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#97782 - 03/21/06 12:10 PM more derating fun
earlydean Offline
Member
Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
Say I feed a multiple gang box with a single 14/2 cable fed by a 15 amp cb, wire-nut seven neutrals, pigtail the hot to 6 switches, then feed six 14/2 cables up the wall and through a single bored hole in the top plate. These six cables continue on to six individual recessed luminaires.
According to code, I have 12 current carrying conductors through this hole. Building code requires me to draft stop this hole, and 334.80 requires me to derate these conductors per 310.15(B)(2)(a).
But, because I feed all six cables from a single 15 amp breaker, am I exempted from the derating requirement? (after all, I am limited to 15 amps shared among all cables)
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Earl
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#97783 - 03/21/06 01:30 PM Re: more derating fun
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
What you describe can happen in pipe systems as well.

In my opinion it should be exempt.

That said I do not believe there is anything in the NEC that allows us to 'exempt it'.

Bob
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#97784 - 03/21/06 03:36 PM Re: more derating fun
George Little Offline
Member
Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
I could not agree more Bob. each wire will carry it's own current and generate it's own heat which will be additive and hence a large amount of heat right at the upper plate where they are closest together.
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George Little
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#97785 - 03/21/06 03:37 PM Re: more derating fun
eprice Offline
Member
Registered: 08/07/03
Posts: 64
Loc: North Logan, Utah, USA
How long is the hole? Even if it is a double top plate and the hole is 3" long, if the total conductor length not in the hole is at least 30" then 310.15(A)(2) exceptiion allows us to use the ampacity of conductors outside of the hole for that portion in the hole.
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#97786 - 03/21/06 07:08 PM Re: more derating fun
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9066
Loc: Estero,Fl,usa
I hate to be the spoil sport here but the code is wrong.
The whole derating issue assumes all conductors will be on separate circuits so the heat adds. If they are on the same circuit it actually subtracts where they are paralleled.
If the total amps is 15 and it is spread out across 1 pair of conductors "in" and 5 "out" pairs of conductors the heat generated in the insulation is ~60% of what it would be with a single Romex in and out. Resistance of the "outs" is 1/5th of a single pair and total heat is still I2R.
The only way this current could add is if this was a series loop and full current comes in and out 3 times.
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Greg Fretwell
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#97787 - 03/21/06 07:24 PM Re: more derating fun
George Little Offline
Member
Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
eprice- I think you are mistakenly using 310.15(A) because 334.80 specifically says that "B" applies and using your approach it would rarely be the case where there would be derating of NM cables. And I do understand the Exception in "A" of 310.15.
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#97788 - 03/22/06 02:13 AM Re: more derating fun
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Quote:
I hate to be the spoil sport here but the code is wrong.


I do not think anyone here is disputing that, I said as much in my post.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#97789 - 03/22/06 07:29 AM Re: more derating fun
George Offline
Member
Registered: 02/23/02
Posts: 380
The code is not wrong ---

(Assuming that the plate is thick enough that derating is required ...)

The code simply says that if you derate this situation then the AHJ will approve this portion of the work.

If you do not wish to derate you need to do engineering. In this case, engineering will show that derating below 15amp is not required.
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#97790 - 03/22/06 07:43 AM Re: more derating fun
dlhoule Offline
Member
Registered: 04/28/05
Posts: 71
Loc: Saline, MI 48197
quote:
--------------------------------------------------------------------------------
I hate to be the spoil sport here but the code is wrong.
--------------------------------------------------------------------------------

I do not think anyone here is disputing that, I said as much in my post.

The code is never wrong. Maybe poorly written in many instances, but not wrong. These examples are where you talk to the AHJ and get permission. When you parrallel conductors you do not significantly increase the heat when they are bundled or in raceway. If the AHJ doesn't know that he shouldn't be inspecting.
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#97791 - 03/22/06 10:47 AM Re: more derating fun
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9066
Loc: Estero,Fl,usa
George, the exception in (A) of 310.15 was discussed in the IAEI news a while ago. The assumption is the thermal conductivity of the conductor will tend to average out the hot spot. In that regard I don't understand the new language of 336.80. I guess the CMP has the same issue you do with the exception but they won't address it.
Sometimes I think the guys who write new language do not rationalize it with old language they let stand.
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Greg Fretwell
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