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#97712 - 03/18/06 05:56 PM derating  
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
Am I reading this right? 310.15(B)(2)(a) requires derating for bundled multiconductor cables "not installed in raceways". Does this mean that if I have a 4 foot piece of 4 inch PVC connected to the top of my basement panel, and I route all the branch circuit NM cables through this "nipple", that I do not have to derate the conductors in the cables in the nipple?


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#97713 - 03/18/06 06:04 PM Re: derating  
bot540  Offline
Joined: Sep 2004
Posts: 174
Vernon Hills, IL
Yes, you would have to derate them. Now if the pipe was 2 feet long, no derating factor would need to be applied.

Jesus may have been a capenter,but God was an electrician.Genesis1:3

#97714 - 03/18/06 06:15 PM Re: derating  
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
So I thought too, but aren't these cables installed in a raceway? And therefore the derating doesn't apply? The 2 foot part is mentioned right there as well:
"multiconductor cables are stacked or bundled longer than 24 inches and not installed in raceways"


#97715 - 03/18/06 06:40 PM Re: derating  
georgestolz  Offline
Joined: Jun 2005
Posts: 91
Fort Collins, CO, US
I don't think you're seeing it quite right, and I can see why.
Where the number of current-carrying conductors in a raceway or cable exceeds three,...

1st scenario: A conduit or a cable, with more than three CCC's in it.

...or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways,...

Second scenario: groups of cables or conduits. I think their point was, there does not have to be a raceway containing the cables to consider them bundled.

I think it would more clearly state their point if it read "(24 in.) without maintaining spacing even when not installed in raceways," IMO.

Also see note 4 to Chapter 9. It reiterates that a nipple less than 24" does not require derating, which seems to infer that over 24" it does.


#97716 - 03/18/06 07:03 PM Re: derating  
luckyshadow  Offline
Joined: Jan 2005
Posts: 308
Maryland USA
24" or LESS = nipple - no derating needed
24 1/4 " or MORE = pipe - better derate

Thats what I've always been taught

#97717 - 03/18/06 08:56 PM Re: derating  
George  Offline
Joined: Feb 2002
Posts: 375
earlydean ---

If it is important, hire a competent engineer and ask him to do engineering.

1) 10% conduit fill never needs derating below the usual 66% (or is it 70%).

2) 200amp service never needs derating below the usual 66%.

#97718 - 03/19/06 01:28 AM Re: derating  
SolarPowered  Offline
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA

Could you give code citations for (1) and (2), please?


[This message has been edited by SolarPowered (edited 03-19-2006).]

#97719 - 03/19/06 08:24 AM Re: derating  
jfwayer  Offline
Joined: May 2005
Posts: 30
Fairmont, WV, USA
I saw something called speedway non-metallic support system for cables at an IAEI meeting recently. It is a system of cable "rings" meant to be mounted under the joists in a residental basement. It also had a solid plastic "cover" that snapped on below, to enclose both sides and the bottom of the rings and hence the cables run through the rings.

The manufacturer's rep told me that no derating of conductors was required. The glossy he handed out said that you could, for example, put up to 23 12-3/G NM-B (for a max of 40% fill).

I'm trying to figure out how this is covered in the NEC. The best guess is that 392.11(A) applies. It refers back to 392.9. I believe that 392.9(F) applies and that speedway's 40% fill is more or less compliant.

Now back to 392.11(A): (1) says derating applies only to cables with more than 3 current carrying conductors [speedway only lists 3 or less conductor cables], (2) talks about covers which complete the enclosure by covering the top [no cover for speedway], and (3) single layer and properly spaced [not mentioned in speedway literature].

What I'm trying to understand is this: does the 392.11(A)(1) no derating required for less that 3 apply even though 392.11(A)(3) requires a single layer and spacing is violated?


#97720 - 03/19/06 03:18 PM Re: derating  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
I think the NEC is pretty strange in this area, at least contradictory.

If I sleeve as many cables as I can stuff in a 4" that is 23.999" long "for physical protection only" I don't see where I need to derate at all but if I have an inch and a half in insulation penetrating a top plate I need to derate. 334.80

... or should I get that pointy hat out again?

Greg Fretwell

#97721 - 03/19/06 04:04 PM Re: derating  
earlydean  Offline
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
I think you guys are missing my point. How do we get past the code language requiring us to derate when "multiconductor cables are stacked or bundled longer than 24 inches and not installed in raceways"?
It seems clear to me this language allows us to install NM cables in a sleeve of any length without derating. But, if we strip the covering off, now there are conductors in the raceway, and derating is required.
We cannot second guess the code, we gotta take it as it is, or submit a code change.
Did I miss a code section?


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