where to find nm kcmils

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maybe I am asking the wrong question. I asked my crew leader how many 12-2 nm or 14-2 wires we can fit in the 3/4'' hole we bore in the wood stud, he didn't know. He said he was told not more than 3 12-2 nm conductors. I want to know how to figure it out for myself for future jobs. Any help would be great.
 
You can stuff in as many as will fit. If they're bundled for more than 24" then derating will apply. Bundling wouldn't apply to the portion within the hole unless it was sealed with a draft or fire stopping compound.
 
infinity said:
You can stuff in as many as will fit. If they're bundled for more than 24" then derating will apply. Bundling wouldn't apply to the portion within the hole unless it was sealed with a draft or fire stopping compound.
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Urm, that makes less than no sense.

You can cram wires in the hole until it's virtually air tight, but if you make it actually air tight you have to derate?

Is there a code section for that, because I'd like to propose that that is a stupid code section and needs to be tossed.
 
tallgirl said:
Is there a code section for that, because I'd like to propose that that is a stupid code section and needs to be tossed.
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How about 334.80 the last paragraph.:smile:
I agree with the tossing comment.
 
tallgirl said:
And the handbook says it's new for '05 stupidity.
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Here's the ROP from 2005 for your reading pleasure:smile:

Recommendation:
Add a new paragraph to 334.80 to read as follows:
"334.80 Where more than two NM cables containing two or more current-carrying conductors are bundled together and pass through
wood framing which is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor
shall be adjusted in accordance with Table 310.15(B)(2)(a)."
Substantiation:
Code-Making Panel 6 Rejected Proposal 6-31 to add the proposed text to 310.15(B)(2)(a) and provided the following Panel statement:
"The Panel agrees with the intent of the Proposal, however, this material is more appropriately addressed in 334.80 since the Proposal
only applies to one type of cable, and Code-Making Panel 6 covers all wiring methods. Therefore, Code-Making Panel 6 has forwarded
this Proposal to Code-Making Panel 7 for action."
The substantiation provided by the submitter, Travis Lindsey, of Proposal 6-31 was:
"Recent experimentation shows the possibility of dangerous conditions when loaded circuits are brought into close proximity to each
other inside a fire- or draft-stop, where the ability to dissipate heat is extremely limited. Cable temperatures well in excess of their 90?C
rating were encountered, with no overcurrent protection present for these conditions. Results indicate that immediate adjustments
should be made to the NEC to apply at least to the specific case represented by the experiment. Such a proposal is being made, with a
supplemental report offered as technical support."
 
The rating for 90 C conductors in NM B is 30 Amps for #12 and 25 Amps for #14 (Reference Table 310.16).

Derating those to 70% for 7 to 9 CCCs gives you 21 and 17.5 Amps respectively.

Since the OCPD limits of 240.4(D) for 12 and 14 AWG are 20 and 15 Amps, you can stuff four (4) pieces of 2-conductor NM B through a hole, complete with firestop foam, without having to reduce the ampacity/OCPD below 20 and 15 Amps for 12 and 14 AWG.
 
Bob NH said:
The rating for 90 C conductors in NM B is 30 Amps for #12 and 25 Amps for #14 (Reference Table 310.16).

Derating those to 70% for 7 to 9 CCCs gives you 21 and 17.5 Amps respectively.

Since the OCPD limits of 240.4(D) for 12 and 14 AWG are 20 and 15 Amps, you can stuff four (4) pieces of 2-conductor NM B through a hole, complete with firestop foam, without having to reduce the ampacity/OCPD below 20 and 15 Amps for 12 and 14 AWG.
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Yeah, but what about framing members above panels where typically a zillion CCCs are passing through a small number of holes on their way into the attic?

That practice has been going on for decades. Has it been safe because residences (where most NM is used) tend to have far more circuits than capacity, or because this is one of those things that only happens under lab conditions, like drinking too much soda causes cancer?

I help rough a house this weekend that had something like 24 circuits on a 125A service. We ran 12 pieces of 12/2, more or less, through 2 Really Big Holes, both of which I'm sure will be foam sealed.

And none of that answers the question -- is it safe if not foam sealed, but still crammed tight as can be?
 
The great thing about this rule on sealing foam is that it brings out that the ampacity of 12-2 NM B is 30 amps for derating.
 
tallgirl said:
And none of that answers the question -- is it safe if not foam sealed, but still crammed tight as can be?
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According the NEC it is permitted. As it stands now the part about sealing with foam does not change the ampacity of the conductors due to 310.15(2)Exception. This exception will be changed in the 2008 NEC so as not to include the provisions of 334.80 regarding draft and fire stopping. So if your under the 2005 NEC you can forget about any derating for NM cables through sealed holes.

310.15(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
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Trevor,
I don't agree that the exception applies for this application. It is my opinion that a specific rule (like that in 334.80) always over rules a general rule (like the exception to 310.12(A)(2)).
Don
 
tallgirl said:
Yeah, but what about framing members above panels where typically a zillion CCCs are passing through a small number of holes on their way into the attic?

That practice has been going on for decades. Has it been safe because residences (where most NM is used) tend to have far more circuits than capacity, or because this is one of those things that only happens under lab conditions, like drinking too much soda causes cancer?

I help rough a house this weekend that had something like 24 circuits on a 125A service. We ran 12 pieces of 12/2, more or less, through 2 Really Big Holes, both of which I'm sure will be foam sealed.

And none of that answers the question -- is it safe if not foam sealed, but still crammed tight as can be?
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I submit that it is safe. Here is the rationale:

Twelve pieces of 12 AWG at 6530 cmils = 78,360 cmils. That is between 1 and 2 AWG. Interpolating Table 310.16, that would permit about 120 Amps. Running your 125 Amp service through those two Really Big Holes (12 cables per hole) would put about half of that through each hole, going both ways.

Those twelve #12s would have a parallel resistance (I know they aren't really paralleled but we need a basis for calculation) of about 0.00018 Ohm per foot, so even if you put 100 Amps going both ways (out and back), you would have only 3.6 Watts per foot (corrected with edit, was 1.8) from the bundle. At half of the 125 Amp service capacity through each bundle, you have only about 1.4 Watts (corrected with edit, was 0.7) per foot.

Copper is also a good conductor of heat. The heat generated at that hole will be conducted along the wires, both ways, and will be dissipated away from the foamed Really Big Hole. That is what supports the "24 inch rule".
 
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tom baker said:
The great thing about this rule on sealing foam is that it brings out that the ampacity of 12-2 NM B is 30 amps for derating.
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Right, but with 24 CCCs (12 circuits from the 24 circuit example I gave earlier) passing through a 2" hole above a panel, the derating factor is 45% (I assume we're using table 310.15), which means that the ampacity of each circuit is reduced to 13.5A.

Working that backwards gives 24 * 13.5A = 324A which is significantly greater than the service size of 125A. Which is why I said its a stupid rule. For a typical residential application there likely isn't the service capacity to create the problem. The 7 to 9 CCCs before derating reduces the ampacity to below 20A would consume enough of the service that I just don't see it happening, as I showed above.
 
tallgirl said:
Working that backwards gives 24 * 13.5A = 324A which is significantly greater than the service size of 125A. Which is why I said its a stupid rule. For a typical residential application there likely isn't the service capacity to create the problem. The 7 to 9 CCCs before derating reduces the ampacity to below 20A would consume enough of the service that I just don't see it happening, as I showed above.
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I like the rational of your arguement.
 
acrwc10 said:
I like the rational of your arguement.
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Yeah, especially since that's only HALF of the panel -- there's another 2" hole with the other 24 CCCs passing through it. The total for that situation would be 648A -- just to get up to what the Code says would be permitted IF derating were applied.

It's the same thing as those stupid rats they fed too much soda -- sure, it's possible to create problems if you can somehow get a 1200 sqft house to consume 77kVA -- which would be permissible if all 24 circuits were loaded to their derated limit of 13.5A.

Likely? Nope. The service lateral, feeder, and everything else involved would likely have long since melted. But those pieces of NMB would still be safe.
 
tallgirl said:
Yeah, especially since that's only HALF of the panel -- there's another 2" hole with the other 24 CCCs passing through it. The total for that situation would be 648A -- just to get up to what the Code says would be permitted IF derating were applied.

It's the same thing as those stupid rats they fed too much soda -- sure, it's possible to create problems if you can somehow get a 1200 sqft house to consume 77kVA -- which would be permissible if all 24 circuits were loaded to their derated limit of 13.5A.

Likely? Nope. The service lateral, feeder, and everything else involved would likely have long since melted. But those pieces of NMB would still be safe.
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ceramic kiln for the pottery class at home, 4 or 5 of them. That may do it.
 
tallgirl said:
Yeah, but what about framing members above panels where typically a zillion CCCs are passing through a small number of holes on their way into the attic?
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I will not be surprised if we see that prohibited as well.


That practice has been going on for decades. Has it been safe because residences (where most NM is used) tend to have far more circuits than capacity, or because this is one of those things that only happens under lab conditions, like drinking too much soda causes cancer?
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IMO your on to it.

I read some of the test criteria, IMO the conditions they created are not what we see in real life.

A number of NMs that all happen to be in the same hole and all happen to be fully loaded (or close to it).

It seems unlikely many homes have more than one NM fully loaded at a time.
 
I believe that this awareness of the 'bundling in holes' issue is the result of some actual experimental work, where bundled cables at 'reasonable' loadings in 'reasonable' ambient conditions exceeded the cable temperature ratings:

http://www.copper.org/applications/electrical/building/pdf/bundle_evaluation_report.pdf

You will need to read into the document to figure out how 'representative' you think their tests were, but they were using cables that were arguably 'bundled' for only a few inches, with fire stopping in a wood top plate. It is not clear just how much insulation batting was around the 'bundled' area.

With all of the cables in the 'bundle' loaded to 80% of the circuit trip rating on a continuous basis, the cable temperature greatly exceeded the 90C temperature rating of the insulation.

We can discuss how realistic the study installation was, but as I read it it would be permitted under the NEC, but for the requirement to derate NM with fire-stopping in 334.80.

-Jon

[edited to add the last half of the last sentence, cut off at posting.]
 
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