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
Reply to
JC
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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
Reply to
Chuck
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.
Reply to
JC
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
Reply to
Chuck
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.
Reply to
JC
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
Reply to
Dave22
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
Reply to
ehsjr
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
Reply to
Chuck
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.
Reply to
Paul Hovnanian P.E.
I also don't see a big problem in just drilling another hole (as you say, properly spaced). The strength of the studs wouldn't be seriously compromised unless you got rediculous about it.
daestrom
Reply to
daestrom
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).
Reply to
bud--
| 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.
Reply to
phil-news-nospam
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.
Reply to
Paul Hovnanian P.E.
There are no rules on this in the 2005 NEC. As long as you keep the cables separated and not bundled and do not apply fire stopping you can put as many cables through a hole in a stud and will fit. If you apply fire stopping you must derate according to the 2008 NEC.
Reply to
Gerald Newton
There seem to be two issues here.
One is that penetrations (holes) in vertical and/or horizontal structural members can provide detrimental airflow and reduce the fire stopping effectiveness of the structure.
Another is that heat buildup in conductors cannot dissipate when surrounded by some fire stopping materials (foam, fiberglass, etc.) and so derating may be appropriate.
Chuck
Reply to
Chuck
|> |> |> | 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.
Reply to
phil-news-nospam
| |> |> > 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 |> > |> > -- |> > JC from Gnat Flats |> |> There are no rules on this in the 2005 NEC. As long as you keep the |> cables separated and not bundled and do not apply fire stopping you |> can put as many cables through a hole in a stud and will fit. |> If you apply fire stopping you must derate according to the 2008 NEC. | | Doesn't that seem sorta silly?
No. A simple short (2 inches) hole does not tend to impact the ability of the wires to carry heat away and dissipate it into air a few inches away. OTOH, a fire stop would be more constraining on the wires and could reduce the heat dissipation.
Reply to
phil-news-nospam
Irrelevant. The point was that you have to derate if you *ADDED* fire stopping.
Bullshit. We're talking about a 2x4. The center of the thing is only 3/4" from free air and the heat conductor is copper (or aluminum).
I'm not buying either reason.
Reply to
krw

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