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.
Click to expand...

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?
 
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?
Click to expand...
Same applies for the previously mentioned 2AWG aluminum cable, it would only be good for 90 amps if not supplying entire dwelling load.
 
hhsting said:
The entire sub panel B does Not feed an entire dwelling unit.
Click to expand...
But what is the size of the service or feeder that supplies the entire dwelling unit? If it's 100A, then it and any feeders within the dwelling unit need only have an ampacity of 83A, and 60C #3 Cu would be fine.

Cheers, Wayne
 
wwhitney said:
But what is the size of the service or feeder that supplies the entire dwelling unit? If it's 100A, then it and any feeders within the dwelling unit need only have an ampacity of 83A, and 60C #3 Cu would be fine.

Cheers, Wayne
Click to expand...

The Service that supply the entire dwelling unit is SER and not subject to NEC 2017 Article 340.80. The service is 200A.
 
hhsting said:
The service is 200A.
Click to expand...
OK, then the 100A feeder needs an ampacity of 100A. Which could be 75C #3 Cu.

So the error here is calling for NM cable, which both doesn't exist in #3, and which would be limited to the 60C ampacity. Use a 75C wiring method, and #3 Cu is the correct size, barring voltage drop concerns.

Cheers, Wayne
 
wwhitney said:
OK, then the 100A feeder needs an ampacity of 100A. Which could be 75C #3 Cu.

So the error here is calling for NM cable, which both doesn't exist in #3, and which would be limited to the 60C ampacity. Use a 75C wiring method, and #3 Cu is the correct size, barring voltage drop concerns.

Cheers, Wayne
Click to expand...

See link below:

www.southwire.com

3-3 CU NMD-90 WHITE JACKET W/G CSA 246 FT | Southwire

www.southwire.com www.southwire.com

If its NM cable and assume #3 awg comes then you amapcity cant be based off of 75C correct based on NEC 2017 Article 340.80?


Sent from my iPhone using Tapatalk
 
this NMD is new to me. i do not find it anywhere in the code book. and it's own spec sheet says the conductors are good for 90°C. not the cable, but the conductors. so, i fail to see the difference between NM-B and NMD. what is the difference and what does the D represent?

edit to add: is it the "damp enclosed location" rating?
 
Master Nater said:
this NMD is new to me. i do not find it anywhere in the code book. and it's own spec sheet says the conductors are good for 90°C. not the cable, but the conductors. so, i fail to see the difference between NM-B and NMD. what is the difference and what does the D represent?
Click to expand...
I think you’ll find that the listing is only CSA, not CSA(US).
 

NM-B vs NM-D

Over the past holidays, I found myself in Canada in the electrical aisle in one of the home-horror stores. I was intrigued that the "romex" wire looked a bit different, and was called type NMD. On the surface, it appeared to be the same thing as our NM-B, except that it did not have any of the...
forums.mikeholt.com

NMD VS NM-B Wire and the Electrical c... - Fine Homebuilding

* I live in Canada, and on a recent trip to the States I picked up some Romex wire to rewire my House (shack… well; Shed… maybe shoe box) so […]
www.finehomebuilding.com www.finehomebuilding.com

jzdcable.com

NMD-90 vs. NM-B Wire: Key Differences, Similarities, and How to Choose - JZD Cable

Confused about NMD-90 and NM-B cables? Our guide explains their specs, applications, differences, and similarities to help you select the right building wire for your project.
jzdcable.com jzdcable.com
 
You must log in or register to reply here.
Top