Romex in residential

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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.
 
hhsting said:
The entire sub panel B does Not feed an entire dwelling unit.
Click to expand...
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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