Someone asked a question about wire damage in an attic in Phoenix, Arizona. Links on the web confirm attic temperatures go above 180 degrees F, or 85 degrees C. Most wires have a 75C or 90C thermal limit.
NEC seems to say the thermal limits for wire start with its column in the usual ampacity chart we use, NEC 310.15(B)(16) — and by the way, it says it is based on ambient temperatures of 30C (86F). That will come up later. Confining ourselves to wires actually rated for 90C, such as THWN-2, you get, say, 55A for #8 wire and 75A for #6 wire.
Then, following what it says in NEC 310.15(A)(3), it says you use the tables such as 310.15(B)(2)(a) "Ambient Temperature Correction Factors Based on 30°C (86°F). That says for 90C conductors operating in an 81-85C environment, you must derate to 0.29, or 29% of nominal value. That means 15.95A in a #8 wire, or on a #6, 21.75A. *And mind you, that's using THHN, not UF (60C) or THWN (75C).
NM (Romex) is a strange one: NEC 334.80 allows NM a 90C rating for the purpose of calculating derates like this, but the final, practical number can't exceed the 60C rating. Thanks Ed Beal.
I just can't believe that all attic wiring in Phoenix is laid with #6 and #8 wire. How is this done in actual practice? What do Phoenix, Riverside, Tuscon and other sunbelt inspectors approve?
Best Answer
I think ThreePhase is right, in that 180 is close to the upper limit of most asphalt roofs (nobody seems to be suggesting anything above 190 is normal). But attics are their own climate and 190 by the top is not 190 all throughout. Also, attics vary depending on the color (a darker roof will absorb more than a light one).
If it's an issue, what I would do is bury the wires in insulation. Most wires sit on top of the joists, and you typically blow-in or batt insulate between the rafters. If you bury the wires in more insulation above that, they should never get close to 180.