I am installing new wiring and need to place 5 lighting switches in one location (yes, I understand the problems/issues of confusion, etc.). These will all be 3-way switches, non-dimmable.
I plan on using 2 dual 3-ways (two 3-way switches on a single strap that fit in a one gang box) and one regular 3-way. The switches are UL listed and rated for the load. I would prefer to use a 3 gang box.
My concern is box fill. Each 3-way has 3 wires (not including the ground): two travelers and one common for each circuit. That means 15 wires for switching, plus at least one white out for each circuit (5), one white in and one ground (2). Each single gang switch is treated as two conductors (6 more). That is 28. Assume all conductors are 14 gauge and connectors are external. The code requires 2 inches per conductor. That adds up to 56 cubic inches.
The largest standard 3 gang box I can find is 54 cubic inches. But there are metal masonry boxes that range in the 60s. Is there any reason I shouldn't use one of those (recognizing that I need to properly secure it to the non-masonry studs wherein it will sit)?
Best Answer
Masonry boxes are metal because concrete can get wet and conduct electricity. The box can be grounded to prevent this becoming a problem. Code allows you to use items that are MORE safe than is required. For instance, if code requires 14 gage wire, you're OK using 12 gage (but not 16). The same holds here. You're proposing using a more safe box than is required. As long as it physically fits, you will be OK.