MCC load calculations

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New member
I have a client (equipment manufacturer) who, if you can believe it, is sizing MCC's for a fairly large greenfield manufacturing facility for which he is supplying the equipment.

He's sent me his motor list with his MCC assignments, and they are all overloaded according to my calculations. He tells me that all the motors are going to run continuously (at least for the length of one shift) so I calculated the connected load thusly: ((sum of connected loads)+(25% of largest motor))*1.25. My boss thinks that there's an exception for multiplying the connected load plus 25% of largest motor by 1.25 if the connected load is more than 800 amps. I've never heard that, but nonetheless am tasked with researching it. I can't find it in NEC 240 or 430. Is it possible? Are there any alternatives to the method I've used? The equipment is not loaded continuously, but the motors run continuously. Thanks for any and all help you can give me. -st


Esteemed Member
Re: MCC load calculations

You are very wise to post this problem. I am not a pro in dealing with heating and ambient temperature compensating.

I do know that when there is numerous items of heat producing equipment, they tend to create their own ambient temperature.

The 125 % of the largest motor is peanuts when considering the extreme heat from continuous running motors, loaded or not.

Proper ventilation, air movement, and cooling, plus monitoring is the key.


Senior Member
Re: MCC load calculations

See 430.24: "Several Motors or a Motor and Other loads". I believe you only need to size the MCC for (sum of the connected loads)+(25% of the largest motor or load). It doesn't matter if it's over 800 Amps or not.

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