Just learned wire gauge drop tables in the FAA handbook are based on 25C, not cabin temps

Found it on page 14-21 of AC 43.13-1B last night, so all my 20 gauge runs on the Gulfstream might be undersized by a full 15% when the panel hits 45C on the ramp, has anyone else been sizing for real heat instead of the standard?

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the_derek23d ago

Man, good catch. I ran into the same thing last summer rewiring a beechcraft. I was sizing at the book numbers and the senior guy on my crew pointed out that same table, said he always adds a wire size up for anything behind the firewall or in the avionics bay. I started measuring actual temps with a cheap IR gun on hot days and yeah, 45C is normal in a closed up panel. My rule now is just use the next gauge up if there's any doubt, it's cheap insurance compared to chasing a voltage drop gremlin later. The FAA table is a starting point, not the whole answer, in my experience.