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Old 12-10-2012, 02:09 PM   #1
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Box fill options for 3-way switches

So I'm ordering a whole bunch of junction boxes for the house and running into an issue with how many wires I can fit in a junction box. So I'm doing retro boxes (maybe that's my first problem), and using 12 gauge for most of the circuits, and the wall depth only allows 3.5" which I found at home depot a 14 cu inch box which fits, the 21 cu inch box doesn't (requires 3.75")

With the 14 cu inch box that basically lets me have a 12-2 coming in, a 12-2 going out, but not another 12-3 for my 3-way switches. I calculated each 12 gauge conductor as 2.25 and assuming 1 switch in a 1 gang box, another 2 cu inch. Basically it looks like I need at least a 16 cu inch box to do a 3 way switch? I had planned on running a seperate 12-3 for the 3 way lights, but not sure if there is another way I can finagle the wiring?

Also are these retro boxes with the tabs junk? Nothing worse than plugging in an outlet to have the box move on you. But the alternative of tearing out enough space to physically attach a new junction box to the stud is not appealing.

*edit* Even if I switched to 14 gauge, I'm still over the limit in a 14 cu inch box.

I used this for the calculations:


Last edited by Ocelaris; 12-10-2012 at 02:13 PM.
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Old 12-10-2012, 02:40 PM   #2
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Use a 2 gang box and you will be fine.
Since you can use a deeper box, this may be the only way to go.
You can then use a blank and a switch 2 gang plate.
The switch or receptacle counts as 2 for the size wire you are using.
So a 3 way switch with #12 will count as 2 wires.


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Old 12-11-2012, 07:45 AM   #3
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I think you left out the 1/2 inch of drywall in your 3.5" measurement, unless you've found dimensional lumber that is 3" wide.

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Old 12-11-2012, 09:48 AM   #4
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Your're exactly right Mark, I didn't count the drywall, it's 3.5" from the back of the wall to the drywall, but 4.25" to the front of the drywall, so I was making a big deal out of nothing! In New York/New Jersey a lot of houses from the 50s-60s have this drywall/plaster combo stuff so it's almost 3/4" thick, real pain but not as bad as plaster/lathe. I bought a bunch an 18 and 21 cu inch retro box and will test fit those in my low voltage cutouts just for fitment, but I think I'm fine. I'm still worried about the holding power of those tabs, but it would be a ton more work to put new work boxes in each of the 50 new receptacle locations! Thanks!
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