How many wires in a hole in a wall stud?

I've searched all through my 2005 NEC looking for some guidance as to how many 12/2 circuits I can run through a single 3/4" hole in the wall studs of
a house I'm building as a retirement project. I can not find anything about it. I've also asked on several over groups. Is there some rule of thumb or is there an actual regulation that I'm missing. I'm way out in the country and we do not have an inspector that I can ask and the one in the county seat said there is no restriction. I'm not concerned about county restrictions, I'm concerned about safety.
TIA
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JC from Gnat Flats




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JC wrote:

I'm not aware of any reference in the NEC to the maximum number of cables in the hole, but from a safety perspective, the issue is obviously heating.
Since the stud thickness is only 1.5" and only two or three 12/2 cables can reasonably fit the hole, I would not expect enough temperature rise to worry about. Of course, with continuous 20A loads on all of the cables there would be more heat buildup. But remember that copper is an excellent conductor of heat and any localized warm spots (in the holes) will quickly dissipate heat in both directions through the wire.
Let us know if you find an authoritative answer.
Chuck
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The best reference I have found seems to be NEC article 310.16 regarding the ampacity of wire. There are charts and footnotes indicating that one must derate bundled cables. One reference is to THHN wire with a 90 degree Celsius rating showing that bundled cable with 10 to 20 current-carrying conductors should be derated at the rate of 50%. The chart shows these conductors carrying capability to be 30Amps, thus when bundled in a group between 10 to 20 current carrying conductors the circuits should be derated to 15Amps. This is for 12/2 wire. The derating factors are 80% for 4 to 6 conductors and 70% for 7 to 9.
So, I'm going to run with that. Thanks.
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JC from Gnat Flats




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That will certainly be conservative. If I'm not mistaken, the Code includes an exception to the effect that derating factors shall not apply to conductors in nipples having a length not exceeding 24 in. Seems difficult to understand why a 1.5" wood stud requires derating but a 24" nipple does not.
The only safety downside to a conservative interpretation is that structural issues may eventually surface if too many holes are bored in the studs.
Chuck
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Yes sir, that's my concern. I have one instance that is going to require 7 12/2 cables in one large hole or a series of holes and it just eats at me to weaken a piece of wood like that. Anyhow, thanks for your attention to my request. I appreciate it.
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In the past I have installed a number of sub-panels in residential construction and had the same concern. I actually dug out my old college textbooks and did a structural load calculation on a vertical 2x4 with one to ten 7/8 inch holes in a row up the middle.
When there was over 2 inches of wood between the holes the difference from 1 to 10 holes was only 5%decrease in available support. When the spacing got down to 1 inch of wood it was up to %15.
So if you only have 7 runs of romex that will take 4 holes so just space them with over one inch of wood between and you should be fine unless there is an extreme load on the wall.
Dave Master Electrician
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JC wrote:

You can't go with a large hole if it leaves too little thickness of wood (1 1/4 minimum) unless you use a guard plate. See 300.4 (A) (1) Note the exceptions - my assumption is that you are using NM.
Using several holes will not appreciably weaken the structure.
Ed
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Chuck wrote:

That's a good point. I don't think the derating factors apply in this case.

Have you ever seen plumbers drill a 2" hole through a 2x4 (effectively 3.5" wide). There's not much wood left. If you need to drill a series of 3/4" holes through a stud, keep them in a vertical line. This will leave a maximum amount of wood on either side in the form of two columns. As long as you separate the holes by a few inches, the stud should retain most of its bending strength.
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Paul Hovnanian P.E. wrote:

In line with what you and Chuck wrote, the derating required in 310.15 (there isn't a 310.16 in the 2005 NEC) is for cables bundled or stacked for more than 24 inches. Just keep the cables loose between studs.
If more than 2 cables are going through wood framing that is required to be fire or draft stopped with insulation or foam, derating is required (334.80).
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wrote: | Chuck wrote: |> |> > The best reference I have found seems to be NEC article 310.16 regarding the |> > ampacity of wire. There are charts and footnotes indicating that one must |> > derate bundled cables. One reference is to THHN wire with a 90 degree |> > Celsius rating showing that bundled cable with 10 to 20 current-carrying |> > conductors should be derated at the rate of 50%. The chart shows these |> > conductors carrying capability to be 30Amps, thus when bundled in a group |> > between 10 to 20 current carrying conductors the circuits should be derated |> > to 15Amps. This is for 12/2 wire. The derating factors are 80% for 4 to 6 |> > conductors and 70% for 7 to 9. |> > |> > So, I'm going to run with that. Thanks. |> > |> > |> |> That will certainly be conservative. If |> I'm not mistaken, the Code includes an |> exception to the effect that derating |> factors shall not apply to conductors in |> nipples having a length not exceeding 24 |> in. Seems difficult to understand why a |> 1.5" wood stud requires derating but a |> 24" nipple does not. | | That's a good point. I don't think the derating factors apply in this | case.
I suspect that it is considered that the extra heating of the bundle is able to reasonably dissipate if that heating is only for a short length. That level of heat can be handled better with a metal nipple up to some length, and in the wood up to some shorter length. The case of a bundle of great length would have no opportunity to have extra dissipation by means of the spread of wires out either end. These are not huge levels of heat, but enough to warrant adjustment to make sure all the other extra margins of safety are maintained.
|> The only safety downside to a |> conservative interpretation is that |> structural issues may eventually surface |> if too many holes are bored in the studs. | | Have you ever seen plumbers drill a 2" hole through a 2x4 (effectively | 3.5" wide). There's not much wood left. If you need to drill a series of | 3/4" holes through a stud, keep them in a vertical line. This will leave | a maximum amount of wood on either side in the form of two columns. As | long as you separate the holes by a few inches, the stud should retain | most of its bending strength.
I would not want such a hole in a load bearing stud. OTOH, I plan to have at least exterior walls double thick with staggered studs, so it would be possible to run wires horizontal, where any runs are needed (not likely), zig-zagging back and forth between the layers. My flooring structure will also be double layer with a 2x4 layer over a 2x16 layer at 90 degree angle so there will also be places to run wires without drilling a single hole. The main support beams will definitely be out of bounds for any drilling. My house design is a post and beam design with walls generally not doing any load bearing, but they may be used for structural stability.
And, FYI, my intent is to use the new combo cable that meets both AC and MC requirements.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net wrote:

The idea here is that heat can travel along the length of the conductor for some distance before being conducted through the insulation to the environment without incurring too high a temperature gain. Copper (and aluminum) conduct heat quite well with a small temp rise.
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| snipped-for-privacy@ipal.net wrote: |>
wrote: |> | Chuck wrote: |> |> |> |> > The best reference I have found seems to be NEC article 310.16 regarding the |> |> > ampacity of wire. There are charts and footnotes indicating that one must |> |> > derate bundled cables. One reference is to THHN wire with a 90 degree |> |> > Celsius rating showing that bundled cable with 10 to 20 current-carrying |> |> > conductors should be derated at the rate of 50%. The chart shows these |> |> > conductors carrying capability to be 30Amps, thus when bundled in a group |> |> > between 10 to 20 current carrying conductors the circuits should be derated |> |> > to 15Amps. This is for 12/2 wire. The derating factors are 80% for 4 to 6 |> |> > conductors and 70% for 7 to 9. |> |> > |> |> > So, I'm going to run with that. Thanks. |> |> > |> |> > |> |> |> |> That will certainly be conservative. If |> |> I'm not mistaken, the Code includes an |> |> exception to the effect that derating |> |> factors shall not apply to conductors in |> |> nipples having a length not exceeding 24 |> |> in. Seems difficult to understand why a |> |> 1.5" wood stud requires derating but a |> |> 24" nipple does not. |> | |> | That's a good point. I don't think the derating factors apply in this |> | case. |> |> I suspect that it is considered that the extra heating of the bundle is |> able to reasonably dissipate if that heating is only for a short length. |> That level of heat can be handled better with a metal nipple up to some |> length, and in the wood up to some shorter length. The case of a bundle |> of great length would have no opportunity to have extra dissipation by |> means of the spread of wires out either end. These are not huge levels |> of heat, but enough to warrant adjustment to make sure all the other |> extra margins of safety are maintained. | | The idea here is that heat can travel along the length of the conductor | for some distance before being conducted through the insulation to the | environment without incurring too high a temperature gain. Copper (and | aluminum) conduct heat quite well with a small temp rise.
Yes, that is what I meant for "a short length". But why a shorter length for in wood than for in metal nipple? I suspect this is because the metal contributes to the ability to dissipate the heat (in all directions) more than wood can. And if things get way too radically hot, it will take a higher temperature to impact the metal than the wood (though at this point I don't think the different really matters much, anymore).
BTW, if the bundle going through the nipple or hole also stays bundled in air beyond it, it would be less able to dissipate heat than the more likely scenario of the individual cables being spread out.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net wrote:

It isn't a shorter length in wood. The rules are the same for "bundled" cables and nipples - derating is not required if the length is under 24". Derating is for over 24" of "bundled" cables - not specifically wood. (310.15)

If the hole in the stud is less 24"long, and the cables are not kept tight together after exiting the stud, derating is not required.
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wrote:

In the 2008 NFPA made it very clear they do mean that NM cables penetrating a framing member with any kind of fire or draft stopping must be derated. They added "caulk" to the list and specifically said you CAN'T use the 310.15 (A)(2) exception (that acknowlages thermal transfer down a wire). They also rejected Jim Pauley's proposal that you didn't have to derate if the fire/draft stopping was only on one side of the member (not actually in the hole). They are serious about this, even though it does sound silly. (not unusual for NFPA) Effectively this simply limits you to 9 current carrying conductors per hole so it is not really that big a deal.
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snipped-for-privacy@aol.com wrote:

>> If the hole in the stud is less 24"long, and the cables are not kept

A separate issue from 'ordinary' holes. It didn't register that this applied to simple seals that weren't fire stops. I wonder if the heat transmission of caulk is lower than fiberglass. What is the heat buildup of 20" of bundled Romex in fiberglass. Foam-in-place insulation should be as bad as firestop.
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wrote:

You start getting into the difference between Draft stopping, Fire Blocking and Fire Stopping The short answer; Foam is draft stop, just stops the free flow of air. Wood is fire block, slows the spread of fire products like smoke and fumes and retards the spread of the fire. Fire Stop is a U/L listed assembly using methods like red caulk and pads that is engineered for a particular burn through time. Simply squirting some red caulk in a hole is not necessarily a fire stop. It depends on the application.
Personally I am not a huge fan of the spray in foam. Some of it burns like those 4th of July "snakes" you scorched the sidewalk with.
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snipped-for-privacy@aol.com wrote:

> Didn't know there were that many kinds of stops/blocks.
I wasn't clear in my intent, which was to compare 20" of Romex bundling in wall cavities that are insulated with either fiberglass batts or spray-in-place foam. Would seem like foam in wall cavities would be worse than fire stopping in framing because there is more length to generate heat for the same heat transfer down the wire. If caulk/fire stop in framing with over 2 cables is a problem it should also be a problem with foam in wall cavities, and only 2 cables should go through framing holes, and cables should be substantially spread between holes.
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wrote:

Installers should not be bundling cables in walls under any circumstances. They should be separated and stapled along the studs or hanging on "stackers". I do find one interesting paradox. You don't have to derate an 18" nipple in 312.5(C) ex that is packed with duct seal. Why is duct seal at the end of a nipple less of a problem than caulk in a framing member hole.
But nobody has ever accused NFPA of making sense
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Remember the 50% conduit fill rule and use that as the rule for this aplication...
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JC wrote:

The idea here is not to find some interpretation of the Code that forces you to compromise some part of your installation as the cost of claiming compliance.
There's no point in any structural compromise at all if there's no benefit in electrical safety and I don't hear any suggestions that three 12/2 NM cables in a 3/4" hole in a stud is a safety issue. Or seven cables in two 3/4" holes (properly spaced).
Chuck
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