g/electrical • Posted by u/journeyman_hopeful • 3 days ago

Running EMT through an unconditioned attic — code compliant in Phoenix?

4th year apprentice here with a code question. On a Phoenix job, the GC wants to run EMT through an attic space (unconditioned, summer temps easily hit 160°F up there).

NEC 358.12 says EMT is not permitted where it will be subject to "severe physical damage." The manufacturer specs on THHN max temp rating is 90°C (194°F) for dry locations which covers 160°F attic temps.

My question is whether the conduit has any temperature limitation I'm missing. EMT is steel — no obvious thermal limit. Is there any NEC restriction on running EMT in a high-temperature attic beyond the wire insulation rating?

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u/breaker_bob • 3 days ago

Been in Phoenix attics in July. EMT is the right choice — it takes the heat. PVC would be a problem (thermal expansion is significant). EMT expands too but it's rigid enough to handle it. Use expansion fittings on runs over 100 feet.

u/sparky420 • 3 days ago

The attic temp concern is real but manageable. THHN in a Phoenix attic is code compliant — just derate. The more practical issue is that 160°F steel conduit in an attic is miserable to work on. Make sure your straps are secured properly because thermal expansion on long EMT runs in extreme heat causes noise and can loosen fittings over time.

u/nec_nerd • 3 days ago

You've done the right homework. EMT itself has no upper temperature limit in the NEC — it's steel conduit. Your constraint is the conductor insulation, which at 90°C/THHN you're fine even in a Phoenix attic. Article 310 Table B.310.15(B)(2)(a) gives correction factors for ambient temp above 30°C — at 60°C ambient (140°F), the derating factor is 0.71. At 70°C (158°F) it's 0.58 for 90°C-rated wire. Run the derating calc for your circuit load — that's your real limit.