what “Part” the NEC rule is relating to. For example, Table 110.34(A) contains the working space clearances for electri-cal equipment. If we aren’t careful, we might think this table applies to all electrical installations, but Table 110.34(A) is located in Part III, which contains the requirements for Over 600 Volts, Nominal installations. The rules for working clearances for electrical equipment for systems 600V, nominal, or less are con-tained in Table 110.26(A)(1), which is located in Part II—600 Volts, Nominal, or Less.

6. Sections and Tables.

Sections. Each NEC rule is called a Code section. A Code sec-tion may be broken down into subsections by letters in paren-theses “(A), (B),” etc. Numbers in parentheses (1), (2), etc., may further break down a subsection, and lowercase letters (a), (b), etc., further break the rule down to the third level. For example, the rule requiring all receptacles in a dwelling unit bathroom to be GFCI protected is contained in Section 210.8(A)(1). Section 210.8(A)(1) is located in Chapter 2, Article 210, Section 8, subsection (A), sub-subsection (1).

Many in the industry incorrectly use the term “Article” when referring to a Code section. For example, they say “Article 210.8,” when they should say “Section 210.8.”

Tables. Many Code requirements are contained within tables, which are lists of NEC requirements placed in a systematic arrangement. The titles of the tables are extremely important; you must read them carefully in order to understand the con-tents, applications, limitations, etc., of each table in the Code. Many times notes are provided in or below a table; be sure to read them as well since they are also part of the requirement. For example, Note 1 for Table 300.5 explains how to mea-sure the cover when burying cables and raceways, and Note 5 explains what to do if solid rock is encountered.

can you use something if you don’t know how it works? The National Electrical Code is organized into ten components.

1. Table of Contents 2. Article 90 (Introduction to the Code) 3. Chapters 1 through 9 (major categories) 4. Articles 90 through 830 (individual subjects) 5. Parts (divisions of an article) 6. Sections and Tables (Code requirements) 7. Exceptions (Code permissions) 8. Fine Print Notes (explanatory material) 9. Annexes (information)10. Index

1. Table of Contents. The Table of Contents displays the layout of the Chapters, Articles, and Parts as well as the page num-bers. It’s an excellent resource and should be referred to peri-odically to observe the interrelationship of the various NEC components. When attempting to locate the rules for a par-ticular situation, knowledgeable Code users often go first to the Table of Contents to quickly find the specific NEC part that applies.

2. Introduction. The NEC begins with Article 90, the introduc-tion to the Code. It contains the purpose of the NEC, what is covered and what is not covered along with how the Code is arranged. It also gives information on enforcement and how mandatory and permissive rules are written as well as how explanatory material is included. Article 90 also includes information on formal interpretations, examination of equip-ment for safety, wiring planning, and information about for-matting units of measurement.

3. Chapters. There are nine chapters, each of which is divided into articles. The articles fall into one of four group-ings: General Requirements (Chapters 1 through 4), Specific Require ments (Chapters 5 through 7), Communications Systems (Chapter 8), and Tables (Chapter 9).

• Chapter 1 General• Chapter 2 Wiring and Protection• Chapter 3 Wiring Methods and Materials• Chapter 4 Equipment for General Use• Chapter 5 Special Occupancies• Chapter 6 Special Equipment• Chapter 7 Special Conditions• Chapter 8 Communications Systems (Telephone,

Data, Satellite, and Cable TV)• Chapter 9 Tables–Conductor and Raceway

Specifications

4. Articles. The NEC contains approximately 140 articles, each of which covers a specific subject. For example:

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Author’s Comment: The number 70 precedes all page numbers because the NEC is NFPA standard number 70.

Index. If you use the Index, which lists subjects in alphabeti-cal order, to look up the term “service disconnect,” you’ll see there’s no listing. If you try “disconnecting means,” then “ser-vices,” you’ll find the Index specifies the rule is located in Article 230, Part VI. Because the NEC doesn’t give a page number in the Index, you’ll need to use the Table of Contents to find the page number, or flip through the Code to Article 230, then con-tinue to flip through pages until you find Part VI.

Many people complain that the NEC only confuses them by taking them in circles. As you gain experience in using the Code and deepen your understanding of words, terms, prin-ciples, and practices, you will find the NEC much easier to understand and use than you originally thought.

Customizing Your Code Book

One way to increase your comfort level with the Code is to cus-tomize it to meet your needs. You can do this by highlighting and underlining important NEC requirements, and by attach-ing tabs to important pages.

Highlighting. As you read through this textbook, be sure you highlight those requirements in the Code that are the most important or relevant to you. Use yellow for general inter-est and orange for important requirements you want to find quickly. Be sure to highlight terms in the Index and Table of Contents as you use them.

Underlining. Underline or circle key words and phrases in the NEC with a red pen (not a lead pencil) and use a six-inch ruler to keep lines straight and neat. This is a very handy way to make important requirements stand out. A small six-inch ruler also comes in handy for locating specific information in the many Code tables.

Tabbing the NEC. By placing tabs on Code articles, sections, and tables, it will make it easier for you to use the NEC. However, too many tabs will defeat the purpose. You can order a custom set of Code tabs online at www.MikeHolt.com, or by calling 1.888.NEC.CODE.

7. Exceptions. Exceptions are Code requirements or allow-ances that provide an alternative method to a specific require-ment. There are two types of exceptions—mandatory and permissive. When a rule has several exceptions, those excep-tions with mandatory requirements are listed before the per-missive exceptions.

Mandatory Exception. A mandatory exception uses the words “shall” or “shall not.” The word “shall” in an exception means that if you’re using the exception, you’re required to do it in a particular way. The phrase “shall not” means it isn’t permitted.

Permissive Exception. A permissive exception uses words such as “shall be permitted,” which means it’s acceptable (but not mandatory) to do it in this way.

8. Fine Print Note (FPN). A fine print note contains explana-tory material intended to clarify a rule or give assistance, but it isn’t a Code requirement [90.5(C)].

9. Annexes. Annexes aren’t a part of the NEC requirements, and are included in the Code for informational purposes only.

10. Index. The Index at the back of the NEC is helpful in locating a specific rule.

Author’s Comment: Changes to the NEC since the previous edition(s), are identified by shading, but rules that have been relocated aren’t identified as a change. A bullet symbol “•” is located on the margin to indicate the location of a rule that was deleted from a previous edition.

How to Locate a Specific Requirement

How to go about finding what you’re looking for in the Code depends, to some degree, on your experience with the NEC. Code experts typically know the requirements so well they just go to the correct rule without any outside assistance. The Table of Contents might be the only thing very experienced NEC users need to locate the requirement they’re looking for. On the other hand, average Code users should use all of the tools at their disposal, and that includes the Table of Contents and the Index.

Table of Contents. Let’s work out a simple example: What NEC rule specifies the maximum number of disconnects per-mitted for a service? If you’re an experienced Code user, you’ll know Article 230 applies to “Services,” and because this arti-cle is so large, it’s divided up into multiple parts (actually eight parts). With this knowledge, you can quickly go to the Table of Contents and see that it lists Service Equipment Disconnecting Means requirements in Part VI.

xii Understanding the National Electrical Code Rules for Backup Power Systems

Title

About the Author

Mike Holt worked his way up through the electrical trade from an apprentice electri-cian to become one of the most recognized

experts in the world as it relates to elec-trical power installations. He was a Journeyman Electrician, Master Electrician, and Electrical Con-tractor. Mike came from the real

world, and he has a unique under-standing of how the NEC relates to

electrical installations from a practi-cal standpoint. You’ll find his writing style to be simple, nontechnical, and practical.

Did you know that he didn’t finish high school? So if you struggled in high school or if you didn’t finish it at all, don’t let this get you down, you’re in good com-pany. As a matter of fact, Mike Culbreath, Master Electrician, who produces the finest electrical graphics in the history of the electrical industry, didn’t finish high school either. So two high school dropouts produced the text and graphics in this text-book! However, realizing success depends on one’s continuing pursuit of education, Mike immediately attained his GED (as did Mike Culbreath) and ultimately attended the University of Miami’s Graduate School for a Master’s degree in Business Administration (MBA).

Mike Holt resides in Central Florida, is the father of seven children, and has many outside interests and activities. He is a five-time National Barefoot Water-Ski Champion (1988, 1999, 2005, 2006, and 2007); he has set many national records and continues to train year-round at a World competition level [www.barefootwaterskier.com].

What sets him apart from some is his commitment to living a balanced lifestyle; he places God first, then family, career, and self.

Acknowledgments

Special Acknowledgments

First, I want to thank God for my godly wife who is always by my side and my children, Belynda, Melissa, Autumn, Steven, Michael, Meghan, and Brittney.

A special thank you must be sent to the staff at the National Fire Protection Association (NFPA), publishers of the NEC—in particular Jeff Sargent for his assistance in answer-ing my many Code questions over the years. Jeff, you’re a “first class” guy, and I admire your dedication and commitment to helping others understand the NEC. Other former NFPA staff members I would like to thank include John Caloggero, Joe Ross, and Dick Murray for their help in the past.

A personal thank you goes to Sarina, my long-time friend and office manager. It has been wonderful working side-by-side with you for over 25 years nurturing this company’s growth from its small beginnings.

Mike Holt Enterprises Team

Graphic Illustrator

Mike Culbreath devoted his career to the electrical industry and worked his way up from an apprentice elec-trician to master electrician. While working as a journeyman electri-cian, he suffered a serious on-the-job knee injury. With a keen interest in continuing education for electri-cians, he completed courses at Mike

Holt Enterprises, Inc. and then passed the exam to receive his Master Electrician’s license. In 1986, after attending classes at Mike Holt Enterprises, Inc. he joined the staff to update mate-rial and later studied computer graphics and began illustrat-ing Mike Holt’s textbooks and magazine articles. He’s worked with the company for over 20 years and, as Mike Holt has proudly acknowledged, has helped to transform his words and visions into lifelike graphics.

Mike Holt Enterprises, Inc. • www.MikeHolt.com • 888.NEC.CODE (888.632.2633) xiii

Acknowledgments

Production TeamI would like to thank Tara Moffitt and Cathleen Kwas who worked as a team to do the layout and production of this book. Their desire to create the best possible product for our customers is appreciated.

Video Team Members

Steve Arne, Mike Culbreath and Ryan Jackson (members of the Mike Holt Enterprises Team) were video team members, along with the following highly qualified professionals.

Tarry Baker

Chief Electrical Code Compliance OfficerBroward County Board of Rules & AppealsFort Lauderdale, Florida

Tarry Baker has been the Chief Electrical Code Compliance Officer for the Broward County Board of Rules and Appeals (Florida) for the last 16 years, standardizing enforcement in 32 municipalities and the unincor-porated area of Broward County. He has served on the Electrical Technical

Committee to the Board of Rules and Appeals for over 10 years, and as an advisor to the committee for 10 years. He was a principal member of the NEC Code-Making Panel-13 (2002, 2005 and 2008 NEC) and 20 (2008 NEC) for the IAEI, Electrical/Alarms Technical Advisory Committee to the Florida Building Commission, and is a former member of the Education, Disciple, and Licensing Work Group for the Governor’s Building Code Study Commission for the State of Florida. He serves as an electrical coordinator and instruc-tor for continuing education for the Broward County Board of Rules and Appeals and IAEI Maynard Hamilton/Fort Lauderdale Division.

He currently serves as Chaplain and past Chairman of the Code Question Committee for the IAEI Southern Section, and is a past President of the IAEI Florida Chapter. Tarry is also currently serving as Chairman of the Central Examining Board of Electricians of Broward County. He has been an Electrical Inspector since 1977 and an Electrical Plans Examiner since 1978. He is a State of Florida Certified Electrical Contractor, Building Code Administrator, Electrical Plans Examiner, Electrical Inspector, and One and Two Family Combination Inspector.

Technical Editorial DirectorSteve Arne has been involved in the electrical industry since 1974 working in various positions from electrician to full-time instructor and depart-ment chair in technical postsecondary education. Steve has developed cur-riculum for many electrical training courses and has developed university

business and leadership courses. Currently, Steve offers occa-sional exam prep and continuing education Code classes. Steve believes that as a teacher he understands the joy of helping others as they learn and experience new insights. His goal is to help others understand more of the technological marvels that surround us. Steve thanks God for the wonders of His creation and for the opportunity to share it with others.

Steve and his lovely wife Deb live in Rapid City, South Dakota where they are both active in their church and community. They have two grown children and five grandchildren.

Technical Code Consultant

Ryan Jackson is a combination inspec-tor for Draper City, Utah. He is certi-fied as a building, electrical, mechanical, and plumbing inspector. He’s also cer-tified as a building plans examiner and electrical plans examiner. Ryan is the senior electrical inspector for Draper City, and also teaches seminars on the

NEC. Ryan is very active in the Utah Chapter of IAEI, where he is currently president. He also enjoys staying active in the NEC change process, and loves to help people with their Code problems. On Mike Holt’s Code Forum, he has been involved in nearly 5,000 topics.

Ryan enjoys reading, going to college football games, and spending time with his wife Sharie and their two children, Kaitlynn and Aaron.

Editorial TeamI would like to thank Toni Culbreath and Barbara Parks who worked tirelessly to proofread and edit the final stages of this publication. Their attention to detail and dedication to this project is greatly appreciated.

xiv Understanding the National Electrical Code Rules for Backup Power Systems

Doug DoutyD & D Resources, Owner Fresno City College Fresno, California

Doug Douty has been a consultant in the electrical and building fields for many years. Doug’s vision is to help you maximize your talents and abili-ties and to provide solutions to diffi-cult problems. He is a member of the National Fire Protection Association, the International Association of Electrical Inspectors, and a 2006

winner of Mike Holt’s Top Gun presentation award.

Doug holds a Master of Science degree in Industrial Technology from California State University, Fresno, and California State Contractor’s license in electrical general con-tracting. He is an instructor in the electronics department at Fresno City College, an Adjunct Professor at California State University, Fresno, and a nationally recognized speaker in the electrical industry.

Doug resides in Fresno in the Central Valley of California with his wife. He has four grown children and five grandchil-dren. Doug has a passion for helping people reach their full potential in life. He also enjoys cycling, running, reading, and continual learning as well as family vacations.

Doug’s motivational style of teaching captures his audience’s attention. At Doug’s seminars, students not only increase their knowledge, but their confidence as well. Seminar topics include Electrical Safety in the Workplace (NFPA 70E), the National Electrical Code (NFPA 70), AC Fundamentals, AC Power Systems, Control Systems, Electrical Certification Preparation, and Continuing Education.

Eric StrombergElectrical Engineer/Instructor Dow Chemical Lake Jackson, TX

Eric Stromberg enrolled in the University of Houston in 1976, with Electrical Engineering as his major. During the first part of his college years, Eric worked for a company that specialized in installing profes-sional sound systems. Later, he worked for a small electrical company and

eventually became a journeyman electrician. After gradua-tion from college in 1982, Eric went to work for an electron-ics company that specialized in fire alarm systems for high-rise buildings. He became a state licensed Fire Alarm Installation Superintendent and was also a member of IBEW local union 716. In 1989, Eric began a career with the Dow Chemical Company as an Electrical Engineer designing power distribu-tion systems for large industrial facilities. In 1997, Eric began teaching National Electrical Code classes.

Eric currently resides in Lake Jackson, Texas, with his wife Jane and three children: Ainsley, Austin, and Brieanna.

Kevin VogelProfessional Engineer/Instructor Crescent Electric Supply Dalton Gardens, ID

Kevin Vogel graduated from Santa Clara University in 1964 with a Bachelor of Science degree in Mechanical Engineering. A licensed Professional Engineer since 1969, he formerly worked as chief engineer for a manufacturer of electric heating prod-ucts and of (the first) thermoplastic

electrical outlet boxes. He has also worked as an electrician and holds a Master Electrician’s license.

In 1978, Kevin was co-founder of an electrical wholesale dis-tribution company in Coeur d’Alene, ID that was sold in 1991 to Crescent Electric Supply Company. Kevin continues to work in that part of the industry. He also provides expert witness testimony in civil and criminal cases, and serves as a certified instructor for National Electrical Code classes at North Idaho College. He also provides occasional assistance to Trindera Engineering, an electrical engineering consulting firm located in Coeur d’Alene.

Kevin married his beloved wife, Linda, in 1966, and they have been blessed with 13 wonderful children and 17 (so far) grand-children. Kevin is extremely grateful to God for all the gifts He has bestowed on him and on his loved ones.

Advisory CommitteeRahe Loftin P.E.Fire Protection Engineer U.S. General Services Administration Fort Worth, TX

Acknowledgments

Mike Holt Enterprises, Inc. • www.MikeHolt.com • 888.NEC.CODE (888.632.2633) 87

AR

TIC

LE 300 Wiring Methods

INTRODUCTION TO ARTICLE 300—WIRING METHODSArticle 300 contains the general requirements for all wiring methods included in the NEC. However, this article doesn’t apply to communica-tions systems, which is covered in Chapter 8, except when Article 300 is specifically referenced in Chapter 8.

This article is primarily concerned with how to install, route, splice, protect, and secure conductors and raceways. How well you conform to the requirements of Article 300 will generally be evident in the finished work, because many of the requirements tend to determine the appearance of the installation.

Because of this, it’s often easy to spot Article 300 problems if you’re looking for Code violations. For example, you can easily see when someone runs an equipment grounding conductor outside a raceway instead of grouping all conductors of a circuit together, as required by 300.3(B).

This is just one of the common points of confusion your studies here will clear up for you. To help achieve that end, be sure to carefully con-sider the accompanying illustrations, and also refer to Article 100 as needed.

PART I. GENERAL REqUIREMENTS

300.1 Scope.

(A) Wiring Installations. Article 300 contains the general requirements for power and lighting wiring methods.

Author’s Comment: The requirements contained in Article 300 don’t apply to the wiring methods for signaling and communica-tions systems. However, the following sections in Chapter 7 and 8 refer back to Article 300 requirements.

• CATV, 820.3• Class 2 and 3 Circuits, 725.3• Communications Cables and Raceways, 800.133(A)(2)• Fire Alarm Circuits, 760.3

(B) Integral Parts of Equipment. The requirements contained in Article 300 don’t apply to the internal parts of electrical equipment. Figure 300–1

(C) Trade Sizes. Designators for raceway trade sizes are given in Table 300.1(C).

Author’s Comment: Industry practice is to describe raceways using inch sizes, such as 1⁄2 in., 2 in., etc.; however, the proper reference (2005 NEC change) is to use “Trade Size 1⁄2,” or “Trade Size 2.” In this textbook we use the term “Trade Size.”

Figure 300–1

88 Understanding the National Electrical Code Rules for Backup Power Systems

300.3 Wiring Methods

(2) Grounding and Bonding Conductors. Equipment ground-ing conductors can be installed outside of a raceway or cable assembly for certain existing installations. See 250.130(C). Equipment grounding jumpers can be located outside of a flexible raceway if the bonding jumper is installed in accor-dance with 250.102(E). Figure 300–4

(3) Nonferrous Wiring Methods. Circuit conductors can be run in different raceways (Phase A in raceway 1, Phase B in raceway 2, etc.) if, in order to reduce or eliminate induc-tive heating, the raceway is nonmetallic or nonmagnetic and the installation complies with 300.20(B). See 300.3(B)(1) and 300.5(I) Ex 2.

300.3 Conductors.

(A) Conductors. Conductors must be installed within a Chapter 3 wiring method, such as a raceway, cable, or enclosure.

Exception: Overhead conductors can be installed in accordance with 225.6.

(B) Circuit Conductors Grouped Together. All conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray, except as permitted by (1) through (4). Figure 300–2

Author’s Comment: All conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray to minimize induction heating of ferrous metal raceways and enclo-sures, and to maintain a low-impedance ground-fault current path [250.4(A)(3)].

(1) Paralleled Installations. Conductors can be run in paral-lel, in accordance with 310.4, and must have all circuit con-ductors within the same raceway, cable tray, trench, or cable. Figure 300–3

Exception: Parallel conductors run underground can be run in different raceways (Phase A in raceway 1, Phase B in race-way 2, etc.) if, in order to reduce or eliminate inductive heating, the raceway is nonmetallic or nonmagnetic and the installation complies with 300.20(B). See 300.3(B)(3) and 300.5(I) Ex 2.

Figure 300–3

Figure 300–2

Figure 300–4


Wiring Methods 300.4

300.4 Protection Against Physical Damage. Conduc-tors must be protected against physical damage [110.27(B)].

(A) Cables and Raceways Through Wood Members. When the following wiring methods are installed through wood mem-bers, they must comply with (1) and (2). Figure 300–8

• ArmoredCable,Article320• ElectricalNonmetallicTubing,Article362• FlexibleMetalConduit,Article348• LiquidtightFlexibleMetalConduit,Article350• LiquidtightFlexibleNonmetallicConduit,Article

356• Metal-CladCable,Article330

(C) Conductors of Different Systems.

(1) Mixing. Power conductors of ac and dc systems rated 600V or less can occupy the same raceway, cable, or enclo-sure if all conductors have an insulation voltage rating not less than the maximum circuit voltage. Figure 300–5

FPN: See 725.136(A) for Class 2 and Class 3 circuit conductors.

Author’s Comments:

• Control, signal, and communications wiring must be sepa-rated from power and lighting circuits so the higher-voltage conductors don’t accidentally energize the control, signal, or communications wiring:

– CATV Coaxial Cable, 820.133(A)– Class 1, Class 2, and Class 3 Control Circuits, 725.48 and

725.136(A). Figure 300–6– Communications Circuits, 800.133(A)(1)(c)– Fire Alarm Circuits, 760.136(A)– Instrumentation Tray Cable, 727.5– Sound Circuits, 640.9(C)

• A Class 2 circuit that has been reclassified as a Class 1 circuit [725.130(A) Ex 2] can be run with associated power conduc-tors [725.48(B)(1)] if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(1)]. Figure 300–7

Figure 300–5

Figure 300–7

Figure 300–6



CAUTION: When drilling or notching wood members, be sure to check with the building inspector to ensure you don’t damage or weaken the structure and violate the building code.

Exception No. 1: A steel plate isn’t required to protect rigid metal conduit, intermediate metal conduit, PVC conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1⁄16 in. thick that provides equal or better protection against nail or screw penetration is permitted. Figure 300–10

• Nonmetallic-SheathedCable,Article334• Service-EntranceCable,Article338• Underground Feeder and Branch-Circuit Cable,

Article 340

(1) Holes in Wood Members. Holes through wood framing members for the above cables or raceways must be not less than 11⁄4 in. from the edge of the wood member. If the edge of the hole is less than 11⁄4 in. from the edge, a 1⁄16 in. thick steel plate of sufficient length and width must be installed to protect the wiring method from screws and nails.

Exception No. 1: A steel plate isn’t required to protect rigid metal conduit, intermediate metal conduit, PVC conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1⁄16

in. thick that provides equal or better protection against nail or screw penetration is permitted. Figure 300–9

Author’s Comment: Hardened steel plates thinner than 1⁄16 in. have been tested and found to provide better protection from screw and nail penetration than the thicker plates.

(2) Notches in Wood Members. Where notching of wood framing members for cables and raceways are permitted by the building code, a 1⁄16 in. thick steel plate of sufficient length and width must be installed to protect the wiring method laid in these wood notches from screws and nails.

Figure 300–8

Figure 300–9

Figure 300–10



• Fire Alarm Cable, 760.7 and 760.8• Optical Fiber Cable, 770.21 and 770.24• Audio Cable, 640.6(B)

(D) Cables and Raceways Parallel to Framing Members and Furring Strips. Cables or raceways run parallel to framing members or furring strips must be protected where they are likely to be penetrated by nails or screws, by installing the wiring method so it isn’t less than 11⁄4 in. from the nearest edge of the framing member or furring strip. If the edge of the framing member or furring strip is less than 11⁄4 in. away, a 1⁄16 in. thick steel plate of sufficient length and width must be installed to protect the wiring method from screws and nails. Figure 300–13

Author’s Comment: This rule doesn’t apply to control, signaling, and communications cables, but similar requirements are con-tained in Chapters 6, 7, and 8 as follows:

• CATV Coaxial Cable, 820.24• Communications Cable, 800.24• Control and Signaling Cable, 725.24• Optical Fiber Cable, 770.24• Fire Alarm Cable, 760.8• Audio Cable, 640.6(B)

Exception No. 1: Protection isn’t required for rigid metal con-duit, intermediate metal conduit, PVC conduit, or electrical metallic tubing.

Exception No. 2: For concealed work in finished buildings, or finished panels for prefabricated buildings where such support-ing is impracticable, the cables can be fished between access points.

(B) Nonmetallic-Sheathed Cable and Electrical Nonmetallic Tubing Through Metal Framing Members.

(1) Nonmetallic-Sheathed Cable (NM). Where Type NM cables pass through factory or field openings in metal fram-ing members, the cable must be protected by listed bushings or listed grommets that cover all metal edges. The protection fitting must be securely fastened in the opening before the installation of the cable. Figure 300–11

(2) Type NM Cable and Electrical Nonmetallic Tubing. Where nails or screws are likely to penetrate Type NM cable or elec-trical nonmetallic tubing, a steel sleeve, steel plate, or steel clip not less than 1⁄16 in. in thickness must be installed to pro-tect the cable or tubing.

Exception: A listed and marked steel plate less than 1⁄16 in. thick that provides equal or better protection against nail or screw penetration is permitted.

(C) Behind Suspended Ceilings. Wiring methods, such as cables or raceways, installed behind panels designed to allow access must be supported in accordance with their applicable article. Figure 300–12

Author’s Comment: This rule doesn’t apply to control, signal, and communications cables, but similar requirements are con-tained in Chapters 6, 7, and 8 as follows:

• CATV Coaxial Cable, 820.21 and 820.24• Communications Cable, 800.21• Control and Signaling Cable, 725.21 and 725.24

Figure 300–11

Figure 300–12



installation by a fitting that provides a smooth, rounded insu-lating surface, such as an insulating bushing. Figure 300–14

Author’s Comment: Where IMC or RMC conduit enters an enclosure without a connector, a bushing must be provided, regardless of the conductor size [342.46 and 344.46].

Exception: Insulating bushings aren’t required where a raceway terminates in a threaded raceway entry that provides a smooth, rounded, or flared surface for the conductors. An example would be a meter hub fitting or a Meyer’s hub-type fitting.

300.5 Underground Installations.

(A) Minimum Burial Depths. When cables or raceways are run underground, they must have a minimum “cover” in accordance with Table 300.5. Figure 300–15

Author’s Comment: Note 1 to Table 300.5 defines “Cover” as the distance from the top of the underground cable or raceway to the surface of finish grade. Figure 300–16

Exception No. 3: A listed and marked steel plate less than 1⁄16

in. thick that provides equal or better protection against nail or screw penetration is permitted.

(E) Wiring Under Roof Decking. Wiring under metal-corru-gated sheet roof decking must not be less than 11/2 in. from the nearest surface of the roof decking.

Exception: Spacing from roof decking doesn’t apply to rigid metal conduit and intermediate metal conduit.

(F) Cables and Raceways Installed in Grooves. Cables and raceways installed in a groove must be protected by a 1⁄16 in. thick steel plate or sleeve, or by 11⁄4 in. of free space.

Author’s Comment: An example is Type NM cable installed in a groove cut into the Styrofoam-type insulation building block structure and then covered with wallboard.

Exception No. 1: Protection isn’t required if the cable is installed in rigid metal conduit, intermediate metal conduit, PVC conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1⁄16 in. thick that provides equal or better protection against nail or screw penetration is permitted.

(G) Insulating Fittings. Where raceways contain insulated conductors 4 AWG and larger that enter an enclosure, the conductors must be protected from abrasion during and after

Figure 300–13

Figure 300–14



Author’s Comment: The cover requirements contained in 300.5 don’t apply to the following signaling, communications and other power limited wiring: Figure 300–17

• CATV, 90.3• Class 2 and 3 Circuits, 725.3• Communications Cables and Raceways, 90.3• Fire Alarm Circuits, 760.3• Optical Fiber Cables and Raceways, 770.3

(B) Wet Locations. The interior of enclosures or raceways installed in an underground installation are considered to be a wet location. Cables and insulated conductors installed in underground enclosures or raceways must be listed for use in wet locations according to 310.8(C). Splices within an under-ground enclosure must be listed as suitable for wet locations [110.14(B)]. Figure 300–18

Author’s Comment: The definition of a “Wet Location,” as con-tained in Article 100, includes installations underground, in concrete slabs in direct contact with the earth, locations sub-ject to saturation with water, and unprotected locations exposed to weather. Where raceways are installed in wet locations above grade, the interior of these raceways is also considered to be a wet location [300.9].

(C) Cables Under Buildings. Cables run under a building must be installed in a raceway that extends past the outside walls of the building.

Table 300.5 Minimum Cover Requirements in Inches

LocationBuried Cables

Metal Raceway

Nonmetallic Raceway

Under Building 0 0 0

Dwelling Unit 24/12* 6 18

Dwelling Unit Driveway 18/12* 6 18/12*

Under Roadway 24 24 24

Other Locations 24 6 18

*Residential branch circuits rated 120V or less with GFCI protection and maximum overcurrent protection of 20A. Note: This is a summary of NEC Table 300.5. See the table in the NEC for full details.

Figure 300–15

Figure 300–16

Figure 300–17



(2) Conductors Entering Buildings. Conductors that enter a building must be protected to the point of entrance.

(3) Service Lateral Conductors. Direct-buried service-lateral conductors that aren’t under the exclusive control of the elec-tric utility, and are buried 18 in. or more below grade, must have their location identified by a warning ribbon placed in the trench not less than 1 ft above the underground installa-tion. Figure 300–20

(4) Enclosure or Raceway Damage. Where direct-bur-ied cables, enclosures, or raceways are subject to physi-cal damage, the conductors must be installed in rigid metal conduit, intermediate metal conduit, or Schedule 80 PVC conduit.

(E) Underground Splices and Taps. Direct-buried conductors or cables can be spliced or tapped underground without a splice box [300.15(G)], if the splice or tap is made in accor-dance with 110.14(B). Figure 300–21

(F) Backfill. Backfill material for underground wiring must not damage the underground cable or raceway, or contribute to the corrosion of the metal raceway.

(D) Protecting Underground Cables and Conductors. Direct-buried conductors and cables such as Types MC, UF and USE must be protected from damage in accordance with (1) through (4).

(1) Emerging from Grade. Direct-buried cables or conductors that emerge from grade must be installed in an enclosure or raceway to protect against physical damage. Protection isn’t required to extend more than 18 in. below grade, and protec-tion above ground must extend to a height not less than 8 ft. Figure 300–19

Figure 300–19

Figure 300–18

Figure 300–20



Exception No. 1: Conductors can be installed in parallel in accordance with 310.4.

Exception No. 2: Individual sets of parallel circuit conduc-tors can be installed in underground PVC conduits, if inductive heating at raceway terminations is reduced by complying with 300.20(B) [300.3(B)(1) and 300.3(B)(3)]. Figure 300–23

Author’s Comment: Installing ungrounded and neutral conduc-tors in different PVC conduits makes it easier to terminate larger parallel sets of conductors, but it will result in higher levels of electromagnetic fields (EMF), which can cause computer moni-tors to flicker in a distracting manner.

Author’s Comment: Large rocks, chunks of concrete, steel rods, mesh, and other sharp-edged objects must not be used for backfill material, because they can damage the underground conductors, cables, or raceways.

(G) Raceway Seals. Where moisture could enter a raceway and contact energized live parts, a seal must be installed at one or both ends of the raceway.

Author’s Comment: This is a common problem for equipment located downhill from the supply, or in underground equipment rooms. See 230.8 for service raceway seals and 300.7(A) for different temperature area seals.

FPN: Hazardous explosive gases or vapors make it neces-sary to seal underground conduits or raceways that enter the building in accordance with 501.15.

Author’s Comment: It isn’t the intent of this FPN to imply that sealing fittings of the types required in hazardous (classi-fied) locations be installed in unclassified locations, except as required in Chapter 5. This also doesn’t imply that the seal-ing material provides a watertight seal, but only that it prevents moisture from entering the conduits or raceways.

(H) Bushing. Raceways that terminate underground must have a bushing or fitting at the end of the raceway to protect emerging cables or conductors.

(I) Conductors Grouped Together. All conductors of the same circuit, including the equipment grounding conductor, must be inside the same raceway, or in close proximity to each other. See 300.3(B). Figure 300–22

Figure 300–21

Figure 300–23

Figure 300–22



(2) Organic Coatings on Boxes or Cabinets. Boxes or cabinets having a system of organic coatings marked “Raintight,” “Rainproof,” or “Outdoor Type,” can be installed outdoors.

(3) In Concrete or in Direct Contact with the Earth. Ferrous metal raceways, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, nipples, fittings, supports, and support hardware can be installed in concrete or in direct contact with the earth, or in areas subject to severe corrosive influ-ences where made of material approved for the condition, or where provided with corrosion protection approved for the condition.

Author’s Comment: Galvanized steel electrical metallic tubing can be installed in concrete at grade level and in direct contact with earth, but supplementary corrosion protection is usually required (UL White Book, product category FJMX). Electrical metallic tubing can be installed in concrete above the ground floor slab generally without supplementary corrosion protection. Figure 300–25

(B) Aluminum Equipment. Aluminum raceways, cable trays, cablebus, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, nipples, fittings, supports, and support hardware embedded or encased in concrete or in direct con-tact with the earth must be provided with supplementary corrosion protection.

(J) Earth Movement. Direct-buried conductors, cables, or raceways that are subject to movement by settlement or frost must be arranged to prevent damage to conductors or equip-ment connected to the wiring.

(K) Directional Boring. Cables or raceways installed using directional boring equipment must be approved by the authority having jurisdiction for this purpose.

Author’s Comment: Directional boring technology uses a direc-tional drill, which is steered continuously from point “A” to point “B.” When the drill head comes out of the earth at point “B,” it’s replaced with a back-reamer and the duct or conduit being installed is attached to it. The size of the boring rig (hp, torque, and pull-back power) comes into play, along with the types of soil, in determining the type of raceways required. For telecom work, multiple poly innerducts are pulled in at one time. At major crossings, such as expressways, railroads, or rivers, outerduct may be installed to create a permanent sleeve for the innerducts.

“Innerduct” and “outerduct” are terms usually associated with optical fiber cable installations, while “unitduct” comes with conductors factory installed. All of these come in various sizes. Galvanized rigid metal conduit, schedule 40 and sched-ule 80 PVC, HDPE conduit and nonmetallic underground conduit with conductors (NUCC) are common wiring methods used with directional boring installations.

300.6 Protection Against Corrosion and Deteriora-tion. Raceways, cable trays, cablebus, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, sup-ports, and support hardware must be suitable for the environ-ment. Figure 300–24

(A) Ferrous Metal Equipment. Ferrous metal raceways, enclo-sures, cables, cable trays, fittings, and support hardware must be protected against corrosion inside and outside by a coat-ing of listed corrosion-resistant material. Where corrosion protection is necessary, such as underground and in wet loca-tions, and the conduit is threaded in the field, the threads must be coated with an approved electrically conductive, cor-rosion-resistant compound, such as cold zinc.

Author’s Comment: Nonferrous metal raceways, such as alu-minum rigid metal conduit, do not have to meet the provisions of this section.

(1) Protected from Corrosion Solely by Enamel. Where fer-rous metal parts are protected from corrosion solely by enamel, they must not be used outdoors or in wet locations as described in 300.6(D).

Figure 300–24



(1) Exposed to Sunlight. Where exposed to sunlight, the materials must be listed or identified as sunlight resistant.

(2) Chemical Exposure. Where subject to exposure to chem-ical solvents, vapors, splashing, or immersion, materials or coatings must either be inherently resistant to chemicals based upon their listing, or be identified for the specific chemical.

(D) Indoor Wet Locations. In portions of dairy processing facilities, laundries, canneries, and other indoor wet loca-tions, and in locations where walls are frequently washed or where there are surfaces of absorbent materials, such as damp paper or wood, the entire wiring system, where installed exposed, including all boxes, fittings, raceways, and cables, must be mounted so there’s at least 1⁄4 in. of airspace between it and the wall or supporting surface.

Author’s Comment: See the definitions of “Exposed” and “Location, Wet” in Article 100.

Exception: Nonmetallic raceways, boxes, and fittings are per-mitted without the airspace on a concrete, masonry, tile, or similar surface.

FPN: Areas where acids and alkali chemicals are handled and stored may present corrosive conditions, particularly when wet or damp. Severe corrosive conditions may also be present in portions of meatpacking plants, tanneries, glue houses, and some stables; in installations immediately adjacent to a seashore or swimming pool, spa, hot tub, and fountain areas; in areas where chemical deicers are used; and in storage cellars or rooms for hides, casings, fertilizer, salt, and bulk chemicals.

300.7 Raceways Exposed to Different Temperatures.

(A) Sealing. Where a raceway is subjected to different tem-peratures, and where condensation is known to be a problem, the raceway must be filled with a material approved by the authority having jurisdiction that will prevent the circulation of warm air to a colder section of the raceway. An explosion-proof seal isn’t required for this purpose. Figure 300–27

(B) Expansion Fittings. Raceways must be provided with expansion fittings where necessary to compensate for ther-mal expansion and contraction. Figure 300–28

FPN: Table 352.44 provides the expansion characteristics for PVC conduit. The expansion characteristics for metal raceways are determined by multiplying the values from Table 352.44 by 0.20, and the expansion characteristics for aluminum raceways is determined by multiplying the values from Table 352.44 by 0.40. Table 354.44 provides the expan-sion characteristics for reinforced thermosetting resin con-duit (RTRC).

(C) Nonmetallic Equipment. Nonmetallic raceways, cable trays, cablebus, boxes, cables with a nonmetallic outer jacket and internal metal armor or jacket, cable sheathing, cabinets, elbows, couplings, nipples, fittings, supports, and support hardware must be made of material identified for the condi-tion, and must comply with (1) and (2). Figure 300–26

Figure 300–25

Figure 300–26



Metal raceways and cable assemblies must be mechanically secured to boxes, fittings, cabinets, and other enclosures.

Exception No. 1: Short lengths of metal raceways used for the support or protection of cables aren’t required to be electrically continuous, nor are they required to be connected to an equip-ment grounding conductor of a type recognized in 250.118 [250.86 Ex 2 and 300.12 Ex]. Figure 300–32

300.8 Not Permitted in Raceways. Raceways are designed for the exclusive use of electrical conductors and cables, and are not permitted to contain nonelectrical com-ponents, such as pipes or tubes for steam, water, air, gas, drainage, etc. Figure 300–29

300.10 Electrical Continuity. Metal raceways, cables, boxes, fittings, cabinets, and enclosures for conductors must be metallically joined together to form a continuous, low-impedance fault current path capable of carrying any fault current likely to be imposed on it [110.10, 250.4(A)(3), and 250.122]. Figure 300–31

Figure 300–27

Figure 300–28

Figure 300–29

Figure 300–31



Author’s Comment: Outlet boxes [314.23(D)] and luminaires can be secured to the suspended-ceiling grid if securely fas-tened to the ceiling-framing member by mechanical means such as bolts, screws, or rivets, or by the use of clips or other secur-ing means identified for use with the type of ceiling-framing member(s) [410.36(B)].

(1) Fire-Rated Assembly. Electrical wiring within the cavity of a fire-rated floor-ceiling or roof-ceiling assembly can be supported by independent support wires attached to the ceil-ing assembly. The independent support wires must be distin-guishable from the suspended-ceiling support wires by color, tagging, or other effective means.

(2) Nonfire-Rated Assembly. Wiring in a nonfire-rated floor-ceiling or roof-ceiling assembly is not permitted to be secured to, or supported by, the ceiling assembly, including the ceil-ing support wires. Independent support wires used for sup-port can be attached to the nonfire-rated assembly.

Author’s Comment: Support wires within nonfire-rated assemblies aren’t required to be distinguishable from the sus-pended-ceiling framing support wires. Most suspended ceiling systems are not part of a fire-resistance rated assembly.

(B) Raceways Used for Support. Raceways must not be used as a means of support for other raceways, cables, or non-electrical equipment, except as permitted in (1) through (3). Figure 300–34

300.11 Securing and Supporting.

(A) Secured in Place. Raceways, cable assemblies, boxes, cabi-nets, and fittings must be securely fastened in place. The ceil-ing-support wires or ceiling grid must not be used to support raceways and cables (power, signaling, or communications). However, independent support wires that are secured at both ends and provide secure support are permitted. Figure 300–33

Figure 300–32

Figure 300–33Figure 300–34



Author’s Comment: When raceways are stubbed into an open-bottom switchboard or other apparatus, the raceway, including the end fitting, can’t rise more than 3 in. above the bottom of the switchboard enclosure [408.5].

(1) Identified. Where the raceway or means of support is identified for the purpose.

(2) Class 2 and 3 Circuits. Class 2 and 3 cable can be sup-ported by the raceway that supplies power to the equipment controlled by the Class 2 or 3 circuit. Figure 300–35

(3) Boxes Supported by Raceways. Raceways are permitted as a means of support for threaded boxes and conduit bodies in accordance with 314.23(E) and (F), or to support luminaires in accordance with 410.36(E).

(C) Cables Not Used as Means of Support. Cables must not be used to support other cables, raceways, or nonelectrical equipment. Figure 300–36

300.12 Mechanical Continuity. Raceways and cable sheaths must be mechanically continuous between boxes, cabinets, and fittings. Figure 300–37

Exception No. 1: Short sections of raceways used to provide support or protection of cable from physical damage aren’t required to be mechanically continuous [250.86 Ex 2 and 300.10 Ex 1]. Figure 300–38

Exception No. 2: Raceways at the bottom of open-bottom equipment, such as switchboards, motor control centers, and transformers, are not required to be mechanically secured to the equipment. Figure 300–39

Figure 300–35

Figure 300–36

Figure 300–38



Author’s Comment: The NEC doesn’t limit the number of exten-sion rings permitted on an outlet box, and the UL White Book (product category QCIT) specifically states that multiple exten-sions can be used. When multiple extensions are used, however, there must be not less than 3 in. of free conductors outside the opening of the final extension ring.

Exception: This rule doesn’t apply to conductors that pass through a box without a splice or termination.

300.17 Raceway Sizing. Raceways must be large enough to permit the installation and removal of conductors without damaging the conductor’s insulation.

Author’s Comment: When all conductors in a raceway are the same size and of the same insulation type, the number of con-ductors permitted can be determined by Annex C.

Question: How many 12 THHN conductors can be installed in trade size ¾ electrical metallic tubing? Figure 300–54

(a) 12 (b) 13 (c) 14 (d) 16

Answer: (d) 16 conductors [Annex C, Table C1]

Author’s Comment: When different size conductors are installed in a raceway, conductor fill is limited to the percentages of Table 1 of Chapter 9. Figure 300–55

300.14 Length of Free Conductors. At least 6 in. of free conductor, measured from the point in the box where the conductors enter the enclosure, must be left at each outlet, junction, and switch point for splices or terminations of luminaires or devices. Figure 300–44

Boxes that have openings less than 8 in. in any dimension, must have at least 6 in. of free conductor, measured from the point where the conductors enter the box, and at least 3 in. of free conductor outside the box opening. Figure 300–45

Figure 300–39

Figure 300–44

Figure 300–45



Author’s Comment: Where a raceway has a maximum length of 24 in., it can be filled to 60 percent of its total cross-sectional area [Chapter 9, Table 1, Note 4]. Figure 300–56

Step 1: When sizing a raceway, first determine the total area of conductors (Chapter 9, Table 5 for insulated conductors and Chapter 9, Table 8 for bare conductors). Figure 300–57

Step 2: Select the raceway from Chapter 9, Table 4, in accor-dance with the percent fill listed in Chapter 9, Table 1. Figure 300–58

Table 1, Chapter 9

Number Percent Fill

1 Conductor 53%

2 Conductors 31%

3 or more 40%

The above percentages are based on conditions where the length of the conductor and number of raceway bends are within reasonable limits [Chapter 9, Table 1, FPN No. 1].

Figure 300–54

Figure 300–55

Figure 300–56

Figure 300–57



Author’s Comment: When alternating current (ac) flows through a conductor, a pulsating or varying magnetic field is created around the conductor. This magnetic field is constantly expand-ing and contracting with the amplitude of the ac current. In the United States, the frequency is 60 cycles per second (Hz). Since ac reverses polarity 120 times per second, the magnetic field that surrounds the conductor also reverses its direction 120 times per second. This expanding and collapsing magnetic field induces eddy currents in the ferrous metal parts that sur-round the conductors, causing the metal parts to heat up from hysteresis.

Magnetic materials naturally resist the rapidly changing mag-netic fields. The resulting friction produces its own additional heat - hysteresis heating - in addition to eddy current heating. A metal which offers high resistance is said to have high mag-netic “permeability.” Permeability can vary on a scale of 100 to 500 for magnetic materials; nonmagnetic materials have a per-meability of one.

Simply put, the molecules of steel and iron align to the polar-ity of the magnetic field and when the magnetic field reverses, the molecules reverse their polarity as well. This back-and-forth alignment of the molecules heats up the metal, and the more the current flows, the greater the heat rises in the ferrous metal parts. Figure 300–63

When conductors of the same circuit are grouped together, the magnetic fields of the different conductors tend to cancel each other out, resulting in a reduced magnetic field around the conductors. The lower magnetic field reduces induced currents in the ferrous metal raceways or enclosures, which reduces hys-teresis heating of the surrounding metal enclosure.

300.20 Induced Currents in Ferrous Metal Enclo-sures and Raceways.

(A) Conductors Grouped Together. To minimize induction heating of ferrous metal raceways and ferrous metal enclo-sures for alternating-current circuits, and to maintain an effective ground-fault current path, all conductors of a cir-cuit must be installed in the same raceway, cable, trench, cord, or cable tray. See 250.102(E), 300.3(B), 300.5(I), and 392.8(D). Figure 300–62

Figure 300–58

Figure 300–63

Figure 300–62



WARNING: There has been much discussion in the press on the effects of electromagnetic fields on humans. According to the Institute of Electrical and Electronics Engineers (IEEE), there’s insufficient information at this time to define an unsafe electromagnetic field level.

(B) Single Conductors. When single conductors are installed in nonmetallic raceways as permitted in 300.5(I) Ex 2, the inductive heating of the metal enclosure must be mini-mized by the use of aluminum locknuts and by cutting a slot between the individual holes through which the conductors pass. Figure 300–64

FPN: Because aluminum is a nonmagnetic metal, aluminum parts don’t heat up due to hysteresis.

Author’s Comment: Aluminum conduit, locknuts, and enclo-sures carry eddy currents, but because aluminum is nonferrous, it doesn’t heat up [300.20(B) FPN].

Figure 300–64


310.1 Scope. Article 310 contains the general requirements for conductors, such as insulation markings, ampacity rat-ings, and their use. This article doesn’t apply to conductors that are an integral part of equipment [90.7 and 300.1(B)].

310.2 Conductors.

(A) Insulated. Conductors must be insulated and installed in a recognized wiring method [110.8]. Figure 310–1

Exception: Where covered or bare conductors are specifically permitted elsewhere in this Code. Figure 310–2

310.3 Stranded Conductors. Conductors 8 AWG and larger must be stranded when installed in a raceway. Figure 310–3

Author’s Comment: Solid conductors are often used for the grounding electrode conductor [250.62] and for the bonding of pools, spas, and outdoor hot tubs [680.26(C)]. Technically, the practice of installing 8 AWG and larger solid conductors in a raceway for protection of grounding and bonding conductors is a violation of this rule.

AR

TIC

LE 310 Conductors for General Wiring

INTRODUCTION TO ARTICLE 310—CONDUCTORS FOR GENERAL WIRINGThis article contains the general requirements for conductors, such as insulation markings, ampacity ratings, and conditions of use. Article 310 doesn’t apply to conductors that are part of flexible cords, fixture wires, or to conductors that are an integral part of equipment [90.7 and 300.1(B)].

People often make errors in applying the ampacity tables contained in Article 310. If you study the explanations carefully, you’ll avoid common errors such as applying Table 310.17 when you should be applying Table 310.16.

Why so many tables? Why does Table 310.17 list the ampacity of 6 THHN as 105 amperes, yet Table 310.16 lists the same conductor as having an ampacity of only 75 amperes? To answer that, go back to Article 100 and review the definition of ampacity. Notice the phrase “conditions of use.” What these tables do is set a maximum current value at which you can ensure the installation won’t undergo prema-ture failure of the conductor insulation in normal use, in the conditions described in the tables.

The designations THHN, THHW, RHH, and so on, are insulation types. Every type of insulation has a heat withstand limit. When current flows through a conductor, it creates heat. How well the insulation around a conductor can dissipate that heat depends on factors such as whether that conductor is in free air or not. Think what happens to you if you put on a sweater, a jacket, and then a coat—all at the same time. You heat up. Your skin can’t dissipate heat with all that clothing on nearly as well as it dissipates heat in free air. The same principal applies to conductors.

Conductor insulation also fails with age. That’s why we conduct cable testing and take other measures to predict failure and replace certain conductors (for example, feeders or critical equipment conductors) while they’re still within design specifications. But conductor insulation failure takes decades under normal use—and it’s a maintenance issue. However, if a conductor is forced to exceed the ampacity listed in the appropriate table, and as a result its design temperature is exceeded, insulation failure happens much more rapidly—often catastrophi-cally. Exceeding the allowable ampacity is a serious safety issue.


310.4 Conductors for General Wiring

(2) Be made of the same conductor material (copper/aluminum).

(3) Be the same size in circular mil area (minimum 1/0 AWG).

(4) Have the same insulation (like THHN).

(5) Terminate in the same method (set screw versus compres-sion).

Author’s Comment: Conductors aren’t required to have the same physical characteristics as those of another phase or neu-tral conductor to achieve balance [310.14(C)].

310.4 Conductors in Parallel.

(A) General. Ungrounded and neutral conductors sized 1/0 AWG and larger can be connected in parallel.

(B) Conductor Characteristics. When circuit conductors are run in parallel, the current must be evenly distributed between the individual parallel conductors by requiring all circuit conductors within a parallel set to: Figure 310–4

(1) Be the same length.

Figure 310–1

Figure 310–3

Figure 310–2

Figure 310–4


Conductors for General Wiring 310.4

(D) Conductor Ampacity Adjustment. Each current-carry-ing conductor of a paralleled set of conductors must be counted as a current-carrying conductor for the purpose of conductor ampacity adjustment, in accordance with Table 310.15(B)(2)(a). Figure 310–7

(E) Equipment Grounding Conductors. The equipment grounding conductors for circuits in parallel must be iden-tical to each other in length, material, size, insulation, and termination. In addition, each raceway, where required, must have an equipment grounding conductor sized in accordance

(C) Separate Raceways or Cables. Raceways or cables con-taining parallel conductors must have the same electrical characteristics and the same number of conductors. Figure 310–5

Author’s Comment: If one set of parallel conductors is run in a metallic raceway and the other conductors are run in PVC conduit, the conductors in the metallic raceway will have an increased opposition to current flow (impedance) as com-pared to the conductors in the nonmetallic raceway. This results in an unbalanced distribution of current between the parallel conductors.

Paralleling is done in sets. Parallel sets of conductors aren’t required to have the same physical characteristics as those of another set to achieve balance.

Author’s Comment: For example, a 400A feeder with a neutral load of 240A can be paralleled as follows. Figure 310–6.

• Phase A, Two—250 kcmil THHN aluminum, 100 ft• Phase B, Two—3/0 THHN copper, 104 ft• Phase C, Two—3/0 THHN copper, 102 ft• Neutral, Two—1/0 THHN aluminum, 103 ft• Equipment Grounding Conductor, Two—3 AWG copper,

101 ft*

*The minimum 1/0 AWG size requirement doesn’t apply to equipment grounding conductors [310.4(E)].

Figure 310–6

Figure 310–7

Figure 310–5



Author’s Comment: These conductor types are only the most common. Refer to the Code for a complete list of conductors that may be installed in a wet location.

(D) Locations Exposed to Direct Sunlight. Insulated conduc-tors and cables exposed to the direct rays of the sun must comply with (1) or (2):

(1) Listed as sunlight resistant or marked as being sunlight resistant. Figure 310–9

Author’s Comment: SE cable and the conductors contained in the cable are listed as sunlight resistant. However, according to the UL listing standard, the conductors contained in SE cable aren’t required to be marked as sunlight resistant.

(2) Covered with insulating material, such as tape or sleev-ing materials that are listed as being sunlight resistant or marked as being sunlight resistant.

310.9 Corrosive Conditions. Conductor insulation must be suitable for any substance to which it may be exposed that may have a detrimental effect on the conductor’s insulation, such as oil, grease, vapor, gases, fumes, liquids, or other sub-stances. See 110.11.

310.10 Insulation Temperature Limitation. Con-ductors must not be used where the operating temperature exceeds that designated for the type of insulated conductor involved.

with 250.122. The minimum 1/0 AWG rule of 310.4 doesn’t apply to equipment grounding conductors. See 250.122(F)(1) for more information on equipment grounding conductors installed in parallel. Figure 310–8

310.5 Minimum Size Conductors. The smallest conduc-tor permitted for branch circuits for residential, commercial, and industrial locations is 14 AWG copper, except as permit-ted elsewhere in this Code.

Author’s Comment: There’s a misconception that 12 AWG copper is the smallest conductor permitted for commercial or industrial facilities. Although this isn’t true based on NEC rules, it may be a local code requirement.

310.8 Location.

(B) Dry and Damp Locations. Insulated conductors used in dry and damp locations must be Types THHN, THHW, THWN, or THWN-2.

Author’s Comment: These conductor types are only the most common. Refer to the Code for a complete list of conductors that may be installed in a dry or damp location.

(C) Wet Locations. Insulated conductors used in wet loca-tions must be:

(2) Types THHW, THWN, THWN-2, XHHW, or XHHW-2

Figure 310–8

Figure 310–9



• The NEC doesn’t require color coding of ungrounded con-ductors, except for the high-leg conductor when a neutral conductor is present [110.15 and 230.56]. Although not required, electricians often use the following color system for power and lighting conductor identification:

– 120/240V, single-phase—black, red, and white

– 120/208V, three-phase—black, red, blue, and white

– 120/240V, three-phase, delta-connected system —black, orange, blue, and white

– 277/480V, three-phase, wye-connected system —brown, orange, yellow, and gray; or, brown, purple, yellow, and gray

310.13 Conductor Construction. Insulated conductors as permitted by the Code can be used in any of the wiring methods recognized in Chapter 3.

Author’s Comment: The following explains the lettering on con-ductor insulation: Figure 310–11

• F Fixture wires (solid or 7 strands) [Table 402.3]• FF Flexible fixture wire (19 strands) [Table 402.3]• No H 60°C insulation rating [Table 310.13(A)]• H 75°C insulation rating [Table 310.13(A)]• HH 90°C insulation rating [Table 310.13(A)]• N Nylon outer cover [Table 310.13(A)]• R Thermoset insulation [Table 310.13(A)]• T Thermoplastic insulation [Table 310.13(A)]• U Underground [Table 310.13(A)]• W Wet or damp locations [Table 310.13(A)]• X Cross-linked polyethylene insulation [Table 310.13(A)]

FPN No. 1: The insulation temperature rating of a conduc-tor is the maximum temperature a conductor can withstand over a prolonged time without serious degradation. The main factors to consider for conductor operating tempera-ture include ambient temperature, heat generated internally from current flow through the conductor, the rate at which heat can dissipate, and adjacent load-carrying conductors.

310.12 Conductor Identification.

(A) Neutral Conductor. Neutral conductors must be identi-fied in accordance with 200.6.

(B) Equipment Grounding Conductor. Equipment grounding conductors must be identified in accordance with 250.119.

(C) Ungrounded Conductors. Ungrounded conductors must be clearly distinguishable from neutral and equipment grounding conductors. Figure 310–10


• Where the premises wiring system has branch circuits or feeders supplied from more than one nominal voltage system, each ungrounded conductor of the branch circuit or feeder, where accessible, must be identified by system. The means of identification can be by separate color coding, marking tape, tagging, or other means approved by the authority having jurisdiction. Such identification must be permanently posted at each panelboard. See 210.5(C) and 215.12 for specific requirements.

Figure 310–11

Figure 310–10



(B) Ampacity Table. The allowable conductor ampacities listed in Table 310.16 are based on conditions where the ambi-ent temperature isn’t over 86°F, and no more than three cur-rent-carrying conductors are bundled together. Figure 310–14

Equipment grounding conductors can be sectioned within a listed multiconductor cable, such as SE cable, provided the combined circular mil area complies with 250.122.

310.15 Conductor Ampacity.

(A) General Requirements.

(1) Tables for Engineering Supervision. The ampacity of a conductor can be determined either by using the tables in accordance with 310.15(B), or under engineering supervision as provided in 310.15(C).

FPN No. 1: Ampacities provided by this section don’t take voltage drop into consideration. See 210.19(A) FPN No. 4, for branch circuits and 215.2(D) FPN No. 2, for feeders.

(2) Conductor Ampacity—Lower Rating. If a single length of conductor is routed in a manner that two or more ampacity ratings apply to a single conductor length, the lower ampac-ity must be used for the entire circuit. See 310.15(B). Figure 310–12

Exception: When different ampacities apply to a length of con-ductor, the higher ampacity is permitted for the entire circuit if the reduced ampacity length doesn’t exceed 10 ft and its length doesn’t exceed 10 percent of the length of the higher ampacity. Figure 310–13

Figure 310–12

Figure 310–13

Figure 310–14



Question: What is the corrected ampacity of 3/0 THHN/THWN conductors in a wet location if the ambient temperature is 108°F?

(a) 164A (b) 196A (c) 213A (d) 241A

Answer: (a) 164A

Conductor Ampacity [75°C] = 200ACorrection Factor [Table 310.16] = 0.82

Corrected Ampacity = 200A x 0.82Corrected Ampacity = 164A

Author’s Comment: When adjusting conductor ampacity, the ampacity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].

(2) Adjustment Factors.

(a) Conductor Bundle. Where the number of current-carry-ing conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in., the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(a). Figure 310–15

Author’s Comment: When conductors are installed in an ambi-ent temperature other than 78°F to 86°F, ampacities listed in Table 310.16 must be corrected in accordance with the multipli-ers listed in Table 310.16.

Table 310.16 Ambient Temperature Correction

Ambient Temperature

°F

Ambient Temperature

°C

Correction Factor 75°C Conductors

Correction Factor 90°C Conductors

70–77°F 21–25°C 1.05 1.04

78–86°F 26–30°C 1.00 1.00

87–95°F 31–35°C 0.94 0.96

96–104°F 36–40°C 0.88 0.91

105–113°F 41–45°C 0.82 0.87

114–122°F 46–-50°C 0.75 0.82

123–131°F 51–-55°C 0.67 0.76

132–140°F 56–60°C 0.58 0.71

141–158°F 61–70°C 0.33 0.58

159–176°F 71–80°C 0.00 0.41

Author’s Comment: When correcting conductor ampacity for elevated ambient temperature, the correction factor used for THHN conductors is based on the 90°C rating of the conduc-tor in a dry location and 75°C rating of the conductor in a wet location, based on the conductor ampacity listed in Table 310.16 [110.14(C) and Table 310.13(A)].

Question: What is the corrected ampacity of 3/0 THHN/THWN conductors in a dry location if the ambient temperature is 108°F?

(a) 173A (b) 196A (c) 213A (d) 241A

Answer: (b) 196A

Conductor Ampacity [90°C] = 225ACorrection Factor [Table 310.16] = 0.87

Corrected Ampacity = 225A x 0.87Corrected Ampacity = 196A

Figure 310–15



Question: What is the adjusted ampacity of 3/0 THHN/THWN conductors in a wet location if the raceway contains a total of four current-carrying conductors?

(a) 160A (b) 196A (c) 213A (d) 241A

Answer: (a) 160A

Table 310.16 ampacity if 3/0 THHN/THWN in a wet location is 200A

Adjustment Factor [Table 310.15(B)(2)(a)] = 0.80

Adjusted Ampacity = 200A x 0.80Adjusted Ampacity = 160A


• When correcting or adjusting conductor ampacity, the ampac-ity is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].

• Where more than three current-carrying conductors are pres-ent and the ambient temperature isn’t between 78 and 86°F, the ampacity listed in Table 310.16 must be corrected and adjusted for both conditions.

Each current-carrying conductor of a paralleled set of con-ductors must be counted as a current-carrying conductor. Figure 310–16

Table 310.15(B)(2)(a)

Number of Current–Carrying Adjustment Factor

1–3 Conductors 1.00


7–9 Conductors 0.70*


Author’s Comment: The neutral conductor might be a current-carrying conductor, but only under the conditions specified in 310.15(B)(4). Equipment grounding conductors are never con-sidered current carrying [310.15(B)(5)].

Question: What is the adjusted ampacity of 3/0 THHN/THWN conductors in a dry location if the raceway contains a total of four current-carrying conductors? Figure 310–17

(a) 180A (b) 196A (c) 213A (d) 241A

Answer: (a) 180A

Table 310.16 ampacity if 3/0 THHN/THWN in a dry location is 225A

Adjustment Factor [Table 310.15(B)(2)(a)] = 0.80

Adjusted Ampacity = 225A x 0.80Adjusted Ampacity = 180A

Figure 310–16

Figure 310–17



Exception No. 3: The conductor ampacity adjustment factors of Table 310.15(B)(2)(a) don’t apply to conductors installed in raceways not exceeding 24 in. in length. Figure 310–18

Exception No. 5: The conductor ampacity adjustment factors of Table 310.15(B)(2)(a) don’t apply to Type AC or Type MC cable when: Figure 310–19

(1) Each cable has not more than three current-carrying conductors,

Question: What is the ampacity of 3/0 THHN/THWN conductors in a dry location at an ambient temperature of 108°F if the race-way contains four current-carrying conductors?

(a) 157A (b) 176A (c) 199A (d) 214A

Answer: (a) 157A

Table 310.16 ampacity if 3/0 THHN/THWN in a dry location is 225A

Ambient Temperature Correction [Table 310.16] = 0.87Conductor Bundle Adjustment [310.15(B)(2)(a)] = 0.80Adjusted Ampacity = 225A x 0.87 x 0.80Adjusted Ampacity = 157A

Author’s Comment: When adjusting or correcting conductor ampacity, the ampacity of THHN/THWN conductors in a dry loca-tion is based on the 90°C rating of the conductor [110.14(C) and Table 310.13(A)].

Question: What is the ampacity of 3/0 THHN/THWN conduc-tors in a wet location at an ambient temperature of 108°F if the raceway contains four current-carrying conductors?

(a) 131A (b) 176A (c) 199A (d) 214A

Answer: (a) 131A

Table 310.16 ampacity if 3/0 THHN/THWN in a wet location is 200A

Ambient Temperature Correction [Table 310.16] = 0.82

Conductor Bundle Adjustment [310.15(B)(2)(a)] = 0.80

Adjusted Ampacity = 200A x 0.82 x 0.80Adjusted Ampacity = 131.20A

Author’s Comment: When adjusting or correcting conduc-tor ampacity, the ampacity of THHN/THWN conductors in a wet location is based on the 75°C rating of the conductor [110.14(C) and Table 310.13(A)].

FPN No. 2: See 376.22(B) for conductor ampacity adjust-ment factors for conductors in metal wireways.

Author’s Comment: Conductor ampacity adjustment only applies when more than 30 current-carrying conductors are installed in any cross-sectional area of a metal wireway.

Figure 310–19

Figure 310–18



When adjusting conductor ampacity, use the conductor ampacity as listed in Table 310.16 based on the conductors’ insulation rating; in this case, it’s 75A at 90°F. Conductor ampacity adjustment is not based on the temperature termi-nal rating as per 110.14(C).

(4) Neutral Conductors.

(a) Balanced Circuits. The neutral conductor of a 3-wire, sin-gle-phase, 120/240V system, or 4-wire, three-phase, 120/208V or 277/480V wye-connected system, isn’t considered a cur-rent-carrying conductor. Figure 310–21

(2) The conductors are 12 AWG copper, and

(3) No more than 20 current-carrying conductors (ten 2-wire cables or six 3-wire cables) are bundled.

Author’s Comment: When eleven or more 2-wire cables or seven or more 3-wire cables (more than 20 current-carrying conductors) are bundled or stacked for more than 24 in., an ampacity adjustment factor of 60 percent must be applied.

(c) Raceways Exposed to Sunlight on Rooftops. The ambient temperature adjustment contained in Table 310.15(B)(2)(c) is added to the outdoor ambient temperature for conductors or cables that are installed in raceways exposed to direct sun-light on or above rooftops when applying ampacity adjust-ment correction factors contained in Table 310.16.

FPN No. 1: See ASHRAE Handbook—Fundamentals (www.ashrae.org) as a source for the average ambient tem-peratures in various locations.

FPN No. 2: The temperature adders in Table 310.15(B)(2)(c) are based on the results of averaging the ambient temperatures.

Table 310.15(B)(2)(c) Ambient Temperature Adder for Raceways On or Above Rooftops

Distance of Raceway Above Roof

C° F°

0 to 1⁄2 in. 33 60

Above 1⁄2 in. to 3 1⁄2 in. 22 40

Above 3 1⁄2 in. to 12 in. 17 30

Above 12 in. to 36 in. 14 25

Author’s Comment: This section requires the ambient tem-perature used for ampacity correction to be adjusted where conductors or cables are installed in conduit on or above a roof-top and the conduit is exposed to direct sunlight. The reasoning is that the air inside conduits in direct sunlight is significantly hotter than the surrounding air, and appropriate ampacity cor-rections must be made in order to comply with 310.10.

For example, a conduit with three 6 THWN-2 conductors with direct sunlight exposure that is 3/4 in. above the roof will require 40°F to be added to the correction factors at the bottom of Table 310.16. Assuming an ambient temperature of 90°F, the tem-perature to use for conductor correction will be 130°F (90°F + 40°F), the 6 THWN-2 conductor ampacity after correction will be 57A (75A x 0.76). Figure 310–20

Figure 310–20

Figure 310–21



(6) Dwelling Unit Feeder/Service Conductors. For individ-ual dwelling units of one-family, two-family, and multifamily dwellings, Table 310.15(B)(6) can be used to size 3-wire, sin-gle-phase, 120/240V service or feeder conductors that supply all loads that are part of, or associated with, the dwelling unit. Figure 310–24

Author’s Comment: Table 310.15(B)(6) cannot be used for ser-vice conductors for two-family or multifamily buildings. Figure 310–25

(b) 3-Wire Circuits. The neutral conductor of a 3-wire circuit from a 4-wire, three-phase, 120/208V or 277/480V wye-con-nected system is considered a current-carrying conductor.

Author’s Comment: When a 3-wire circuit is supplied from a 4-wire, three-phase, 120/208V or 277/480V wye-connected system, the neutral conductor carries approximately the same current as the ungrounded conductors. Figure 310–22

(c) Wye 4-Wire Circuits That Supply Nonlinear Loads. The neutral conductor of a 4-wire, three-phase, 120/208V or 277/480V wye-connected system is considered a current-car-rying conductor where more than 50 percent of the neutral load consists of nonlinear loads. This is because harmonic currents will be present in the neutral conductor, even if the loads on each of the three phases are balanced. Figure 310–23

Author’s Comment: Nonlinear loads supplied by a 4-wire, three-phase, 120/208V or 277/480V wye-connected system can produce unwanted and potentially hazardous odd triplen harmonic currents (3rd, 9th, 15th, etc.) that can add on the neutral conductor. To prevent fire or equipment damage from excessive harmonic neutral current, the designer should con-sider increasing the size of the neutral conductor or installing a separate neutral for each phase. For more information, visit www.MikeHolt.com, click on the ‘Technical’ link, then the ‘Power Quality’ link. Also see 210.4(A) FPN, 220.61 FPN No. 2, and 450.3 FPN No. 2.

(5) Grounding Conductors. Grounding and bonding conduc-tors aren’t considered current carrying.

Figure 310–23

Figure 310–22

Figure 310–24



Neutral Conductor Sizing. Table 310.15(B)(6) can be used to size the neutral conductor of a 3-wire, single-phase, 120/240V service or feeder that carries all loads associated with the dwelling unit, based on the calculated load in accor-dance with 220.61.

CAUTION: Because the service neutral conductor is required to serve as the effective ground-fault current path, it must be sized so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accom-plished by sizing the neutral conductor in accordance with Table 250.66, based on the area of the largest ungrounded service conductor [250.24(C)(1)].

Question: What size service conductors are required if the cal-culated load for a dwelling unit equals 195A, and the maximum unbalanced neutral load is 100A? Figure 310–27

(a) 1/0 AWG and 6 AWG (b) 2/0 AWG and 4 AWG (c) 3/0 AWG and 2 AWG (d) 4/0 AWG and 1 AWG

Answer: (b) 2/0 AWG and 4 AWG

Service Conductor: 2/0 AWG rated 200A [Table 310.15(B)(6)]

Neutral Conductor: 4 AWG is rated 100A in accordance with Table 310.15(B)(6). In addition, 250.24(C) requires the neu-tral conductor to be sized no smaller than 4 AWG based on 2/0 AWG service conductors in accordance with Table 250.66.

Feeder conductors are not required to have an ampacity rating more than the service conductors [215.2(A)(3)].

Table 310.15(B)(6) Conductor sizes for 120/240v, 3-Wire, single-Phase

Dwelling services and Feeders

Amperes Copper Aluminum

100 4 AWG 2 AWG

110 3 AWG 1 AWG

125 2 AWG 1/0 AWG

150 1 AWG 2/0 AWG

175 1/0 AWG 3/0 AWG

200 2/0 AWG 4/0 AWG

225 3/0 AWG 250 kcmil

250 4/0 AWG 300 kcmil

300 250 kcmil 350 kcmil



WARNING: Table 310.15(B)(6) doesn’t apply to 3-wire feeder/service conductors connected to a three-phase, 120/208V system, because the neutral conductor in these systems always carries neutral current, even when the load on the phases is balanced [310.15(B)(4)(b)]. For more information on this topic, see 220.61(C)(1). Figure 310–26

Figure 310–25 Figure 310–26



Figure 310–27


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TIC

LE 312Cabinets, Cutout Boxes, and Meter Socket Enclosures

INTRODUCTION TO ARTICLE 312—CABINETS, CUTOUT BOxES, AND METER SOCkET ENCLOSURESThis article addresses the installation and construction specifications for the items mentioned in its title. In Article 310, we observed that you need different ampacities for the same conductor, depending on conditions of use. The same thing applies to these items—just in a different way. For example, you can’t use just any enclosure in a wet location or in a hazardous (classified) location. The conditions of use impose special requirements for these situations.

For all such enclosures, certain requirements apply—regardless of the use. For example, you must cover any openings, protect conductors from abrasion, and allow sufficient bending room for conductors.

Part I is where you’ll find the requirements most useful to the electrician in the field. Part II applies to manufacturers. If you use name brand components that are listed or labeled, you don’t need to be concerned with Part II. However, if you are specifying custom enclosures, you need to be familiar with these requirements to help ensure that the authority having jurisdiction approves the enclosures.

312.1 Scope. Article 312 covers the installation and con-struction specifications for cabinets, cutout boxes, and meter socket enclosures. Figure 312–1

Author’s Comment: A cabinet is an enclosure for either surface mounting or flush mounting provided with a frame in which a door may be hung. A cutout box is designed for surface mount-ing with a swinging door [Article 100]. The industry name for a meter socket enclosure is “meter can.”

PART I. INSTALLATION

312.2 Damp or Wet Locations.

Enclosures in damp or wet locations must prevent moisture or water from entering or accumulating within the enclosure, and must be weatherproof. When the enclosure is surface mounted in a wet location, the enclosure must be mounted with not less than a 1⁄4 in. air space between it and the mount-ing surface. See 300.6(D).

Where raceways or cables enter above the level of uninsulated live parts of an enclosure in a wet location, a fitting listed for wet locations must be used for termination.Figure 312–1


Cabinets, Cutout Boxes, and Meter socket Enclosures 312.3

(C) Cable Termination. Cables must be secured to the enclo-sure with fittings designed and listed for the cable. See 300.12 and 300.15. Figure 312–5

Author’s Comment: Cable clamps or cable connectors must be used with only one cable, unless that clamp or fitting is identified for more than one cable. Some Type NM cable clamps are listed for two Type NM cables within a single fitting (UL White Book, product category PXJV).

Author’s Comment: A fitting listed for use in a wet location with a sealing locknut is suitable for this application.

Exception: The 1⁄4 in. air space isn’t required for nonmetallic equipment, raceways, or cables.

312.3 Installed in Walls. Cabinets or cutout boxes installed in walls of concrete, tile, or other noncombustible material must be installed so that the front edge of the enclo-sure is set back no more than 1⁄4 in. from the finished surface. In walls constructed of wood or other combustible material, cabinets or cutout boxes must be flush with the finished sur-face or project outward.

312.4 Repairing Gaps. Gaps around cabinets and cutout boxes that are recessed in noncombustible surfaces (plaster, drywall, or plasterboard) having a flush-type cover, must be repaired so that there will be no gap more than 1⁄8 in. at the edge of the cabinet or cutout box. Figure 312–2

312.5 Enclosures.

(A) Unused Openings. Openings intended to provide entry for conductors must be adequately closed. Figure 312–3

Author’s Comment: Unused openings for circuit breakers must be closed by means that provide protection substantially equiva-lent to the wall of the enclosure [408.7]. Figure 312–4

Figure 312–3

Figure 312–2

Figure 312–4


312.8 Cabinets, Cutout Boxes, and Meter socket Enclosures

(d) The raceway is sealed.

(e) Each cable sheath extends not less than 1⁄4 in. into the panelboard.

(f) The raceway is properly secured.

(g) Conductor fill is limited to Chapter 9, Table 1 percentages.

312.8 Used for Raceway and Splices. Cabinets, cutout boxes, and meter socket enclosures can be used as a raceway for conductors that feed through if the conductors don’t fill the wiring space at any cross section to more than 40 percent. Figure 312–7

Author’s Comment: Service conductors and other conductors can be installed in the same enclosure [230.7].

Splices and taps can be installed in cabinets, cutout boxes, or meter socket enclosures if the splices or taps don’t fill the wiring space at any cross section to more than 75 percent. Figure 312–8

Exception: Cables with nonmetallic sheaths aren’t required to be secured to the enclosure if the cables enter the top of a sur-face-mounted enclosure through a nonflexible raceway not less than 18 in. or more than 10 ft long, if all of the following con-ditions are met: Figure 312–6

(a) Each cable is fastened within 1 ft from the raceway.

(b) The raceway doesn’t penetrate a structural ceiling.

(c) Fittings are provided on the raceway to protect the cables from abrasion.

Figure 312–5

Figure 312–7

Figure 312–6


Cabinets, Cutout Boxes, and Meter socket Enclosures 312.8

Figure 312–8


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TIC

LE 408 Switchboards and Panelboards

INTRODUCTION TO ARTICLE 408—SWITCHBOARDS AND PANELBOARDSArticle 408 covers the specific requirements for switchboards, panelboards, and distribution boards that control power and lighting circuits. Some key points to remember:

• One objective of Article 408 is that the installation prevents contact between current-carrying conductors and people orequipment.

• Thecircuitdirectoryofapanelboardmustclearlyidentifythepurposeoruseofeachcircuitthatoriginatesinthepanelboard.

• Youmustunderstandthedetailedgroundingandovercurrentprotectionrequirementsforpanelboards.

PART I. GENERAL

408.1 Scope.

(1) Article 408 covers the specific requirements for switch-boards, panelboards, and distribution boards that control power and lighting circuits. Figure 408–1

(2) Article 408 covers battery-charging panels supplied from light or power circuits.

Author’s Comment: For the purposes of this textbook, we’ll only cover the requirements for panelboards.

408.4 Circuit Directory or Circuit Identification. All circuits, and circuit modifications, must be legibly identified as to their clear, evident, and specific purpose. Spare posi-tions that contain unused overcurrent devices must also be identified. Identification must include sufficient detail to allow each circuit to be distinguished from all others, and the identification must be on a circuit directory located on the face or inside of the door of the panelboard. See 110.22. Figure 408–5

Circuit identification must not be based on transient condi-tions of occupancy, such as Steven’s, or Brittney’s bedroom. Figure 408–6

408.5 Clearance for Conductors Entering Bus Enclo-sures. Where raceways enter a switchboard, floor-standing panelboard, or similar enclosure, the raceways, including end fittings, must not rise more than 3 in. above the bottom of the enclosure.

Figure 408–1


switchboards and Panelboards 408.7

PART III. PANELBOARDS

408.40 Equipment Grounding Conductor. Metal pan-elboard cabinets and frames must be connected to an equip-ment grounding conductor of a type recognized in 250.118 [215.6 and 250.4(A)(3)].

Where the panelboard cabinet is used with nonmetallic race-ways or cables, or where separate equipment grounding conductors are provided, a terminal bar for the circuit equip-ment grounding conductors must be bonded to the metal cabinet. Figure 408–12

Exception: Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] can pass through the pan-elboard without terminating onto the equipment grounding ter-minal of the panelboard cabinet. Figure 408–13

Equipment grounding conductors must not terminate on the neutral terminal bar, and neutral conductors must not ter-minate on the equipment grounding terminal bar, except as permitted by 250.142 for services and separately derived sys-tems. Figure 408–14

Author’s Comment: See the definition of “Separately Derived System” in Article 100.

408.7 Unused Openings. Unused openings for circuit breakers and switches must be closed using identified clo-sures, or other means approved by the authority having juris-diction, that provide protection substantially equivalent to the wall of the enclosure. Figure 408–7

Figure 408–5

Figure 408–6

Figure 408–12


408.41 Neutral Conductor Terminations. Each neutral conductor within a panelboard must terminate to an individ-ual terminal. Figure 408–15

Author’s Comment: If two neutral conductors are connected to the same terminal, and someone removes one of them, the other neutral conductor might unintentionally be removed as well. If that happens to the neutral conductor of a multiwire cir-cuit, it can result in excessive line-to-neutral voltage for one of the circuits, as well as undervoltage for the other circuit. See 300.13(B) of this textbook for details. Figure 408–16

CAUTION: Most panelboards are rated as suitable for use as service equipment, which means they are supplied with a main bonding jumper [250.28]. This screw or strap must not be installed except when the panelboard is used for ser-vice equipment [250.24(A)(5)] or separately derived systems [250.30(A)(1)]. In addition, a panelboard marked “suitable only for use as service equipment” means the neutral bar or terminal of the panelboard has been bonded to the case at the factory, and this panelboard is restricted to being used only for service equipment or on separately derived systems according to 250.142(B).

408.41 switchboards and Panelboards

Figure 408–13

Figure 408–14

Figure 408–15

Figure 408–16


switchboards and Panelboards 408.41

This requirement doesn’t apply to equipment grounding con-ductors, because the voltage of a circuit is not affected if an equipment grounding conductor is accidentally removed. Figure 408–17

Figure 408–17


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LE 700 Switchboards and Panelboards

INTRODUCTION TO ARTICLE 700—EMERGENCY STANDBY POWER SYSTEMSEmergency standby power systems are legally required, often as a condition of an operating permit for a given facility. The authority having jurisdiction makes the determination as to whether an emergency standby power system is necessary for a given facility and what it must entail. Sometimes, an emergency standby power system simply provides power for exit lighting and exit signs upon loss of main power or in the case of fire. Its purpose isn’t to provide power for normal business operations, but rather to provide lighting and controls essential for human life.

This background information will help you understand that not all emergency actions to save human life fall under Article 700. The gen-eral goal is to keep the emergency operation as reliable as possible. In an emergency, it’s difficult to administratively control loads. Thus, the emergency standby power system must be able to supply all emergency loads simultaneously. When the emergency power supply also supplies power for load shedding or other nonemergency loads, the emergency loads take priority over the other loads, and those other loads may be dropped to support the emergency loads.

As you study Article 700, keep in mind that emergency standby power systems are essentially lifelines for people. The entire article is based on keeping those lifelines from breaking.

PART I. GENERAL

700.8 Signs.

(A) Emergency Sources. A sign must be placed at the service-entrance equipment indicating the type and location of on-site emergency power sources. Figure 700–4

PART II. CIRCUIT WIRING

700.9 Wiring.

(A) Identification. Boxes and enclosures, including transfer switches, generators, and power panels for emergency circuits must be permanently marked as components of an emer-gency standby power system. Figure 700–5

(B) Wiring. To ensure that a fault on the normal wiring cir-cuits will not affect the performance of emergency wiring or equipment, all wiring to emergency loads must be kept entirely independent of all other wiring, except:

(1) Wiring in transfer equipment. Figure 700–6

(2) Luminaires supplied from two sources of power.

(3) A junction box attached to luminaires supplied from two sources of power.

Figure 700–4


Emergency standby Power systems 700.12

Exception to (5)(b): Overcurrent protection is permitted at the source or for the equipment, provided the overcurrent pro-tection is selectively coordinated with the downstream overcur-rent protection.

c. Legally required and optional standby circuits don’t originate from the same vertical switchboard section, panelboard enclosure, or individual discon nect enclo-sure as emergency circuits.

Author’s Comment: Separation of the circuits served by a generator source for emergency, legally required, and optional standby circuits may be accomplished by running feeders from a single generator to individual overcurrent devices or to a distribution switchboard that separates emergency circuits in different vertical sections from other loads.

(C) Wiring Design and Location. Emergency wiring circuits must be designed and located to minimize the hazards that might cause failure due to flooding, fire, icing, vandalism, and other adverse conditions.

PART III. SOURCES OF POWER

700.12 General Requirements. In the event of fail-ure of the normal supply to the building or structure, emer-gency power must be available within 10 seconds. Emergency equipment must be designed and located so as to minimize the hazards that might cause complete failure due to flood-ing, fires, icing, and vandalism. The emergency power supply must be one of the following:

(A) Storage Battery. Storage batteries must be of suitable rating and capacity to supply and maintain the total load for a period of 11/2 hours, without the voltage applied to the load falling below 871/2 percent of normal.

(B) Generator Set.

(1) Prime Mover-Driven. A generator approved by the authority having jurisdiction and sized according to 700.5 must have means to automatically start the prime mover when the normal service fails.

(2) Internal Combustion Engines as Prime Movers. Where internal combustion engines are used as the prime mover, an on-site fuel supply must be provided for not less than 2 hours of full-demand operation of the system. Figure 700–7

(4) Wiring within a common junction box attached to unit equipment, containing only the branch circuit supply ing the unit equipment and the emergency circuit sup plied by the unit equipment.

(5) Wiring from an emergency source is permitted to supply any combination of emergency, legally required, or optional standby loads in accordance with (a), (b), and (c).

a. From separate vertical switchboard sections or from individual disconnects mounted in separate enclosures.

b. By single or multiple feeders without overcurrent pro-tection at the source.

Figure 700–5

Figure 700–6


700.12 Emergency standby Power systems

(C) Uninterruptible Power Supplies. Uninterruptible power supplies serving as the emergency power source must comply with the applicable requirements of 700.12(A) and (B).

(F) Unit Equipment. Individual unit equipment (emer-gency lighting battery pack) must consist of the following: Figure 700–9

(1) A rechargeable battery,

(2) A battery charging means,

(3) Provisions for one or more lamps mounted on the equip-ment, or terminals for remote lamps (or both), and

(4) A relaying device arranged to energize the lamps auto-matically upon failure of the supply to the unit equip-ment. Emergency lighting battery pack equipment must be permanently fixed in place. Flexible cord-and-plug connection (twist lock not required) is permitted for emergency lighting battery pack equipment designed for this purpose, provided the cord doesn’t exceed 3 ft. Figure 700–10

The branch-circuit wiring that supplies emergency light-ing battery pack equipment must be the same branch-cir-cuit wiring that supplies the normal lighting in the area, but the emergency lighting battery pack equipment must be con-nected ahead of any local switches. The branch circuit that feeds the emergency lighting battery pack equipment must be clearly identified at the distribution panel in accordance with 110.21 and 408.4.

(6) Outdoor Generator Sets. Where a generator located out-doors is equipped with a readily accessible disconnecting means located within sight (within 50 ft) of the building or structure, an additional disconnecting means isn’t required on or at the building or structure for the generator feeder conductors that serve or pass through the building or struc-ture. Figure 700–8

Figure 700–7

Figure 700–8

Figure 700–9


Emergency standby Power systems 700.12

PART VI. OVERCURRENT PROTECTION

700.25 Accessibility. The branch-circuit overcurrent devices for emergency circuits must be accessible to autho-rized persons only.

700.26 Ground-Fault Protection of Equipment. The alternate power supply for emergency standby power systems isn’t required to have ground-fault protection of equipment, but 700.7(D) requires ground-fault indication of the emer-gency power supply.

700.27 Coordination. Overcurrent devices for emergency power systems must be selectively coordinated with all sup-ply-side overcurrent devices.

Author’s Comment: Selective coordination means the overcur-rent protection scheme confines the interruption to a particular area rather than to the whole system. For example, if a short cir-cuit or ground fault occurs with selective coordination, the only breaker/fuse that will open is the one protecting just that branch circuit. Without selective coordination, an entire floor of a build-ing can go dark. See Article 100 for the definition of the term “Coordination, Selective.”

Author’s Comment: There are two reasons why the emergency lighting battery packs must be connected ahead of the switch controlling the normal area lighting: (1) in the event of a power loss to the lighting circuit, the emergency battery lighting packs will activate and provide emergency lighting for people to exit the building, and (2) the emergency lighting battery packs won’t turn on when the switch controlling normal lighting is turned off.

CAUTION: Individual unit equipment must not be connected to the emergency circuit, because it will not operate when normal power is lost, since the equipment is being supplied by the emergency standby power system.

PART IV. CIRCUITS FOR LIGHTING AND POWER

700.15 Loads on Emergency Branch Circuits. Emer-gency circuits must supply only emergency loads.

700.16 Emergency Illumination. Emergency lighting systems must be designed and installed so that the failure of any individual lighting element, such as the burning out of a lamp, will not leave in total darkness any space that requires emergency illumination.

Author’s Comment: This means that a single remote head is never sufficient for an area. A minimum of two lighting heads is always required. This is why individual unit equipment (some-times called “lunchboxes” in the field) always has two lighting heads.

Figure 700–10


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LE 701Legally Required Standby Power Systems

NTRODUCTION TO ARTICLE 701—LEGALLY REqUIRED STANDBY POWER SYSTEMSIn the hierarchy of electrical systems, Article 700 Emergency Power Systems receives first priority. Taking the number two spot are Legally Required Standby Power Systems, which fall under Article 701. Legally required systems must supply standby power in 60 seconds or less after a power loss, instead of the 10 seconds or less required for emergency standby power systems.

Article 701 systems don’t serve the purpose of directly protecting human life. They supply specific loads that, if shut down, will create haz-ards or impede rescue operations. Thus, hospital communications systems fall under Article 701; evacuation instructions announced over the public address system are part of a rescue operation.

Article 700 basically applies to systems or equipment required to protect people who are in an emergency and trying to get out, while Article 701 basically applies to systems or equipment needed to aid the people responding to the emergency. For example, Article 700 lighting pro-vides an exit path. But, Article 701 lighting might illuminate the fire hydrants and switchgear areas.

PART I. GENERAL

701.1 Scope. The provisions of Article 701 apply to the installation, operation, and maintenance of legally required standby systems consisting of circuits and equipment intended to supply illumination or power when the normal electrical supply or system is interrupted.

Author’s Comment: Legally required standby systems provide electric power to aid in firefighting, rescue operations, control of health hazards, and similar operations.

701.2 Definitions.

Legally Required standby systems. Legally required standby systems are those systems classified as legally required by any governmental agency having jurisdiction. These systems are intended to automatically supply power to selected loads (other than those classed as emergency loads) in the event of failure of the normal power source.

FPN: Legally required standby systems typically supply loads such as heating and refrigeration systems, commu-nications systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes that, when stopped, could create hazards, or hamper rescue or firefighting operations.

701.3 Application of Other Articles. Except as modified by this article, all applicable requirements of the NEC apply.

701.4 Equipment Approval. The authority having juris-diction must approve all equipment.

701.5 Tests and Maintenance. Legally required standby system testing consists of acceptance testing and operational testing. Written records of both types of testing and mainte-nance must be maintained.

(A) Conduct or Witness Test. To ensure that the legally required standby system meets or exceeds the original instal-lation specification, the authority having jurisdiction must conduct or witness an acceptance test of the legally required system upon completion of the installation.

(B) Periodic Testing. Legally required standby systems must be periodically tested to ensure that adequate maintenance has been performed and that the systems are in proper oper-ating condition.

Author’s Comment: Running the legally required standby system to power the loads of the facility is often considered an acceptable method of operational testing.


Legally Required standby Power systems 701.6

PART III. SOURCES OF POWER

701.11 General Requirements. If the normal supply fails, legally required standby power must be available within 60 seconds. The supply system for the legally required standby power supply must be one of the following:

(A) Storage Battery. Storage batteries must be of suitable rating and capacity to supply and maintain the total load for a period of 11/2 hours, without the voltage applied to the load falling below 871/2 percent of normal.

(B) Generator Set.

(1) Prime Mover-Driven. A generator approved by the authority having jurisdiction and sized according to 701.6 must have the means to automatically start the prime mover on failure of the normal service.

(2) Internal Combustion Engines as Prime Movers. Where internal combustion engines are used as the prime mover, an on-site fuel supply must be provided for not less than 2 hours of full-demand operation of the system. Figure 701–2

(5) Outdoor Generator Sets. Where a generator located out-doors is equipped with a readily accessible disconnecting means located within sight (within 50 ft) of the building or structure, an additional disconnecting means isn’t required on or at the building or structure for the generator feeder conductors that serve or pass through the building or struc-ture. Figure 701–3

(C) Battery Systems Maintenance. Where batteries are used, the authority having jurisdiction must require periodic maintenance.

(D) Written Record. A written record must be kept of all required tests and maintenance.

Author’s Comment: The NEC doesn’t specify the required record retention period.

701.6 Capacity and Rating. A legally required standby system must have adequate capacity to carry all loads expected to operate simultaneously. The legally required standby alternate power supply is permitted to supply legally required standby and optional standby system loads if:

(1) The alternate power supply has adequate capacity to handle all connected loads

(2) There is automatic selective load pickup and load shed-ding to ensure adequate power to the legally required standby circuits

701.7 Transfer Equipment.

(A) General. Transfer equipment must be identified for legally required standby use, and be approved by the author-ity having jurisdiction.

Author’s Comment: Legally required standby systems and optional standby systems are permitted to be on the same trans-fer switch, but emergency standby power systems must have their own.

(C) Automatic Transfer Switches. Automatic transfer switches must be electrically operated, mechanically held, and listed for legally required standby system use.

701.9 Signs.

(A) Mandated Standby. A sign must be placed at the service-entrance equipment indicating type and location of on-site legally required standby power sources. Figure 701–1

PART II. CIRCUIT WIRING

701.10 Wiring. Legally required standby system wiring is permitted to occupy the same raceways, cables, boxes, and cabinets with other general wiring.

Figure 701–1


701.11 Legally Required standby Power systems

To minimize the possibility of simultaneous interruption of the legally required standby supply, a separate service drop or lateral must be electrically and physically separated from all other service conductors.

(E) Connection Ahead of Service Disconnecting means. Where approved by the authority having jurisdiction, connection ahead of, but not within, the same cabinet, enclosure, or ver-tical switchboard section as the service disconnecting means is permitted. Figure 701–5

To prevent simultaneous interruption of supply, the legally required standby service disconnecting means must be suf-ficiently separated from the normal service disconnection means.

FPN: See 230.82 for equipment permitted on the supply side of a service disconnecting means.

(C) Uninterruptible Power Supplies. Uninterruptible power supplies serving as the legally required standby power source must comply with 701.11(A) and (B).

(D) Separate Service. Where approved by the authority having jurisdiction, an additional service installed in accor-dance with Article 230 is permitted to serve as a legally required source of power. Figure 701–4

Figure 701–3

Figure 701–2

Figure 701–4


Legally Required standby Power systems 701.15

PART IV. OVERCURRENT PROTECTION

701.15 Accessibility. The branch-circuit overcurrent devices for legally required standby circuits must be accessi-ble to authorized persons only.

701.17 Ground-Fault Protection of Equipment. The alternate source for legally required standby power systems isn’t required to have ground-fault protection of equipment.

701.18 Coordination. Overcurrent devices for legally required power systems must be selectively coordinated with all supply-side overcurrent devices.

Author’s Comment: See the definition of “Coordination, Selective” in Article 100.

Figure 701–5


PART I. GENERAL

702.2 Definition.

Optional standby systems. Optional standby systems are intended to supply power to public or private facilities or property where life safety doesn’t depend on the performance of the system. Optional standby systems are intended to supply on-site generated power to selected loads either auto-matically or manually.

FPN: Optional standby systems are typically installed to provide an alternate source of electric power for such facili-ties as industrial and commercial buildings, farms and resi-dences, and to serve loads such as heating and refrigeration systems, data processing and communications systems, and industrial processes that, when stopped during any power outage, can cause discomfort, economic loss, serious inter-ruption of the process, damage to the product or process, or the like.

702.3 Application of Other Articles. Except as modified by this article, all applicable NEC requirements apply.

702.4 Equipment Approval. The authority having juris-diction must approve all equipment used for the optional standby power system.

702.5 Capacity and Rating.

(B) System Capacity. The calculated load on the standby source must be in accordance with Article 220 or by a method approved by the authority having jurisdiction.

(1) Manual Transfer Equipment. The optional standby power source must have adequate capacity for all equip ment intended to operate at one time as determined by the user.

Author’s Comment: When a manual transfer switch is used, the user of the optional standby system selects the loads to be connected to the system, which determines the system kVA/kW rating.

(2) Automatic Transfer Equipment.

(a) Full Load. The optional standby power source must have adequate capacity to supply the full load transferred.

Author’s Comment: For a new installation, the load is deter-mined by Article 220 or an alternate method approved by the AHJ. For existing facilities, the maximum demand data for 1 year or the average power demand of a 15-minute period over a min-imum of 30 days can be used to size the power source [220.87]. Figure 702–3

AR

TIC

LE 702 Optional Standby Power Systems

INTRODUCTION TO ARTICLE 702—OPTIONAL STANDBY POWER SYSTEMSTakingthirdpriorityafterEmergencyandLegallyRequiredSystems,OptionalStandbyPowerSystemsprotectpublicorprivatefacilitiesorproperty where life safety doesn’t depend on the performance of the system. These systems aren’t required for rescue operations.

Supposeaglassplantlosespower.Onceglasshardensintheequipment—whichitwilldowhenprocessheatislost—theplantisgoingtosuffer a great deal of downtime and expense before it can resume operations. An optional standby system can prevent this loss.

You’ll see these systems in facilities where loss of power can cause economic loss or business interruptions. Data centers can lose millions of dollars from a single minute of lost power. A chemical or pharmaceutical plant can lose an entire batch from a single momentary power glitch. In many cases, the lost revenue can’t be recouped.


Optional standby Power systems 702.6

PART IV. SOURCES OF POWER702.11 Outdoor Generator Sets. Where a generator located outdoors is equipped with a readily accessible dis-connecting means located within sight (within 50 ft) of the building or structure, an additional disconnecting means isn’t required on or at the building or structure for the generator feeder conductors that serve or pass through the building or structure. Figure 702–5

702.6 Transfer Equipment. A transfer switch is required for all fixed or portable optional standby power systems.

Exception: Temporary connection of a portable generator without transfer equipment is permitted where written safety procedures are in place and conditions of maintenance and supervision ensure that only qualified persons will service the installation, and where the normal supply is physically isolated by a lockable disconnecting means or by the disconnection of the normal supply conductors.

702.8 Signs.

(A) Standby Power Sources. A sign that indicates type and location of on-site optional standby power sources must be placed at the service-entrance equipment. Figure 702–4

PART II. CIRCUIT WIRING702.9 Wiring. Optional standby system wiring can occupy the same raceways, cables, boxes, and cabinets with other general wiring.

Figure 702–3 Figure 702–4

Figure 702–5

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