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#90198 11/02/04 08:10 PM
Joined: Apr 2001
Posts: 507
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.


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#90199 11/02/04 08:16 PM
Joined: May 2004
Posts: 697
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).]

#90200 11/02/04 09:39 PM
Joined: Feb 2004
Posts: 133
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?

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)

#90201 11/02/04 09:48 PM
Joined: May 2004
Posts: 697
I understand that portion of the NEC to apply to multiwire branch circuits. As it is described as 7 hots and 7 neutrals for 7 circuits, I don't think it applies.


#90202 11/02/04 09:56 PM
Joined: Sep 2004
Posts: 86
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?

#90203 11/02/04 10:19 PM
Joined: Feb 2004
Posts: 133
You guys are right. I was thinking 3 wire circuits with shared neutral. Separate neutral with each phase wire - all neutrals are indeed current carrying conductors.

#90204 11/02/04 10:21 PM
Joined: May 2003
Posts: 2,876
e57 Offline
Nailed on derating, and fill factor.

358.22 to Chapter 9 Table 1, and notes to Tables. 40% fill, not 88%.

The only way I could see 15 #12's being allowed, is if were in a 23" conduit, and even then, I wouldn't do it.

[This message has been edited by e57 (edited 11-02-2004).]

Mark Heller
"Well - I oughta....." -Jackie Gleason
#90205 11/02/04 10:36 PM
Joined: May 2004
Posts: 697
I use Annex C, Table C1 Maximum Number of EMT.

Type THHN, THWN for 3/4" EMT is 16 conductors for 12 ga. Of course there would be some derating involved.


#90206 11/02/04 10:42 PM
Joined: Feb 2004
Posts: 133
Up to 16 THHN conductors in 3/4" EMT allowed per table C1 in annex C.

#90207 11/02/04 11:06 PM
Joined: Apr 2002
Posts: 558
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


Curt Swartz
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