Wire ratings in very hot attics, e.g. Phoenix where they go to 180F?

Question

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?

Answer 1

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.

Answer 2

Fix the issue at its source: the deficient attic

An attic that is hitting those temperatures throughout, even in a hot climate such as Phoenix, has a design flaw. Either the ceiling plane needs to be air-sealed and insulated to allow the attic to be vented at the soffits and ridge to provide convection cooling of the space, or the underside of the roof deck needs to be air-sealed and insulated, forming a hot roof, to allow the attic to be conditioned and treated as interior space.

With proper venting, the attic should stay closer to 120°F (~50°C) worst-case at the top of the joist space, which is where most of the wiring lives in any case. Of course, in a conditioned hot roof system, the attic will be kept at living-space temperatures instead; this is vastly superior to even a vented attic when there is equipment and ductwork in the attic, not just the occasional cable or vent pipe. "Hot roofs" do have the downside that they can put more stress on temperature-sensitive roofing materials in hot climates, though, so there is that to keep in mind as well.

If fixing the attic isn't an option, then one could bury the wires in insulation given that they are at or near the joist space, but this does pose downsides for future maintenance.