Romex in residential

H

hhsting

Senior Member
Location
Glen bunie, md, us
Occupation
Junior plan reviewer
I have 100A feeder from Panel A feeding sub panel B all inside town home. Engineer provided #3 awg and using romex. However is that not undersized for 100A breaker since your suppose to use 60C for it?
 
i believe you are correct. NM cable must not be rated any higher than the 60°C column, unless 310.12(B) applies.

i've never seen #3 NM CU cable, but it would have an ampacity of 85a. 240.4(B) allows that feeder to be rounded up to the next size so must be on a 90a OCPD.

310.12(B) allows dwelling feeders to be de-rated by Table 310.12(A), but only in the instance that it feeds the entire load of the dwelling unit. so is the entire sub-panel B feeding an entire "dwelling unit"? if not, this derating does not apply. if so, Table 310.12(A) derating applies, and as small as #4 NM would be sufficient.
 
Master Nater said:
i believe you are correct. NM cable must not be rated any higher than the 60°C column, unless 310.12(B) applies.

i've never seen #3 NM CU cable, but it would have an ampacity of 85a. 240.4(B) allows that feeder to be rounded up to the next size so must be on a 90a OCPD.

310.12(B) allows dwelling feeders to be de-rated by Table 310.12(A), but only in the instance that it feeds the entire load of the dwelling unit. so is the entire sub-panel B feeding an entire "dwelling unit"? if not, this derating does not apply. if so, Table 310.12(A) derating applies, and as small as #4 NM would be sufficient.
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The entire sub panel B does Not feed an entire dwelling unit.
 
Master Nater said:
then it does not qualify for the Table 310.12(A) derating. there are many folks who think that table applies to any feeder in a dwelling. i know, i was one of them. :) maybe your engineer is one too?
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Same applies for the previously mentioned 2AWG aluminum cable, it would only be good for 90 amps if not supplying entire dwelling load.
 
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