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Derating Application #90198 11/02/04 08:10 PM
Joined: Apr 2001
Posts: 507
golf junkie Offline OP
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.


2017 / 2014 NEC & Related Books and Study Guides
Re: Derating Application #90199 11/02/04 08:16 PM
Joined: May 2004
Posts: 697
Dave55 Offline
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).]

Re: Derating Application #90200 11/02/04 09:39 PM
Joined: Feb 2004
Posts: 133
Electric Ian Offline
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)

Re: Derating Application #90201 11/02/04 09:48 PM
Joined: May 2004
Posts: 697
Dave55 Offline
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.


Re: Derating Application #90202 11/02/04 09:56 PM
Joined: Sep 2004
Posts: 86
Norstarr Offline
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?

Re: Derating Application #90203 11/02/04 10:19 PM
Joined: Feb 2004
Posts: 133
Electric Ian Offline
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.

Re: Derating Application #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
Re: Derating Application #90205 11/02/04 10:36 PM
Joined: May 2004
Posts: 697
Dave55 Offline
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.


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

Re: Derating Application #90207 11/02/04 11:06 PM
Joined: Apr 2002
Posts: 558
caselec Offline
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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