NEC 430.24 for 100% rated breakers

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circuitrider

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Upstate SC
I have a 3MVA, 3-phase transformer, with a 480V secondary from which I am using a 4000A, 100 percent rated breaker set to 3600A for switchgear main. I'm thinking NEC 430.24 should apply, but there is no exception for the 100% breaker rating.

If I calculate the load based strictly on this code section, I am in excess of 3600A, but if I base it on 100 percent of continuous, non continuous, and motor load plus 25% of largest motor, I'm fine. I think I'm using the right calculation, but is there a section that confirms it?
 
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circuitrider

Member
Location
Upstate SC
petersonra said:
Motor feeder protection is covered in 430.61-430.63.
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This is kind of a laborious path for having to reach a conclusion, but 240.4(C) requires the conductors protected by an OCPD rated over 800A to have an ampacity greater than or equal to that rating. So in that respect I am looking for an OCPD size that will meet my load requirement and then I provide the cables to give me that ampacity. So yes, 430.24 is relevant.

It would be nice if 430.24 had an exception allowing 100 percent of the continuous non-motor load if the breaker is rated for 100 percent-rated load, but it doesn’t. However, the wording in 420.63 seems to allow for “other load(s)” to be calculated according to the exception for 100 percent rated and the motor loads added (except these are maximum allowed, not minimum required).

I can make my case; I just wish there were an easier way to do it.

Thanks for your input.
 
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wwhitney

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Berkeley, CA
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Looks like an oversight in the language to me. I would submit a PI, but unfortunately the deadline for the 2023 cycle was two weeks ago, so it will have to wait a couple years.

Cheers, Wayne
 
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