How to Specify the Right Motor Control Center for the Job

8/3/2019 How to Specify the Right Motor Control Center for the Job

http://slidepdf.com/reader/full/how-to-specify-the-right-motor-control-center-for-the-job 1/4

advanced search

power quality design / engineer ing ops & maintenance market t rends produc t depot light ing voice / data / video const ruc tion grounding NEC tra ining

It’s no surprise that as the interrupting rating of a device

increases, so does its cost. This chart gives you a simple view as

to how much it increases.

How to Specify the Right Motor Control

Center For the JobJun 1, 2005 12:00 PM, By Patrick Daley, P.E., Devon Engineering

When purchasing a new or replacement MCC be sure to address a few important

items in your specification

Motor control centers (MCCs) were first introduced in 1937 as a way to conserve wall

and/or floor space in industrial facilities. Before 1960, wall-mounted motor starters were

used even if only a few motors were involved. Relays were housed in separate controlcabinets. Today, even if a few motor starters are needed, they're typically installed within

a standardized vertical enclosure with all the required relays, instruments, and controls.

Overall, the same requirement today often dictates the use of MCCs in new installations.

However, unlike the '50s, when the vast majority of loads served by MCCs were electric

motors with across-the-line starters, today's MCCs can accommodate a wide variety of

different devices required in modern facilities. But specification of this popular piece of

equipment shouldn't be taken for granted. A few key line items in your spec, including

ampacity, bussing material, and feeder cables, could mean the difference between a

long, reliable service life and an early, abrupt failure.

Ampacity. One of the first items to be determined is the ampacity of the MCC. Basically,

the ampacity is the maximum amount of current that the main horizontal bus can

accommodate without overheating. The minimum ampacity of an MCC main horizontal

bus is 600A. Smaller sizes are generally not available and are considered “special

order” items. Often, these special order MCCs with smaller bus ampacity simply contain

600A bussing with a revised nameplate.

Following the horizontal bus ampacity is the specification of the vertical bus ampacity.

The minimum ampacity of a vertical bus is 300A. For most manufacturers, the maximumampacity of an MCC horizontal bus is 2,000A and the maximum ampacity of a vertical

bus is 1,600A, although some manufacturers claim an ampacity up to 5,000A for a

horizontal bus. Generally, if a group of loads requires more than 2,000A, then it would be

best to use two separately fed MCC line-ups as opposed to one.

Another issue to

consider when

specifying an MCC is

the available fault

current levels on the

power distribution

system. T he

available fault

current is simply the

amount of electric

current that would

flow if a direct short

were to occur

between phases or

from phase toground (enclosure)

within the MCC.

A couple of

definitions are in order at this point. The interrupting rating of an electrical device applies

primarily to circuit breakers and fuses and other devices designed to interrupt the flow of

current during a ground fault and/or a short circuit. This rating signifies that the current

interrupting device will open and break the flow of current to the piece of equipment it's

protecting without rupturing and damaging nearby equipment or personnel. The short-

circuit rating of electrical equipment is the maximum fault current that the device can

handle without extensive damage to the electrical components of the circuit.

Per 110.10 of the 2005 NEC, “Listed products applied in accordance with their listing

shall be considered to meet the requirements of this section.” Although an MCC may

have a nameplate that indicates the short-circuit withstand rating as 42,000A, for

example, the overall short-circuit rating of an MCC is based on the installed circuit

interrupting device with the lowest current-interrupting rating.

The amount of current available varies from installation to installation, depending on the

impedance of the electric source. For example, if the available fault current isdetermined to be 12,000A, the MCC must be rated to withstand 12,000A of fault current

during the time it takes the overcurrent protective device (OCPD) immediately upstream

of the fault to open and clear the fault. Fault-current ratings of electrical devices are

available in semi-standard ratings. Some typical ratings are 10,000A, 14,000A, 18,000A,

and 22,000A.

The fault current withstand rating of an MCC is determined by the lowest rated device

in this issue

Power

Monitoring

Equipment

Trends

Electricians in

Antarctica

Heat Tracing

More from this issue

Subscribe to Digital Edition

advertisement

what's wrong here?

What's Wrong

Here?Oct 20, 2011 11:38 AM

What's Wrong

Here?Oct 6, 2011 11:46 AM

What's Wrong

Here?Sep 22, 2011 4:58 PM

View all What's Wrong Here?

What's Wrong Here

Sponsored By:

product spotlight

Light tower diffuser Oct 28, 2011 7:00 AM

View all 2011 Product Spotlights

Free Product Info

Our Product Information site is the

ultimate online resource for products

and services offered by Advertisers

featured in our Magazine. Thisservice is provided as a quick and

easy way to request Product

Information online. Get FREE

product information now.

More...

advertisement

Arlington CP9000

Arlington's new 9"

diameter ceiling cover

plate is the neatest way to

cover unused "high hat"

cans. With no tools

needed, installation

couldn't be faster!...

Arlington Gangable

Boxes

Designed for new or

retrofit applications, these

snap-together gangable

plastic boxes eliminate the

need to construct a

multiple-gang steel box outof several individual

ones...

advertisement

2008 NEC Books

2011 NEC Books

Technical Books

About Us Contact Us Subscribe Advertise Login Classifieds

Home > Magazine Articles

09Share 0 2 0Digg

w to Specify the Right Motor Control Center For the Job http://ecmweb.com/mag/electric_specify_right_motor/

4 28/10/2011 22:0



within the MCC. If an MCC bus structure is rated to withstand 42,000A and a circuit

breaker is installed with a fault-withstand rating of 18,000A, then the entire MCC

assembly is rated 18,000A. Bear in mind that most NEMA class motor contactors are

rated for a fault-current withstand of 5,000A; some are rated for 10,000A. In order to

account for the lower fault-current withstand rating of motor contactors (and avoid costly

fault damage), a current-limiting motor protector circuit breaker or current-limiting fuses

should be considered.

Bear in mind that the higher the interrupting rating of a device, the higher the cost

(Figure on page 18). Therefore, significant cost savings can be obtained by doing a

short-circuit analysis of an electric power circuit.

Bussing material. The material used for the electrical bussing within the MCC is

another item you should carefully consider during specification.

The most common type of bus material is copper. Aluminum can be used to reducecosts, but aluminum has its own set of problems. For example, its expansion and

contraction is much greater than copper, which can result in the gradual loosening of

bolts and other fasteners. To account for this effect, special washers with spring-type

tensioning are used to connect the aluminum bus to the insulated bus supports. These

fasteners must be properly torqued per the manufacturer's recommendations, or the

spring effect will be lost and subsequent loosening of the fasteners will occur over time.

Also, due to its lower conductivity than copper, aluminum bus must be larger to handle

the same level of current.

Copper bus, on the other hand, can be bolted tightly to the bus supports, and fasteners

seldom loosen over time. However, copper does have its own shortcomings. Some

environments may present a corrosive atmosphere to the copper bussing, which will

lead to eventual MCC failure. For example, ammonia gas, which is often used in

refrigerant or large chiller loops, will attack copper. It will also cause stress corrosion

cracking of copper alloys.

It's possible to protect the copper bussing from corrosion by coating the bus with tin.

Although a tin coating will corrode, the corrosion products of tin are relatively thin and

friable and will break down mechanically, allowing for a continuously reliable metal-

to-metal connection. Tin is also anodic with respect to copper. In other words, if atin-coated copper bus is scratched and the underlying copper is exposed, the tin will

corrode while the copper bus won't. Other examples of corrosive industrial environments

that you should pay attention to are pulp plants, wastewater treatment facilities, fossil

fuel-based power generating plants, and marine environments.

Coating copper bus with silver is effective in some environments, but it can form

relatively thick, insulating layers of silver sulfide (tarnish). It can also be aggressively

attacked by chloride sources. Copper is also anodic to silver, meaning that given a

copper/silver metallic junction, the copper will corrode while the silver wont.

Feeder cables. One of the commonly overlooked issues involved in specifying and

purchasing an MCC is how the main feeder cables will enter the MCC. You typically

have two choices: overhead or underground. Due to the fact that the electric power

cables that feed an MCC are typically numerous and large, knowing beforehand how an

MCC will be fed will eliminate difficult wire bends and/or costly field changes required to

properly terminate an awkward cable feed. One can specify an MCC that will

accommodate both types of feeds, but again, costs will increase and some MCC space

will be wasted.

If automatic controls are going to be implemented for operation of the motor loads, thenthe type of control wiring should be indicated. The various types of control wiring

available are as follows:

Type A — No terminal blocks within the MCC. Only wiring on the motor starter itself is

required. No wiring external to the control device.

Type B — Control wiring to terminal blocks within motor starter buckets while user field

load wiring terminates to the device adjacent to the vertical wire-way.

Type B-D — For Size 3 and smaller starters, field load wiring connects directly to the

device terminals.

Type B-T — For Size 3 and smaller starters, field load wiring connects to factory-wired

power terminal blocks located in or adjacent to each unit.

Type C — Same as B except all factory-wired to master terminal boards at the top or

bottom of the MCC.

Class I — Doesn't include interwiring or interlocking between units or to remotely

mounted devices. Doesn't include control system engineering diagrams if individual units

are supplied. Types A, B, or C wiring available.

Class II — Factory-wired controls and interlocks. Full wiring diagrams are provided,

factory system engineering. Types B or C. Class II type C wiring is expensive and

requires a longer lead time.

By no means are the preceding items meant to be an all-encompassing list of

specifications necessary to successfully purchase or spec an MCC for a specific

application. However, it's a good starting point. Other issues that must be considered

when specifying an MCC include:

Enclosure type: indoor (NEMA 1) or outdoor (NEMA 3R)

Ground-fault detection

Metering

Wire marking

Indication lights

Hands-off auto switches

Voltage rating

Additional help is available through a review of pre-written specifications. However, the

selected document will need to be carefully reviewed and possibly altered in order toproduce the specification that will meet you and your client's specific needs. If the task

seems time consuming or difficult, it's recommended that the services of a professional

engineer be sought for expertise in this area. The MCC will be in service for the next 25

to 30 years, so such an investment will be well worth the effort.

Daley is an electrical engineer with Devon Engineering in Colorado Springs, Colo.

More...

Recent Comments

More...

Case Studies

The Backup Plan

Lessons Learned from

Large-Scale Fire Alarm

Installation at Two California

Hospitals

Century Aluminum Saves

Thousands with Upgrade to LED

Obstruction Lights

1capybara all equipment shall be

installed as the manufacturer

intends. in the specs of the

disconnect it...

Stumped by the Code? June

2011 6 days ago

Matrk03 Was looking at moving

violations #10 .wouldn't the

installation be exempt from the

NEC if it was...

Whats Wrong Here? 1 week ago

Gordon Tesch I would request to

have it removed and probably

going to have to replace every

thing before there...

Whats Wrong Here? 1 week ago

Pjbeacom18 where in the code

book can I find out about the law

of two wires under a single lug on

load side...

Stumped by the Code? June

2011 1 week ago

Tlcgizer Per National Electric

Code, what is the maximum

allowable non-fire rated (outdoor)

cable length...

Stumped by the Code?

September 2011 1 week ago

Social Media

More ways to stay informed...

More...

More...

Moving Violations 10 Moving Violations 9

Understanding NEC

Requirements for Solar

Photovoltaic Systems

During this session, Mike Holt will

address possible conflicts or confusing NEC

requirements, provide tips on proper electrical

installation techniques, and discuss the dangers

related to improper installations. View this On Demand

Webinar!

Top Code Changes: 2011 NEC

EC&M has partnered with NEC

expert Mike Holt to present a one

hour Webinar covering the top 2011

NEC Code Changes. This event will

be broadcast live on the Web with

streaming audio and PowerPoint slides allowing the

audience to have real-time interaction with the guest

speaker, Mike Holt. View this On Demand Webinar!

resources

product info

tradeshow

research

industry links

rss


4 28/10/2011 22:0



Latest Articles

European Green Building Space to

Almost Quadruple by 2016Oct 28, 2011 2:34 PM

Nonresidential Building

Construction Spending Inches

UpwardOct 28, 2011 2:13 PM

North American Business

Conditions Improve in October Oct 28, 2011 2:01 PM

Trending Articles

Insulation Resistance

Testing: How and Why?

Electrical connections

using threaded couplings or

threaded hubs on

Like

Want to use this article? Click here for options!

© 2011 Penton Business Media, Inc.

Acceptable Use Policy

Add New Comment

Showing 0 comments

Sort by Subscribe by email Subscribe by RSS

Trackback URL

blog comments powered by DISQUS

Back to Top

Browse Back Issues

October 2011 September 2011 August 2011 July 2011 June 2011 May 2011 April 2011 March 2011

Browse Back Issues

0

1 person liked this.

Type your comment here.

Image

Siemens answers www.siemens.com/answers

The intelligent factory Answers for industry.

Motor Control Tutorials www.Galilmc.com

Free Web Tutorials from Galil, The World Leader inMotor Control.

Eaton Circuit Breakers www.PocoSales.com

In stock for fast delivery specs, drawings, designguides

National Electrical Code NFPA.org/NationalElectricalCode

2011 National Electrical Code (NEC) Order now for safety & compliance.

9Share 0 2 0Digg


4 28/10/2011 22:0

How to Specify the Right Motor Control Center for the Job

Documents

Transcript of How to Specify the Right Motor Control Center for the Job