NM Subpanel Feeder

[roc]

I'm curious if 6/3 MN cable can be used for a 60A subpanel feeder [which has multiple outlet circuits] according to the NEC.

NEC 334.80 would require 90C insulation on the wires, which would normally be a 75A capacity per 310.15 for #6 wire with say THHN insulation. But then 334.80 goes on to state that ampacity shall be based on 60C insulation which results in 55A capacity.

You are permitted to derate the 90C value ? and if you look at say an ambient temperature rise of 86F to 104F for unconditioned space, this would result in a 68A cable capacity. But that NEC section says you still cant exceed the 60C capacity of 55A. Why restrict the ampacity like that?

I understand that the wires may have a 90C rating, but that the jacket is only rated for 60C. Why then allow the higher insulation value for the wires to be derated if the jacket is only good for 60C. Is that so the entire cable assembly does not have a rating that exceeds the 60C value ... I'm not really following that.

Rob

[ October 29, 2003, 12:59 AM: Message edited by: roc ]

 

[iwire]

Re: NM Subpanel Feeder

Originally posted by roc:
I'm curious why 6/3 MN cable can't be used for a 60A subpanel feeder according to the NEC.

Actually it can be if you apply 240.4(B), it allows the use of the next standard size breaker in many instances.

As for the rest of your question I do not know why.

But I will not complain about a gift like being able to start my derating figures at 90C.

Imagine how low the ampacity of the cable would be if you started derating at the 60C rating.

 

[tom baker]

Re: NM Subpanel Feeder

Roc the advantage is for temp derating due to ambient. Instead of starting at the 60 Deg C rating, we start at the 90 deg C rating, same procedure as THHN, except that THHN we can operate at 75 Deg C

 

[roc]

Re: NM Subpanel Feeder

Bob ... good point about allowing the next size breaker in certain cases ... I fixed the original post.

I'm not complaining about starting at the 90C value for derating, just trying to understand that section.

Why not allow either 75C or 90C conductor insulation, and then the derating would start from there ... with the same 60C overall limitation. For this case the derated 75C wires would give you 59A, which would be more in line with the 55A overall cable limit.

Rob

 

[iwire]

Re: NM Subpanel Feeder

Rob

You may use the 60 amp breaker on your panel feeder regardless of the panel having multi outlet branch circuits.

240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1)The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

The feeder conductors are not "part of" the branch circuit.

The branch circuits will have their own OCP sized accordingly.

My comment about not complaining was more of a way to cover up the fact that I do not know the reason why you where looking for.

[ October 26, 2003, 09:46 AM: Message edited by: iwire ]

 

[roc]

Re: NM Subpanel Feeder

Thanks again Bob ... after I went over that section again I agree that the next std size breaker may be used on a subpanel feeder, even if there are multiple receptacle circuits ...

Still curious why 334.112 requires the 90C conductor insulation for NM cable, and you are not permitted the option to use either 75C or 90C insulation ... not that it's a big deal, just trying to understand that.

Maybe because the typical derating starting from 90C would not usually drop the cable ampacity below the 60C value ... so it's kind of "fool proof" for most installations?

[ October 31, 2003, 08:43 PM: Message edited by: roc ]

 

[Guest]

Re: NM Subpanel Feeder

 

[roc]

Re: NM Subpanel Feeder



[ October 31, 2003, 08:42 PM: Message edited by: roc ]

 

[iwire]

Re: NM Subpanel Feeder

Rob you are more than welcome, I am glad I could help with that part of your question.

The other part I still can not help you with.

Bob