How many wires in a hole in a wall stud?

[JC]

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]

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

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.

 

[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

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.

 

[ehsjr]

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.

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

 

[bud--]

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

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).

 

[[email protected]]

| 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.

 

[krw]

[email protected]>,
alt.engineering.electrical, [email protected] says...

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?

 

[[email protected]]

| [email protected] 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.

 

[[email protected]]

| In article <37adfe4a-b085-4dd3-a291-
| [email protected]>,
| alt.engineering.electrical, [email protected] 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.

 

[krw]

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.

Irrelevant. The point was that you have to derate if you *ADDED*
fire stopping.

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.

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.

 

[krw]

| In article <37adfe4a-b085-4dd3-a291-
| [email protected]>,
| alt.engineering.electrical, [email protected] 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.

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

 

[krw]

In the NEC, derating of bundled
conductors is required, with some
exceptions. With fire stopping in hollow
spaces, some of those exceptions
disappear and you must derate.

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.

The requirements on conductors
penetrating walls, studs, floors, etc.
with fire stopping all but dictate that
the conductors pass through metal conduit.

What does "all but" mean?

If you think a bundle of conductors in
free air will dissipate heat as well as
the same bundle encased in fiberglass or
foam, then it is understandable you find
the explanation wanting.

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?).

 

[bud--]

| [email protected] 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).

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)

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.

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.

 

[[email protected]]

| In article <[email protected]>,
| alt.engineering.electrical, [email protected] says...
|> | In article <37adfe4a-b085-4dd3-a291-
|> | [email protected]>,
|> | alt.engineering.electrical, [email protected] 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).

 

[bud--]

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.

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.

 

[krw]

Keith, I think what's confusing you is
that the NEC simply doesn't address
derating conductors passing through a
hole in a 2x4. Anywhere. Period.

Unless that 2x4 is a fire block. Silly.

The NEC does talk about derating bundled
conductors. Multiple conductors in a
hole in a stud is a bundle and therefore
would have to be derated.

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.

But an exception in the NEC says a
bundle doesn't have to be derated if it
is less than 24 inches in length.

Unless it's a fire block, then it must be derated.

Another exception applies to nipples
less than 24 inches in length (think
conduit).

Well, a nipple less than 24" *is* less than 24", so I guess that
works.

But if the bundle goes through fire
stopping, the exceptions don't apply and
you do have to derate.

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.

Again, the 2x4 itself never enters into
any discussion of derating a bundle of
conductors in the NEC. It only addresses
bundling, derating, and fire stopping.

It *certainly* does! Fireblocks *are* made out of 2x4s.

Does that help?

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.

 

[krw]

| In article <[email protected]>,
| alt.engineering.electrical, [email protected] says...
|> | In article <37adfe4a-b085-4dd3-a291-
|> | [email protected]>,
|> | alt.engineering.electrical, [email protected] 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 only limits the radial dissipation for 3/4" of the wire, I.e.
not much.

| 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).

If you have a constant load... Well, you finish the sentence.

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).

Nonsense.

 

[bud--]

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.

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.

 

[bud--]

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.

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.