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Found out Romex derating rules are way stricter than I thought
I was running 6 current-carrying conductors in a 3/4 EMT for a remodel in Austin last month. Checked the code book and found out I have to derate to 80% of the 60C column for THHN inside conduit. That dropped my 12 AWG from 20 amps to just 16 amps. I had to upsize to 10 AWG on two circuits. Has anyone else been burned by this on a tight conduit run?
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sandrat2429d ago
Man, that's a rough one! I've been there. Austin's got its own code quirks too. Derating for conduit fill catches a lot of people. Best tip: always run a spare conduit or two from the panel. Future proof it. Also, 90C column applies for derating if you're not over 100 amps, so check that. Saves you from upsizing wire more than you gotta.
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felixfisher29d ago
Read a recent thread where a master electrician in Phoenix said they always check the local amendments on temperature derating. Apparently some counties treat rooftop conduit like a straight up oven, so your wire gauge gets jacked up way more than the standard table says. Caught me off guard because I'd been using the same derating factors for years. Also heard about guys running EMT instead of PVC for rooftop runs because the metal helps with heat dissipation. Makes sense but adds to the cost. Always good to cross reference with someone who's been working the area a while.
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