I have heard that many people consider the nuetral of the circuit you describe as current carrying, due to the non-linear loads of flourescant lights. You might ask your electrical engineer if 310.15(4) applies. The thing to remember, however, is that at 90 degrees a #12 conductor is good for 30 amps, but only for the purposes of derating. Since you are describing a situation that may be 4 current carrying conductors in a raceway, you are permitted to use 30amp rating to derate...80% of which is 24 amps. Since 240.4(D) requires that a #12 conductor be supplied from a breaker not larger than 20 amps, it really is a moot point until you have 10 current carrying conductors in a raceway.

Hope this helps [Linked Image]

Ryan Jackson,
Salt Lake City