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Old 01-17-2011, 01:43 AM   #1
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Derating???? How many wires through hole.


I'm doing some remodeling and would like to run some branch circuits for recessed lighting in my 2nd floor attic. I'm grabbing my power from a bedroom light switch that currently operates a wall outlet. Right now there is a 14-3 at the switch. The wire goes through the top plate into the attic and the hole in the top plate is 1 1/4" which would easily accommodate another 14-3 and a 14-2. The hole is sealed with foam.

Is this a situation where I have to be concerned about derating? This is a 15amp circuit and I plan to use LED recessed lighting which will be very low wattage.

I have the 2008 NEC book and read about derating. But I'm confused and don't quite understand if I have a situation that requires derating. I know about bundling and I've been careful not to do that during this remodel.

I can easily drill another hole in the top plate to split up the number of wires that go into the attic. But I hate to weaken my framing unnecessarily.


Thanks
Rick

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Old 01-17-2011, 08:05 AM   #2
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Derating???? How many wires through hole.


How many wires are going through the bottom & top plate in that section now?

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Old 01-17-2011, 01:38 PM   #3
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Derating???? How many wires through hole.


There is only one 14-3 now and it goes from the switch box through the top plate into the attic. No wires are going downward. I will have two 14-3 and one 14-2 in a triple gang box. All wires will go up through the top plate. I would like to put these three cables through the same 1 1/4" hole, which will be sealed with foam insulation.

Last edited by RM575; 01-17-2011 at 01:40 PM.
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Old 01-17-2011, 02:12 PM   #4
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Derating???? How many wires through hole.


I see no problems. You are only passing through maybe a 2x4 & 3/4" subfloor.
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Old 01-17-2011, 02:56 PM   #5
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Derating???? How many wires through hole.


I don't see any problems either. But the NEC seems to address this situation. I can't find the code at this time, but I've read on numerous posts that you must derate the circuit if a certain number of current carrying conductors pass through a bored hole that will be sealed with insulation. I think some people use general rules such as no more than 50 percent of the bored hole should be filled with cables, etc. I think this is done to reduce the likelyhood of heat buildup when multiple cables are bundled together for long distances and when going through penetrations in framing.

It is very confusing and I was hoping to get some simpler explanation of this.

Upon examining the electrical in my home I noticed the original electrician bored a 1 1/4" hole in the top plate at the circuit breaker panel and ran 4 cables that appear to be two 14-2, one 12-3 and one 6-2 in the same hole. Then it was insulated with foam. I think those cables are filling up much more than 50 percent of that hole. So, if they did that back in 1985, and there has been no problem with heat buildup, I think I'll be fine.
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Old 01-17-2011, 03:13 PM   #6
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Derating???? How many wires through hole.


Here is the section you are talking about:


334.80 Ampacity.

The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60C (140F) conductor temperature rating. The 90C (194F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60C (140F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.

Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.

Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).
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Old 01-17-2011, 03:20 PM   #7
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Derating???? How many wires through hole.


Since I have the 2008 NEC on my iPhone, I am unable to pull the info from it for here, I found this in the 2008 draft that is avail. I always go by the rule of thumb, I always pull no more than 2 cables through a 1" - 1 1/4" for #12/2 or #14/2.

334.80 Ampacity. The ampacity of Types NM, NMC, and
NMS cable shall be determined in accordance with 310.15.
The ampacity shall be in accordance with the 60C (140F)
conductor temperature rating. The 90C (194F) rating
shall be permitted to be used for ampacity derating purposes,
provided the final derated ampacity does not exceed
that for a 60C (140F) rated conductor. The ampacity of
Types NM, NMC, and NMS cable installed in cable tray
shall be determined in accordance with 392.80(A).
Where more than two NM cables containing two or more
current-carrying conductors pass through the same opening in
wood framing that is to be fire- or draft-stopped using thermal
insulation, caulk, or sealing foam, the allowable ampacity of
each conductor shall be adjusted in accordance with Table

310.15(B)(2)(a) and the provisions of 310.15(A)(2), Exception,
shall not apply. [ROP 7–72, 7–70, 7–73]
Where more than two NM cables containing two or
more current-carrying conductors are installed in contact
with thermal insulation without maintaining spacing between
cables, the allowable ampacity of each conductor
shall be adjusted in accordance with Table 310.15(B)(2)(a).
[ROP 7–74]
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Old 01-17-2011, 03:20 PM   #8
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Derating???? How many wires through hole.


You are quick Petey.
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Old 01-17-2011, 03:26 PM   #9
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Derating???? How many wires through hole.


Yes. That is the section I am concerned about. Now what, in simpler terms does it mean for me? What am I supposed to do or not do?

Given the fact the the bored hole is presently 1 1/4" now, does this mean I can put only two 14-2 cables through the hole? Can I put two 14-3 and one 14-2?

Yes. The hole will be filled with insulation foam. So, that apparently applies to this situation.

I'm wondering if what I propose to do is a violation of this code section? And, if so, how do I proceed so I don't violate the section?
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Old 01-17-2011, 03:35 PM   #10
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Derating???? How many wires through hole.


Also, as I mentioned earlier, back in 1985 the original electrician filled up a 1 1/4" hole with 4 cables of various ampacity and then it was foamed. See my reference to the circuit panel area. Complying with this code then would mean that multiple holes would have been drilled in the framing that most of the framing would have been removed in that area.

So, it doesn't seem appropriate to restrict three cables from going through the same (large) hole when the cables won't be bundled prior to or after penetrating the wood framining. Maybe I'm not interpreting the code right.

Does it mention the restriction of the size of the hole and the number and gauge of conductors passing through?
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Old 01-17-2011, 03:43 PM   #11
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Derating???? How many wires through hole.


If only two conductors, either a 3/4" or 1" will do. I did not fill mine up with foam when I redid our bathroom, for the two romex that passed through to the basement. Now, up in the attic when I get a chance to do so this summer, I have to go up and use the Fireblock foam or silicone for the places that warm air from our main living area is getting through, this includes a couple of places that had old BX that we used the holes to pull a single pull of 14/3 through a couple.
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Old 01-17-2011, 04:01 PM   #12
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Derating???? How many wires through hole.


I have no problem drilling another hole next to the one there now. But, in another part of the home I ran some individual bathroom circuits back to the panel. At the framing near the panel I was able to send them through the same holes drilled by the original electrician. After busting out some of the foam insulation the new 12-2 and 14-2 fit easily through the holes. Again, these are 1 1/4" holes.

So, in one of those holes I have three 12-2 and one 14-2. Still plenty of room for more cables.

In another hole there are two 14-2 and one 12-2. Again plenty more room for more cable.

The way the framing is constructed near the panel, I would not be able to safely drill anymore holes without the possibility of damaging existing wiring.
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Old 01-17-2011, 04:37 PM   #13
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Derating???? How many wires through hole.


I personally would not put any foam into the holes on the joists. Only place I would personally seal if pulling up from an unconditioned space, or required to not allow spread of smoke, would be at the bottom plate where the wire comes up, or the top plate if entering the attic.

As for the joist by the panel, you may have to find a hole big enough to pull that cable through it. Keep in mind that it sometimes looks like some fool went crazy with a drill if you have a lot of romex going to the panel through one joist. If anything, you may have to disconnect a couple Romex from the panel to backpull if you need to make that one hole larger.

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