Question on Romex (NM wire)

[Matt the electrician]

This is something that I have wondered for quite some time.
When you run Romex wire, whether it is 12/2, 14/3, 6/2, etc.. being that it is NM, I have been told that you always size it to the 60 degree column of table 310.16 due to the code rule of 334.80 is this correct?
thanks,
Matt

 

[LarryFine]

Correct. It compensates for being embedded in insulation and similar installations.

 

[infinity]

I agree. Hopefully this dumb requirement leaves the code soon.

 

[wwhitney]

When you run Romex wire, whether it is 12/2, 14/3, 6/2, etc.. being that it is NM, I have been told that you always size it to the 60 degree column of table 310.16 due to the code rule of 334.80 is this correct?

Mostly correct. The final ampacity is limited to the 60 degree column value. But if any derating is required you are allowed to use the 90C column ampacity as the starting point for the derating, and then your final ampacity is the lesser of the 60 degree column value and the derating calculation.

So for example, say you bundle together (4) 12/2 NM cables so you have to adjust for 8 CCCs. That adjustment factor is 0.7, but the 90C ampacity of #12 Cu is 30A. That makes the adjusted ampacity 21A. As that's still above the 60C table value of 20A, those 12/2 NM still have an ampacity of 20A (assuming no temperature correction applies).

Cheers, Wayne

 

[kwired]

keep in mind if you are working with some older cables, if they do not have the "NM-B" designation then the starting point for ampacity adjustments is the 60C rating.

All NM cable manufactured after sometime in mid to late 1980's has been type NM-B.

 

[Eddie702]

So let's say you have a heat pump with an MOCP of 50 an MCA of 35. Your run NM-B out to the disconnect from the main panel it would have to be #8. The whip to the AC from the disconnect could be #10 (rated 35A @75C)??

 

[wwhitney]

The whip to the AC from the disconnect could be #10 (rated 35A @75C)??

If it's a 75C rated wiring method and wire. That rules out LFNC or LFMC in wet locations.

Cheers, Wayne

 

[Eddie702]

I agree. Hopefully this dumb requirement leaves the code soon.

I think it is dumb as well. Why is SE cable not held to this standard. Are they just trying to protect the NM installed in older homes?

 

[infinity]

So let's say you have a heat pump with an MOCP of 50 an MCA of 35. Your run NM-B out to the disconnect from the main panel it would have to be #8. The whip to the AC from the disconnect could be #10 (rated 35A @75C)??

Yup stupid but that's what the book says.