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Derating Application #90198
11/02/04 09:10 PM
11/02/04 09:10 PM
G
golf junkie  Offline OP
Member
Joined: Apr 2001
Posts: 507
York, NE
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.

GJ

2017 / 2014 NEC & Related Books and Study Guides
Re: Derating Application #90199
11/02/04 09:16 PM
11/02/04 09:16 PM
D
Dave55  Offline
Member
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
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.)

Dave

[This message has been edited by Dave55 (edited 11-02-2004).]

Re: Derating Application #90200
11/02/04 10:39 PM
11/02/04 10:39 PM
E
Electric Ian  Offline
Member
Joined: Feb 2004
Posts: 133
MA, USA
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)

Re: Derating Application #90201
11/02/04 10:48 PM
11/02/04 10:48 PM
D
Dave55  Offline
Member
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
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.

Dave

Re: Derating Application #90202
11/02/04 10:56 PM
11/02/04 10:56 PM
N
Norstarr  Offline
Member
Joined: Sep 2004
Posts: 86
Wi
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

Re: Derating Application #90203
11/02/04 11:19 PM
11/02/04 11:19 PM
E
Electric Ian  Offline
Member
Joined: Feb 2004
Posts: 133
MA, USA
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 11:21 PM
11/02/04 11:21 PM
E
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
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 11:36 PM
11/02/04 11:36 PM
D
Dave55  Offline
Member
Joined: May 2004
Posts: 697
Crystal Lake, Illinois, USA
I use Annex C, Table C1 Maximum Number of Conductors....in EMT.

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

Dave

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

Re: Derating Application #90207
11/03/04 12:06 AM
11/03/04 12:06 AM
C
caselec  Offline
Member
Joined: Apr 2002
Posts: 558
San Jose, CA
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

Curt


Curt Swartz
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