Rooftop Ampacity Derating - AC vs DC?


So I'm working on an install for a developer and a QC guy questioned a couple of pipes that are less than 3-1/2" from the rooftop. Given we are on 2017 for this jurisdiction I gave the go-ahead, not seeing the note in the print that said minimum 3-1/2" from the roof, but this was obviously just some outdated part of the template so I was able to convince their engineering team to allow it to stay installed as-is.

So I preface with the story above to tell you that the inspector said he was fine with it too, but his reasoning was because it was DC wires only and not AC. I was not there, but when that message was relayed to me I went cross-eyed.

When I derate the ampacity of a conductor due to ambient temperature I do not consider whether it's AC or DC current on the line. The only time I would exclude those calcs is for the XHHW-2, which is exempt from direct sunlight on a roof derating. But the 2017 took out the ambient temp adder chart and made a single line in the sand that says if it's over 7/8" then no need to derate. And if it's under 7/8" then its all the way to 60°F adder. So just "all or none" basically.

But other than the above mentioned which is 310.15(B)(3)(c), I don't find anything in the NEC, nor do I understand why it would be the case that on the physics level, that DC current would create less heat than AC current, given the same resistance.

Did I miss something, or did the inspector's statements get change from the "phone-game" effect.


Senior Member
Springfield, MA, USA
Electric motor research
Nothing different about AC or DC ampacity calculations until you get into the skin effect realm of very large conductors or very high frequency.