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
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.
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.
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
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
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
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.
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
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).
| 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.
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.
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.
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
|>
|> |> | 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.
| |> |> > 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.
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.
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