running wires?

[Don Schwartz]

Can I drill one hole in a 2"x4" stud wall, to run several 12-2 wires?

[Jim Port]

As long as the cables are not damaged. Too many and derating will need to be considered, but 2 cables thru a 3/4 is fine.

[Frank P. Newman]

There are other considerations as well, such as making certiain that the holes are drilled as close to the center of the stud as possible, or are properly protected from damage.

[Paul W. Abernathy]

Along with what Jim has stated.....You could actually run (3) 12-2 and (1) 12-3 thru a stud and even after deratings be fine. Now I am sure someone can explain why this is ok...but if they can't I will be glad to do so.

hint...90 Degree column is allowed -334.80

[Rick Marton]

Paul, this is a big one to cause many arguments. Not counting the neutral conductor in the derate calculations in 310.15(B)(2)A is going to cause a lot of controversy. When have we ever not counted the neutral as a current carrying conductor? I believe this is for three wire circuits only because it mentions the unbalanced load in that article.Rick

[Jim Port]

Rick,

I am not sure where you think Paul said the neutral was not a CCC. In his example above;

3 x 2 = 6, + 3 = 9 CCC. The ampacity of #12 is 30 from the 90 degree column. 30 x .7 = 21 amps.

[Rick Marton]

Jim, I think that the 3 wire or 12/3 the neutral is not suppose to be counted??. look at 310.15 (4) Let me know what you think. Its a little tricky. Thanks Rick

[Robert Meier]

Derating is a rather complicated issue. For the questions proposed here there are a few different parameters involved. One being the number of conductors through a bored hole and how that is related to derating. The other involves when and if the neutral conductor is counted as a current carrying conductor (CCC). Starting with the latter the neutral conductor in a multi-wire branch circuit (MWBC) is sometimes counted as a CCC and at other times it is not. For this question regarding NM cable in a dwelling, where the system is almost always 120/240 volts, the neutral conductor in a 3 wire MWBC is not considered a CCC. Here's the rule from the 2008 NEC:

310.15(B)(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

For a MWBC on a 120/240 volt system 310.15(B)(4)(a) would apply since the neutral conductor carries only the unbalanced current from the other conductors and is not required to be counting when calculating derating.

The second issue in this thread is regarding cables through a bored hole in wood. Where the bored hole does not contain fire or draft stopping materials multiple cables through a bored hole do not require any derating. Derating would apply to cables that are bundled together for more than 24" but it would not apply to cables installed through a single hole.

When derating NM cables in a hole that contains fire or draft stopping material the cables can be derated by using their 90° C ampacity in table 310.16. For general use with #14 AWG and #12 AWG cables that means that up to 9 CCC's can be installed in that fire or draft stopped hole before derating would have any affect on the cables derated ampacity. That leaves us with the possibility of installing 4-14/3 or 4-12/3 wire cables (8 CCC's) in that hole before derating would actually lower the ampacity level of those conductors below the typical [240.4(D)] 15 amps for #14AWG and 20 amps for #12 AWG cables.

One last caveat in derating is 310.15(A)(2)Exception which tells us that we can disregard the lower adjusted ampacity of the derated cable if it meets the so called 10' or 10% rule but we can leave that for another day.