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# How Many Wires through joists?

#1
06-09-17, 04:27 PM
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Join Date: Dec 2011
Location: USA
Posts: 69
How Many Wires through joists?

I'm reading up and getting myself confused on what I can do to meet code.

In wiring recessed lighting in a basement project, I have setup 3 'sets' of lights (blocks of 4) on 3 way light switches, with the wiring all going to the bottom of my stairs and the other side, far end of the basement.

Is it ok to run the wiring for all 3 light switches to the lights through 1 hole in each joist?

Wiring is on a 15 amp breaker with 14/3 wire for the 3 way switches. I'm not sure if can squeeze 3 14/3 lines through a 3/4 hole, or would need to drill 1 inch.

The ceiling will eventually be sheet rocked over as well.

#2
06-09-17, 05:07 PM
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Location: Twin Cities, MN
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You didn't say what kind of cable you are using (Romex or MC) but I would just drill 2 3/4" holes or 1 - 1". 2 holes might be handy for that cable you might add later. (before the rock)

#3
06-09-17, 06:52 PM
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Join Date: Dec 2011
Location: USA
Posts: 69
Sorry about that. It's Romex 14/3 wire for the 3 way switches out to the lights.

But based on your suggestion, I'll probably just drill 2 holes through the joists, and run 2 cables through 1 of them, which will then leave some extra room in the other like you mentioned. It really doesn't take all of an extra 5 minutes to drill the extra holes. Just a bunch more dust and clean up.

But in meeting code. From what I have read, I should be ok including 2 or even 3 cables of 14/3 through 1 hole? It got confusing when reading about 'derating'.

#4
06-10-17, 04:44 AM
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Derating applies when 3 or more current carrying conductors are in a tight space such as conduit or drilled holes through joists. While the derating starts at 3 conductors it really doesn't have a "real" affect until you reach 10 conductors because the wires installed in Romex cable are THHN. #14 THHN wires are rated 25 amps at 90 degrees C. At 7-9 wires you have to derate 70% which is 17.5 amps. However, Romex cable is limited to 60 degrees C which is 15 amps. When you reach 10 wires you have to derate 50% which is 12.2 amps. Now you are lower then the 15 amp rating of the breaker. Make sense?

In your case, you are running cables for 3 ways which include a neutral or hot, two travelers, and a ground wire. The ground doesn't count because it only carries current during a fault and only one traveler will be carrying current at a time. So, really you have a total of 6 current carrying conductors in your group of 3 cables.

I hope that helps.

Last edited by Tolyn Ironhand; 06-10-17 at 02:45 PM.
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