Hole size to pass 3 x 12/2 romex through top plate?


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Old 11-25-20, 09:00 AM
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Hole size to pass 3 x 12/2 romex through top plate?

Looking to drop 3 x 12/2 romex down from attic through a top plate. 1 cable will be bringing power down to two light switches. 2 cables back up to two sets of LED lights. I'll have to foam this hole, so there won't be air flow around the romex for the thickness of the top plate (will either be 1.5 or 3"). It will be a straight drop down to the switch box, so I don't have to worry about pulling through turns.

What's the hole size I should make?

I know there is an ampacity adjustment table that applies, table 310.15(B)(3)(a), but since this is for LED lights connected by 12/2 romex, this isn't a problem.

Not sure if there is a NEC spec for hole size in this situation.
 
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Old 11-25-20, 09:16 AM
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On a related note, I'll be picking up power from a junction box ~20ft away. So I'd run romex from that j-box to the hole in top plate and down to switch box. Then have 2 romex run from switch box back up through top plate and each runs to a set of LED lights.

I could run it just as described. Or I could put a j-box near that top plate and splice all 3 cables. Any reason to install a j-box other than for future proofing (i.e., if I wanted to add lights or outlets in future that would tap this j-box)?
 
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Old 11-25-20, 09:20 AM
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I'd use a 3/4" bit and drill two holes. One for your 120 VAC wire and the other for your two low voltage wires.

If you want to be code compliant use fire resistant foam or caulk to seal the holes after you've run your wires.
 
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Old 11-25-20, 09:33 AM
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if I wanted to add lights or outlets in future
No need to add a J-box now. Just leave enough of a cable loop in the attic to add one later if needed.

I think PD is assuming your switch legs are not line voltage. If they are then it shouldn't be necessary to separate them from the power cable except for hole size.
 
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Old 11-25-20, 10:27 AM
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The LED lights use line voltage, not low voltage, so I'm using 12/2 to feed the lights. 7/8" hole would probably physically work for all 3, just not sure if anything in NEC requires going larger. And as mentioned in the OP, I'll be foam sealing the hole afterward.
 

Last edited by cartman; 11-25-20 at 10:54 AM.
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Old 11-25-20, 04:32 PM
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7/8" for three #12 is a good choice. I typically use 1".

Keep in mind...... you don't have to use #12 wiring unless you are connecting to an existing #12 circuit.
I don't put any of my light wiring on #12...... it's all on #14. Much easier to work with and doesn't fill up the junction boxes. Especially easier to work with in some of those tiny LED junction/driver boxes.
 
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Old 11-25-20, 06:18 PM
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7/8" for three #12 is a good choice. I typically use 1".

Keep in mind...... you don't have to use #12 wiring unless you are connecting to an existing #12 circuit.
I don't put any of my light wiring on #12...... it's all on #14. Much easier to work with and doesn't fill up the junction boxes. Especially easier to work with in some of those tiny LED junction/driver boxes.
Thanks.

I'd use 14/2, but I happen to have plenty of 12/2 on hand and no 14/2. I've used 12/2 in those LED light boxes before and yeah, it's a bit of a hassle, but as a DIY'er, if it takes a few minutes longer for each box, it's not a big deal. Obviously for a pro, that would be a very good reason to use 14/2. As a potential side benefit, this is probably the only circuit in the attic that I can access all parts of (since it gets to the attic from outside the house), and some parts of it are already 12/2, so if I wanted/needed down the road, I could upgrade the rest to 12/2 and put it on a 20A breaker.
 
 

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