MCC Load Calculations

[hou628]

Hello,

I am working on a design project for a new MCC that as 1200 A main CB. I've created a preliminary load calculation using continuous (125% of Amp rating) and noncontinous (100% of Amp rating) loads and am at 1220 A total. As the process will not require all equipment to be started all at the same time, would the 20A over the main CB rating be acceptable or does NEC code not allow an overages in load calculations?

 

[Smart $]

No overages are permitted.

Does that total include all demand factoring? Even nitpicking items such as general lighting in halls, corridors, closets, stairways calculated at 0.5VA/ft?

To reduce the calculated load for equipment not operating concurrently, there has to be an interlocking method?mechanical, electrical, or combination?to prevent concurrent operation.

 

[hou628]

Thanks for the reply. Yes that includes lighting and other misc power draws as well. It looks like I'll be pulling power from an existing MCC to satisfy this requirement. Do you know the NEC code for the interlock requirement off the top of your head? If not, Ill search for it. Thanks again

 

[don_resqcapt19]

Are you using the continuous and non-continuous loads for the motor loads, of just the other loads?

 

[hou628]

Are you using the continuous and non-continuous loads for the motor loads, of just the other loads?

Im using continuous and non-continuous loads...

 

[Smart $]

Thanks for the reply. Yes that includes lighting and other misc power draws as well. It looks like I'll be pulling power from an existing MCC to satisfy this requirement. Do you know the NEC code for the interlock requirement off the top of your head? If not, Ill search for it. Thanks again

Judging by your reply, I'm uncertain you got the gist of my post.

General Lighting: many calculate based on area of entire project but fail to calculate some of that area permitted to be calculated at a lower power density.

For instance, 100,000 ft? under industrial commercial (loft) building is 200kVA, but if 3,000 ft? of that area is halls, corridors, closets, stairways, then you have 97,000x2+3,000x0.5= 195.5kVA... and 4.5kVA savings at 480V 3? is 5.4A. True, it is not substantial and the reason most don't bother, but if you combine this 5.4A with other possible reductions, you may achieve the needed -20A without changing any loads or getting power elsewhere.

Code doesn't actually say loads must be interlocked to reduce the calculated load, just a common interpretation. The actual Code section is 220.60.

 

[Smart $]

Im using continuous and non-continuous loads...

Motors loads are technically neither, but typically summed as noncontinuous because all the 125% factoring is included in Article 430/440 requirements and never factored an additional 125% for running continuously for more than 3 hours.

 

[hou628]

Motors loads are technically neither, but typically summed as noncontinuous because all the 125% factoring is included in Article 430/440 requirements and never factored an additional 125% for running continuously for more than 3 hours.

Thanks for the feedback. How are motors not considered either? If the motors runs for 3 continuous hours shouldn't its amperage be factored into the conductor and circuit breaker sizing at 125%?

 

[Smart $]

Thanks for the feedback. How are motors not considered either? If the motors runs for 3 continuous hours shouldn't its amperage be factored into the conductor and circuit breaker sizing at 125%?

The thing about continuous loads is operation at maximum current is required continuously for 3 hours or more. Most motors do not run at the maximum current continuously.

See 220.50. That takes you to 430.24, -.25, and -.26. Then go to 430.61, -.62, and -.63

When you get to feeder and service level, you only factor the largest motor* by 125% [430.62]. Article 220 and 230 don't state explicitly how to incorporate motor loads into the sizing determinations, but they have to be included per Article 430 Part V. So if you want, you can consider the largest motor a continuous load, and all other motors noncontinuous. I realize Code is a bit lacking in text to easily arrive at this conclusion. I'm certain if you research the issue, you will arrive at the same conclusion.

*Determined by line amps, not kVA.

 

[GoldDigger]

As has been mentioned in other threads as well as above, motor conductors and OCPD are already oversized beyond the Full Load Amperage (potentially continuous current) to allow for starting surges.
Applying an additional correction for continuous load is not necessary.
You can either multiply FLA BT 125% to allow for starting or you can multiply FLA by 125% to allow for continuous load. The result is the same and the Code has chosen the first method as its standard.

 

[petersonra]

the 125% is a requirement for the size of the wires, not the size of the main CB.

you can go to 100% on the CB if you use a 100% rated CB.

 

[petersonra]

the 125% is a requirement for the size of the wires, not the size of the main CB.

you can go to 100% on the CB if you use a 100% rated CB.

I realized this came out a little strange.

The size of the feeder wires for an MCC is determined by this provision.

430.24 Several Motors or a Motor(s) and Other
Load(s). Conductors supplying several motors, or a motor(s)
and other load(s), shall have an ampacity not less
than the sum of each of the following:
(1) 125 percent of the full-load current rating of the highest
rated motor, as determined by 430.6(A)
(2) Sum of the full-load current ratings of all the other
motors in the group, as determined by 430.6(A)
(3) 100 percent of the noncontinuous non-motor load
(4) 125 percent of the continuous non-motor load.

The size of the feeder CB is determined by 430.62.

 

[don_resqcapt19]

In my experience the rating of an industrial MCC is not normally based on the rules found in the MCC. It is not unusual to find an MCC with a connected load that far exceeds the rating of the MCC. The size is often based on what equipment is likely to run at the same time, based on what the process engineer has told the electrical engineer. In these designs the control system does not have specific interlocks that limit the maximum load that could be running at the same time, just that it would be unlikely.

I think this is one of those cases where the code rules could be changed using the substantiation that has been successfully used a number of times for rules related to industrial installations...."we have been doing it that way for years without problems".