We had one of our crews run into a situation yesterday with derating.
Final inspection, a new office in a converted industrial space. The guys had run 7 ckts. in a 3/4" emt. 7 hots, 7 neutrals, one ground, #12 thhn, 20a ocpd.
The state inspector failed them for derating. He said that, "Since this was a single phase service all neutrals are counted as current carrying conductors." These circuits are all general use plugs and lighting. No Non-linear loads, no HID lighting.
My repsonse is WTF??????? This is definitely a new one on me. I consider that conduit to have 7 current carrying conductors per the NEC. Please educate me if I'm wrong.
I don't know how other guys figure this, but I'm pretty sure if I took the neutral out of the service and pinched it's end between my fingers, then, with a different finger, touched the screw where it came from I'd find it to be "current-carrying", so I count it as such.
(Don't try this at home.)
[This message has been edited by Dave55 (edited 11-02-2004).]
Current may indeed flow through your fingers but I dare say it would be "unbalanced current" and thus 310.15(B)(4)(a)would apply, no?
310.15(B)(4)(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a). (derating table)
Junkie, I'm under the impression that you have 7 two wire circuits which means they are all current carrying conductors. Looking at 310.15(B)(2)(a) you find that you must derate to 50% of the thhn value of 30A and the ocpd would be 15A. Why aren't you installing 3 wire circuits which would reqire you to derate to 70% and your ocpd would be 20A? Ron
Mark, The tables in Annex C are not 100% fill, they are based on Chapter 9 Table 1. (16) #12 THHN conductors would meet the 40% fill requirement for ¾” EMT according to Annex C Table C1. The area of #12 THHN is .0133” 16 X .0133 = .2128” ¾” EMT is .213 @ 40% fill