W-Cummins said:
Some thing else to watch is that you can't fill the conduit with more than 3 current carring conductors per run w/o de-rating the wires running in it.
William....
I hate to object, but this is a rather blanket statement that doesn't tell the whole story. True, four or more conductors in a single conduit require derating of the wires. It is commonly accepted that #12 wire is 20 amp. well, it is, sorta, because the max Overlload Current Protection Device allowed is 20 amp. However, the wire commonly used in conduit is THHN and that is a 90 degree C rated wire and #12 THHN wire is rated for 30 amps, thus, you have to have more than 9 current carrying wires in a single conduit to cause that #12 THHN wire to be derated to less than 20 amps. Since we were using a 20 amp breaker anyhow (the rule mentioned above on the use of 20 amp breaker on #12 wire), the derating of the wire, in this case hasn't caused us any grief.
For that matter, you also need to derate the wire for "high ambient temp". If the maximinum ambient temperature that the wire will see is greater than 86F degrees, then you also have to start derating for temperature.
Example: We put between 7 and 9 current carrying wires in a conduit. we have sized the conduit so that we meet the fill limits of it. All of the wires are #12 THHN being run for various receptacle circuits in our shop. A #12 THHN wire is rated at 30 amps, derated by mutiplying by .7 (70%) gives us 21 amps. We live in a moderate climate and the temps exceed 86F but do not exceed 95F, so we derate by .96 for the high temps, giving us 20.16 amps. We still protect this circuit with a 20 amp breaker. If we lived in a hotter climate the temp derating would cause us to use 15 amp breakers to protect the circuit.
How does the electrician get away with running open romex wires in the hot attic of your house? Well, even lowly romex #12 wire is rated at 25 amps, and we can go up to 122F before our temp derating takes us below 20 amps.
Again, the best thing to do is have a whole bookshelf of good reference books and the current NEC, when doing any sort of complex wiring job.
Confused? Ask ten electricians the same question and you will get a dozen different answers!
Charles