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MCC Feeder Size (NEC 430.24 vs. 215.2)

Question asked by Jeff Tootle on Nov 7, 2017
Latest reply on Nov 14, 2017 by Jeff Tootle

NEC 430.24 allows conductors feeding several motors and other loads to be sized based on 125% FLC of the largest motor, 100% FLC of all other motors, 125% of continuous non-motor loads and 100% of noncontinuous non-motor loads. Reading the paragraph in context with the exceptions and the following two paragraphs, it seems apparent that 430.24 is speaking of continuous motor loads.

 

NEC 220.3 and NEC 220.50 point to 430.24 for calculating feeder, and service loads. I have often seen 430.24 used to determine the conductor size of Motor Control Center Feeders.

 

However, today I saw this challenged because 215.2(A)(1)(a) requires feeder ampacity to be not less than the non-continuous load plus 125% of the continuous loads (i.e. 125% of all the continuous loads including all of the motors calculated at 100% based on 430.24). There is no exception for motors in 215.2(A)(1)(a). 

 

Am I missing something here or should NEC be clarified? For an MCC feeder, is it 125% of the largest motor or 125% of all continuous motors? If it is 125% of all continuous motors, then what is the point of 430.24?

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