I'm running several new circuits through 1.5" EMT to a large junction box where I split off to 3 - 3/4" EMT runs. All of this is external to the building. Most of this is under eaves and shaded, but the 1.5" is in direct sunlight for a good part of the day and gets very hot to touch. I have not tried to measure the exact temperature.
According to Ugly's Electrical References (Not NEC, I know, but a good reference book), I need to derate conductors according to ambient temp and # of current carrying conductors in a run. Ugly's says a 12 AWG THHN is rated to 25 AMPS, but at an ambient temp of 150F (which I'm assuming the EMT is capable of getting to on a hot summer day) I have to derate the conductor to 58%. So 25 x .58 = 14.5 Amps.
Now I need to further derate the conductor based on # inside the conduit. I'm running 8 circuits. Not counting grounds, I'm assuming I need to figure 16 current carrying conductors (8 hot + 8 returns). Ugly's tells me that for 10-20 conductors, I need to further derate to 50%. Now I'm down to 7.25 amps for a 12AWG THHN wire.
All runs are below 100 ft., so I'm not worried about voltage drop.
Does this all sound right? Am I missing something? This sounds like I'm going to need to run bridge cables just to run a few light bulbs.
One thing I'll do is paint the EMT so it will collect less solar heat. I don't want to run a bunch of 1/2" EMT conduit when I already have a 1.5" going to where it needs to be, and a 5 amp circuit breaker isn't going to cut it for the load on some of these circuits.