If I run 12 ga THHN in closed cell foam under roof insulation, must I derate that circuit? I find nothing in NEC.
I had a similar question regarding the structural insulated panels I'm using to insulate and finish my basement. They're 4" thick, 3.5" styrofoam plus 1/4" of MgO board on each side. They have formed 1" channels for wiring in the center from top to bottom.
My worst case would be NM-B dropping down to a receptacle, then up to go to the next receptacle, which would be 4 conductors encased in insulation.
I setup a test with a length of 14 gage NM-B folded and going up and back through one panel, and a thermocouple data logger to monitor the wire temperature 3 ft from one end. With 12 A "load" through the 4 conductors, the temperature rise was about 34 C, stable after 2 hours.
That would mean the ambient can't be more than 90-34 = 56 C = 133 F, certainly okay for my basement.
Disclaimer: This is intended as an example only, your results will be different. It does make me wonder about houses with old 60 C wiring and new spray foam in the attic.
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