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?

Yes, you would have to derate them. Now if the pipe was 2 feet long, no derating factor would need to be applied.

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"

I don't think you're seeing it quite right, and I can see why.
1st scenario: A conduit or a cable, with more than three CCC's in it.

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.

24" or LESS = nipple - no derating needed
24 1/4 " or MORE = pipe - better derate

Thats what I've always been taught

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%.

George,

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

Thanks!


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

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?

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?

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?

No, IMO, whether the covering is stripped off or not, the conductors are still in the sleeve/conduit and the "where the number of current-carrying conductors in a raceway or cable exceeds three" part of the section would apply. The "or where single conductors or multiconductor cables are stacked or bundled" part is there for the cases where there is no raceway involved.

IMO, the exception to 310.15(A)(2) will usually allow us to forget about derating in the above situations.

Have you looked at 334.80 in the 2005 code? There seems to be no wiggle room on the "insulation" derating.

I do not see that 334.80 has a bearing on this discussion.

Common sense tells me I must derate, but the wording seems to contradict that.

If I install cables in a raceway I don't need to derate per 310.15(B)(2)(a). Doesn't make sense, but the wording is there.

Would you guys all agree with me that derating is a good idea? And, that the wording needs to be made clearer?

"Where the number of current-carrying conductors in a raceway or cable exceeds three," does not say those CAN'T be cable conductors in a raceway, the following text you quote refers to cables not in raceways.

I agree it is poor wording but I believe they mean what you are saying is right.

SolarPowered ---

The NEC allows engineering. As do all national building codes.

The engineering to support my statement is very simple.

quote:
--------------------------------------------------------------------------------
...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,...

Dean, I agree; better wording would be nice.
How about instead of "and not installed in raceways", "it just said even if not installed in raceways."

Not only that; but if they are bundled how do you maintain spacing. IMO if they're bundled you haven't maintained spacing.

I did consult with Jeff Sargent of NFPA about this. Yes, we must derate (as common sense would tell us) for the number of conductors in the conduit, including the conductors in the cables.
The phrase: "and not installed in raceways" was intended to clarify the cables were stacked or bundled, not to confuse the requirement for derating of conductors in confined conditions. Who among us would like to submit clearer language for the next code? Ryan?

312.5(C)ex applies to earlydeans' original post. You cannot put a 2ft piece of PVC in the top of a panel and route all the NM's through it.

312.5(C) ex. does just the opposite. It gives explicit permission to run all your NMs in a single PVC conduit. The FPN also makes it clear derating is needed.

Earlydean
I think you missed an important word in 310.15(B)(2)(a)...OR.

Where more than three current-carrying conductors in a raceway or cable exceeds three, OR where single conductors or multiconductor cables are stacked or bundled longer than (24 in) without maintaining spacing and are not installed in raceways....


I do not see the confusion here.

I did not miss the word "OR".
My point was the confusing (at least to me) wording in the article: "....multiconductor cables are stacked or bundled longer than 24 inches and not installed in raceways".
We all figured we have to derate multiple conductors in conduit, in cables or not.
This is still true, and I never doubted it, I only questioned the choice of words: "and not installed in raceways".
As dlhoule suggested, how about changing the wording to: "whether installed in raceways or not". Then, it would make sense to me.