# How many wires in a hole in a wall stud?

Discussion in 'Electrical Engineering' started by JC, Dec 26, 2007.

1. ### JCGuest

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

TIA

2. ### JCGuest

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.

3. ### JCGuest

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.

4. ### ehsjrGuest

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

5. ### bud--Guest

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).

6. ### Guest

| 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.

7. ### krwGuest

>,
alt.engineering.electrical, says...
Doesn't that seem sorta silly?

8. ### Guest

| 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.

9. ### Guest

| >,
| alt.engineering.electrical, says...
|> > 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.

10. ### krwGuest

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).

11. ### krwGuest

Maybe I misunderstand incorrectly what a "fire stop" is here. I
thought it was stuff to plug the holes so no air could move through
the hole in the 2x. With the hole filled, the center of the wire
is still only 3/4" from free air on either side of the 2x.

It seems silly to derate the wire, since that means more holes in
the fire stop.

12. ### krwGuest

Which is what I'm saying is silly. If you plug a hole that a
bundle of wires through you must derate, but if you don't plug it
you don't? You *must* drill *more holes in the fire block?
*Damned* silly.
What does "all but" mean?
Through a 2x? No, I don't find the explanation wanting, I find is
damned stupid! There has to be something more to the rule than
you're saying (or know?).

13. ### bud--Guest

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.

14. ### Guest

| In article <>,
| alt.engineering.electrical, says...
|> | >,
|> | alt.engineering.electrical, says...
|> |> > 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.
|
| Maybe I misunderstand incorrectly what a "fire stop" is here. I
| thought it was stuff to plug the holes so no air could move through
| the hole in the 2x. With the hole filled, the center of the wire
| is still only 3/4" from free air on either side of the 2x.

Without the fire stop, the ability to dissipate is my multiple means.
It can dissipate _some_ within the hole, and _some_ through the wires
to the free air away from the hole. The fire stop would have the
effect of reducing or eliminating _one_ of these means of heat escape.
That is, afterall, its design purpose.

| It seems silly to derate the wire, since that means more holes in
| the fire stop.

More holes would be if you reduce the bundling rate to _avoid_ derating
or as much derating. Otherwise it means using _larger_ wire that is
treated as rated less. The latter might have to be the way to do it if
the number of holes is an issue (as building construction code or safe
engineering may dictate).

I think the derating could be more flexible, such as a variant rating
factor for the length of the run, and the heat transfer capability of
the containing material. But this can also get complex and would need
to be one of those "under engineering supervision" things (where a PE
puts his license on the line).

15. ### bud--Guest

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

16. ### krwGuest

Unless that 2x4 is a fire block. Silly.
See the rest of this discussion. Apparently there is an exception
for runs under 24" that doesn't include fire blocks. At some point
this cases *more* holes in the fire block. Damned silly.
Unless it's a fire block, then it must be derated.
Well, a nipple less than 24" *is* less than 24", so I guess that
works.
So if you come up against that situation you have to drill another
hole. That is the silliest thing I've heard in a long time.
It *certainly* does! Fireblocks *are* made out of 2x4s.
Helps clarify that I'm *NOT* missing anything and that the NEC is
damned silly! ...and I thought most things in there made sense, if
you looked at it from *some* angle.

17. ### krwGuest

It only limits the radial dissipation for 3/4" of the wire, I.e.
not much.
If you have a constant load... Well, you finish the sentence.
Nonsense.

18. ### bud--Guest

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.

19. ### bud--Guest

I thought someone in this thread went back to the source for derating
for over 2 Romexes in firestop. It was based on "experimentation" that
found "temperatures well in excess of their 90 C ratings". Results
apparently not published, so the applicability to draft stopping not in
the hole, as described by gfretwell, is not know by mere mortals like
us. Also not known if the experimentation was reasonable. The
requirement could make sense from some angle and is still be silly.

As gfretwell pointed out, the effective limitation is "9 current
carrying conductors per hole" since you derate from the 90 C ampacity,
but you can only use Romex at a 60 C ampacity.