ECN Forum Forums Code Related Discussion NEC & other Code issues Ampacity of Branch circuit

Ob- The original post was evolved and it was an attempt to see if the rules for roof top temperature could be followed and if the rule for 310.15(A)(2) could be also incorporated.

Picture this: Ambient temperature based on ASHRAE is 89 degrees F in Detroit. Add to that 40 degrees F for the adder mentioned in T. 310.15(B)(2)(c) for a conduit installed 4" above the roof. Now we have 129 degrees and we need a THWN conductor that will be adjusted to that temperature and still be rated for 34 amperes. Since 10' of wiring in the 100' run example exceeds the limit we can not ignore the temperature correction factor.

I imagine this will still be surprising to the guys who slept through that part of their code update and "never did it that way before".
I suspect the code language was really just trying to recognize you needed an adjustment and they did not want to get into the detail of raceway color, roof color etc. This is complicated enough as it is. How many guys know where to find the ASHRAE data?

I see this biting the guys who should understand, the hardest, the solar installers. I wonder if they know they should derate the conductors in their DC raceways? 690.8 doesn't seem to mention it.

I can also see this getting the HVAC installations and the guys who are installing to the label on the condenser.

Is this really being enforced?

Hello, first post here, been a lurker for awhile now.

This is being enforced in Calif, has been for a few years now in the PV installs at the very least (DC and AC conductors).

Whether by NEC 2005 as 690.31(C) or NEC 2008 as 310.15(B)(2)c and/or 690.31(C).

Each jurisdiction is different and some even have their own correction factor regardless of what the code says.

Oh, the temp in conduit is higher above a white roof than a black roof.

Vindeceptor:

Welcome to ECN forums from one of the 'Jersey Guys'!!

Thanks for your input!

Vindiceptor,

Glad to have you here, don't be afraid to speak up, we won't bite. This is a great bunch of guys here and there are many knowledgeable people with great advice they are willing to share.