US Army Engineer course - Electricity III (Install Electrical Boxes)

7/30/2019 US Army Engineer course - Electricity III (Install Electrical Boxes)

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SUBCOURSE EDITION

EN5142 B

US ARMY ENGINEER SCHOOL

INSTALL ELECTRICAL BOXES



INSTALL ELECTRICAL BOXES

Subcourse EN 5142

EDITION B

United States Army Engineer School

Fort Leonard Wood, Missouri 65473

1 Credit Hour

Edition Date: August 1999

SUBCOURSE OVERVIEW

This subcourse is part of the Electrician, Military Occupational Specialty (MOS) 51R, Skill Levels 1 and 2

course. It is designed to provide the knowledge necessary to identify and install various common typesof electrical boxes, cover plates, and special devices. It also addresses how to calculate the number of conductors allowable in an electrical box. This subcourse is presented in one lesson that corresponds tothe terminal learning objective as indicated below. Work must be accomplished in a manner consistentwith environmental laws and regulations.

There are no prerequisites for this subcourse.

This subcourse reflects the doctrine that was current when this subcourse was prepared. In your work,always refer to the latest official publications.

Unless otherwise stated, the masculine gender of singular pronouns is used to refer to both men andwomen.

TERMINAL LEARNING OBJECTIVE:

ACTION: You will learn to identify and install various common types of electrical boxes, coverplates, and special devices. You will also learn how to calculate the number of conductors allowable in an electrical box.

CONDITION: You will be given the material in this subcourse and an Army Correspondence CourseProgram (ACCP) examination response sheet.

STANDARD: To demonstrate competency of this task, you must achieve a minimum of 70 percenton the subcourse examination.

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TABLE OF CONTENTS

Section Page

Subcourse Overview....................................................................................................................................i

Lesson: Electrical-Box Installation ............................................................................................................1

Part A - Electrical Boxes................................................................................................................2

Part B - Electrical-Box Uses and Electrical Connectors ...............................................................6

Part C - Electrical-Box Conductors ............................................................................................11

Part D - Electrical-Box Covers ...................................................................................................15

Practice Exercise .....................................................................................................................................17

Answer Key and Feedback......................................................................................................................22

Appendix A: List of Common Acronyms.............................................................................................A-1

Appendix B: Recommended Reading List ............................................................................................B-1

Appendix C: Metric Conversion Chart ................................................................................................C-1

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LESSON

ELECTRICAL-BOX INSTALLATION

Critical Task: 051-246-1111

OVERVIEW

LESSON DESCRIPTION:

In this lesson, you will learn to identify and install various electrical boxes, cover plates, and specialdevices. You will also learn how to calculate the number of conductors allowable in an electrical box.

TERMINAL LEARNING OBJECTIVE:

ACTION: You will learn to identify and install electrical boxes, cover plates, and special devices.

You will learn their installation procedures according to the NEC. You will also learnhow to calculate the number of conductors allowable in an electrical box.

CONDITION: You will be given the material contained in this lesson.

STANDARD: You will correctly answer all practice-exercise questions at the end of this lesson.

REFERENCES: The material contained in this lesson was derived from STP 5-51R12-SM-TG, FM 5-424, and the NEC Handbook.

INTRODUCTION

When an electrical circuit is installed, it will begin and end in an electrical box. Electrical boxes areconstructed of various types of materials and come in a variety of sizes and shapes. Electrical boxes canbe installed in many different ways and places for a variety of uses. After the boxes are installed,conductors are installed in the boxes. All electrical boxes must be covered when the work is completed.

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PART A - ELECTRICAL BOXES

1. Types.

a. There are many types of electrical boxes. They are made of either metallic or nonmetallic(NM) material. The metallic box is widely used by the Army. The NM box is becoming more popularbecause it is less expensive and the installation of conductors is easier and faster than with the metallic

box. Figure 1 shows a NM, two-gang box.

Figure 1. Nonmetallic, two-gang box

b. Electrical boxes come in many shapes, sizes, and depths (Figure 2). Box depths range from1/2 inch to 6 inches. Boxes of a special size and depth can be ordered.

Figure 2. Common metallic boxes

c. When required, common metallic boxes can be ganged together to make the box wider(Figure 3). Boxes to be ganged must be of the same type and have a removable side. If a box is notdeep enough, extension rings can be added to increase the cubic-inch capacity as needed (Figure 4.Extension rings are attached to a box by tightening the screws.

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Figure 3. Box ganging

Figure 4. Extension ring

2. Mounting.

a. Electrical boxes are mounted in several different ways. Some boxes come with the mountingbrackets attached. The side-mounting box is attached to a stud with nails or screws (Figure 5).

Figure 5. Side-mounting boxes

b. The front-mounting box is attached to a stud by hammering the spikes of the mountingbracket into the stud (Figure 6, page 4). This procedure is known as setting the box. Nails and screwsare also driven through the mounting bracket to ensure that the box is securely mounted to the stud toprevent movement.

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Figure 6. Front-mounting boxes

c. An adjustable bar hanger is designed for installation between wall studs, ceiling joists, or floor joists. The bar hanger is nailed or screwed to studs or joists, then a box is mounted to the bar hanger(Figure 7).

NOTE: Do not use a bar hanger if anything heavier than a light fixture is to be applied; for example,

do not use a bar hanger for a ceiling fan.

Figure 7. Adjustable bar hanger

d. Electrical boxes must be mounted so that the outer edge of the box will be flush with thefinished surface of the wall. Some boxes come with thickness gauges stamped on them (Figures 8 and9). Align the gauge against the side of the stud (Figure 9). Move the box in or out, depending on thethickness of the proposed finished surface. Nail or screw the box to the stud.

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Figure 8. Gauged boxes

Figure 9. Gauge alignment

e. Electrical boxes installed in walls constructed of noncombustible material can be mounted upto 1/4 inch back from the finished surface. Electrical boxes mounted in walls containing combustiblematerials must be mounted flush with the finished surface.

f. All electrical boxes (including junction boxes) should be mounted so that the conductors areaccessible. Wall covering should never cover boxes.

g. Electrical switch boxes may be mounted 46 to 52 inches from the top of the box to thefinished surface of the floor. Outlet boxes may be mounted 12 to 18 inches from the bottom of the boxto the finished surface of the floor.

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PART B - ELECTRICAL-BOX USES ANDELECTRICAL CONNECTORS

3. Uses. Electrical boxes are used to mount a variety of devices such as switches, light fixtures,and receptacle outlets (Figure 10). Special-purpose rings (Figure 11) can be added to a box to give itadditional volume and versatility as to what type of device can be installed inside the box. Electricalboxes are also used as junction boxes (Figure 12). A junction box is used when a circuit splits into two

or more directions and when splicing of wires is necessary.

Figure 10. Devices

Figure 11. Special-purpose rings

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Figure 12. Junction box

4. Connectors.

a. Electrical conductors/wires are connected to a box with connectors. Connectors come indifferent sizes and shapes and are used to secure all types of cable or conduit to the box (Figures 13through 15, pages 7 and 8).

NOTE: All boxes have a limit to the number of conductors that can be installed in them. These

limits are explained in Part C, page 11.

Figure 13. Nonmetallic-sheathed cable connectors

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Figure 14. Flexible metal conduit and armored cable connectors

Figure 15. Conduit connectors

b. Always use the appropriate connector for the type of cable or conduit being used. The designof the box also dictates the type of connector to be used. When using a round ceiling pan (roundelectrical box), never use conduit or connectors that require a locknut or bushing on the round surface of the box (Figure 16).

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Figure 16. Round ceiling pans

c. Use connectors on the flat back part of the box, not on the round sides of the box. Conduitand connectors are not used on the round surface because there would not be a good, tight connectionbetween the bushing and the box or between the locknut and the box (Figure 17).

Figure 17. Incorrect conduit connection

d. Never leave knockout holes open in an electrical box; seal them with a knockout plug (Figure18). The item marked A is pushed into the hole. The item marked B is installed by placing it half inside the box and half outside the box and then securing the two pieces together with a screw throughthe middle of the fixture.

Figure 18. Knockout plugs

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e. Box-hanging straps are mounted to building studs to support electrical boxes thataccommodate lighting devices (Figure 19). If the box is too deep, you may have to use a fixtureextension.

Figure 19. Lighting outlet box

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PART C - ELECTRICAL-BOX CONDUCTORS

5. Conductor Count

a. When all boxes are installed, installation of conductors can begin. A common problem thatarises is how to determine the number of conductors that can be installed in a box. This problem iseasily solved by consulting the NEC Handbook. A sample NEC table is shown in Table 1. This table

shows the number of conductors that can be installed in a box, based on the American Wire Gauge(AWG) classification of the wire.

Table 1. Sample NEC table

b. Other factors also affect the number of conductors that can be mounted in a box. Differentsize conductors require different amounts of free space. This free space is measured in cubic inches.Table 2 is a sample NEC table showing the amount of free space required per conductor, based on thelargest size conductor in the box. The number of conductors permitted is reduced by one for eachfixture stud, fixture extension, internal connector, or clamp (Figure 20, page 12). Reduce the number of conductors by two for each switch or receptacle. No allowance is required for a connector with aconnecting device outside of the box. Locknuts and bushings do not count.

Table 2. Sample NEC table showing the volume required per conductor

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Figure 20. Box devices

c. Quite often, especially in a junction box, more than one size of conductor will terminate inthe box. If no devices are present, calculating how many conductors can fit into the box isaccomplished by consulting the NEC Handbook. Under normal conditions, there will be some type of cable or internal connector or clamp and possibly a device in the box. Two conductors are subtractedfor each device. The following steps show how to determine which size of conductor will be subtractedwhen various sizes are present in the box:

Step 1. Determine the cubic-inch capacity of the box being used by consulting the appropriate table inthe NEC. For this course, use the sample NEC table in Table 2, page 11.

Step 2. Determine the amount of free space needed for the largest conductor entering the box.

Step 3. Total the number of the largest conductors. Add one for all internal clamps, and add two foreach switch or receptacle.

Step 4. Multiply the number from step 3 by the free space needed for the largest conductor.

Step 5. Total all remaining conductors (by size), and multiply each individual total by the free space

required for its size.

Step 6. Add the answer from step 4 to the answer from step 5.

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Step 7. Subtract the answer to step 6 from the answer to step 1.

If the final answer to step 7 is a positive number, the conductors and device(s) will fit. If the answer isa negative number, a larger box is needed. See the example shown in Figure 21.

Figure 21. Determining conductor size

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6. Grounding Conductor Count

a. Grounding conductors are counted differently. Regardless of how many groundingconductors are coming into a box, the total will be counted as one conductor (Figure 22). The largestgrounding conductor determines how much free space is required. For example, three 14-AWG and one12-AWG grounding conductors will be counted as one 12-AWG conductor.

b. Conductors that terminate inside the box without being connected to a device count as oneeach. Conductors that pass through the box also count as one each. Conductors that originate in thebox and never leave the box do not count. An example of this would be fixture wires. If more freespace is needed, extension rings and/or additional boxes can be added to provide the additional spacerequired.

Figure 22. Conductor count

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PART D - ELECTRICAL-BOX COVERS

After a device has been installed in a box or the work is completed in a box, the box must be covered.Electrical-box covers come in different sizes, shapes, and styles (Figure 23). The type of device and thelocation of the box dictate what type cover to use. Standard plastic box covers are the most commonfor indoor use. When an electrical box is located outdoors or where it is subject to moisture, aWaterproof metal cover should be used. Blank box covers are used to cover junction boxes.

Figure 23. Electrical-box covers

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LESSON

PRACTICE EXERCISE

The following items will test your grasp of the material covered in this lesson. There is only one correctanswer for each item. When you complete the exercise, check your answers with the answer key thatfollows. If you answered any item incorrectly, study again that part of the lesson which contains the

portion involved.

1. Electrical boxes range in depth from _________.

A. 1/2 inch to 4 inches

B. 1/2 inch to 6 inches

C. 1 inch to 4 inches

D. 1 inch to 6 inches

2. Which box can be ganged with a box of the same design?

3. An extension ring can be added to a box to increase the cubic-inch capacity of the box.

A. TrueB. False

4. Which box is used as a side-mounting box?

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5. The device shown below is designed for installation between wall studs.

A. True

B. False

6. For quicker and easier installation, some boxes have thickness gauges stamped on them.

A. True

B. False

7. A box may be recessed 1/4 inch in a wall constructed of combustible material.

A. True

B. False

8. Electrical boxes do not have to be accessible.

A. True

B. False

9. How many inches from the finished floor may electrical switch boxes be installed?

A. 40 to 46

B. 46 to 52

C. 52 to 58

D. 58 to 64

10. How many inches from the finished floor may outlet boxes be installed?

A. 6 to 12

B. 9 to 15

C. 12 to 18

D. 15 to 21

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11. Electrical boxes must always have a device mounted in them.

A. True

B. False

12. Electrical boxes should never be used to mount light fixtures.

A. True

B. False

13. What type connector is shown below?

A. Round or oval cable

B. EMT

C. Armored cable

D. NM cable

14. Which of the following connectors is used with armored cable?

15. The device shown below is used to plug unused knockout holes in boxes.

A. True

B. False

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16. The device shown below is a/an __________.

A. Conduit bushing

B. Fixture extension

C. Fixture stud

D. Internal clamp

17. After a device is installed and all work is completed, a box cover should be installed.

A. True

B. False

18. A junction box should be covered with which type of cover plate?

19. A box installed outdoors does not need a special weatherproof cover.

A. True

B. False

20. Refer to Table 1, page 11. How many 12-AWG conductors can be placed in a box that is 2 1/8x 4 11/16 inches square?

A. 6

B. 14

C. 16

D. 18

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21. Refer to Table 2, page 11. A box that has 42 cubic inches of free space can hold twenty 14-AWG conductors.

A. True

B. False

22. A fixture extension does not reduce the number of conductors permitted in a box.

A. True

B. False

23. Conductors that terminate inside the box without being connected to a device count as only oneconductor.

A. True

B. False

24. What is the total conductor count in the box shown below if an of the box connectors areinternal?

A. 7

B. 8

C. 9

D. 10

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LESSON

PRACTICE EXERCISE

ANSWER KEY AND FEEDBACK

Item Correct Answer and Feedback

1. B. 1/2 inch to 6 inches (page 2

2. D. (page 2)

3. A. True (page 2)

4. B. (page 3)

5. A. True (page 4)

6. A. True (page 4)

7. B. False (page 5)


9. B. 46 to 52 (page 5)

10. C. 12 to 18 (page 5)



13. B. EMT (page 8)

14. A. (page 8)

15. A. True (page 9)

16. C. Fixture stud (page 10)


18. C. (page 15)

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Item Correct Answer and Feedback


20. D. 18 (page 11)




24. D. 10 (page 12)

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US Army Engineer course - Electricity III (Install Electrical Boxes)

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