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#95217 - 09/06/05 02:22 PM Derating NM cable  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
What is the general policy for derating NM cable conductors when they are pulled through wooden studs or basement ceiling joist? Are the inspectors asking that the derating apply after more than 3 NM cables are pulled through the same set of holes? NEC 310.15(B)(2)


George Little

2017 / 2014 NEC & Related Books and Study Guides

#95218 - 09/06/05 02:32 PM Re: Derating NM cable  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
George they do ask for that here in MA.

More than 9 current carrying conductors through the holes in studs and joists will need derating.

However we also have less a less restrictive derating table.

I understand there is much controversy about this and many areas do not ask this. I can sure see it going either way.

Not much help. [Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#95219 - 09/06/05 02:47 PM Re: Derating NM cable  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,113
Estero,Fl,usa
I think this gets down to the "bundling" question. If they fan out between bored holes this is a pretty thin beef. If the installer was very neat and they run in military precision I see the *potential* problem with the ones in the middle.
Rake them out with your fingers. Then they are not bundled. Twisted and kinked is a plus in this case.

It seems strange at first but "neat = heat".

The same thing is true in a panel when you see those "aircraft like" wire harnesses running down the gutters, tywrapped, with breaks at each device. It looks beautiful but derating can rear it's ugly head.


Greg Fretwell

#95220 - 09/06/05 03:02 PM Re: Derating NM cable  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
As I said I can see both sides and my feeling is the NEC is on the same page as our local inspectors.

Consider the following exception to the derating rules.

Quote
310.15(B)(2)(a) Exception No. 5: Adjustment factors shall not apply to Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:
(a) Each cable has not more than three current-carrying conductors.
(b) The conductors are 12 AWG copper.
(c) Not more than 20 current-carrying conductors are bundled, stacked, or supported on “bridle rings.”
A 60 percent adjustment factor shall be applied where the current-carrying conductors in these cables that are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing exceeds 20.


To me this exception demonstrates that the NEC considers MC supported on bridle rings every 5' to be bundled.

If they call this bundling I don't see how we can not call NM through studs bundling.

MC hung on bridle rings always 'fans' out some.

Really what I believe and I know of someone putting in a proposal along these lines is this whole issue should go away for NM in dwelling units as you will never have all the conductors fully loaded for any length of time.

Here in MA we still use a lot of NM in commercial jobs and I have found some pretty warm bundles of NM in these locations.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#95221 - 09/06/05 03:25 PM Re: Derating NM cable  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,113
Estero,Fl,usa
What about the "without maintaining spacing"?
If you have the NM fanned out so air can freely flow around them they are not bundled for greater than 24". MC and AC may not maintain the spacing because of flexibility, shape and weight, but NM will pretty much stay where you put it in a joist bay between stud bores.

I guess a related question is how big a hole are they boring if they are getting more than 3 NM in here (9 conductors).


Greg Fretwell

#95222 - 09/06/05 04:05 PM Re: Derating NM cable  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
We do not 'drill' many holes for NM, generally we punch them and use plastic bushings in the holes. The bushing must be about about 1" ID.

I don't find it is convenient to run to many cables together, I would rather just make more holes.

Where I see the bundles of NM is usually the last 5' to 10' into the electrical panels.

Ever see a bundle of NM with about a 12" OD?


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#95223 - 09/06/05 05:02 PM Re: Derating NM cable  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,113
Estero,Fl,usa
George specifically addressed stud bays and joists but I certainly agree that "bundle" going in the top of the panel can be troubling. I haven't seen 12" bundles but 10 or more RX shoved in a 2" nipple is not unusual.

Here is one from the HI site
[Linked Image]


[This message has been edited by gfretwell (edited 09-06-2005).]


Greg Fretwell

#95224 - 09/06/05 06:06 PM Re: Derating NM cable  
Ryan_J  Offline
Moderator
Joined: Aug 2003
Posts: 1,374
West Jordan, Utah, USA
Does it meet 310.15(A)(2) Exception?


Ryan Jackson,
Salt Lake City

#95225 - 09/06/05 06:38 PM Re: Derating NM cable  
George Little  Offline
Member
Joined: Jan 2004
Posts: 1,489
Michigan USA
Thanks Greg- I agree the "stove piping" happens all the time without any regard for the fill or bushings. Can I pick on your picture for a minite? Let me get your permission first [Linked Image]


George Little

#95226 - 09/06/05 07:23 PM Re: Derating NM cable  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,113
Estero,Fl,usa
It's not my picture. I robbed it from the home inspector BB.
There are plenty of things going on there. I was just pointing out the 2" full of RX


Greg Fretwell

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