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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)?

  • 1
    Since you're open to changing the box out, can you just cut out the wall to put in a wider, 5 gang box? That seems a little less confusing for the user than having a mix of dual and single switches. Even with the slightly larger 60 cu inch box, it sounds like it's going to be a little tight inside the box. – Johnny Feb 10 '15 at 23:33
  • @Johnny Five gang is uber intrusive! I HATE anything above 2 gang, but sometimes we have no choice. This is a home, not a nuclear reactor facility. – bib Feb 10 '15 at 23:58
  • 5 gang is an electrical orgy. You might think it is cool at first but you can only look at it so long without being disgusted. – DMoore Feb 11 '15 at 1:08
  • 4 is pushing it... If I had to do a 5 gang I'd be looking into digitally controlled switches. – Mazura Feb 11 '15 at 1:17
  • I think 5 switches in one box is too many, having them separated into a wide 5 gang box doesn't seem much worse. – Johnny Feb 11 '15 at 1:38
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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.

  • it needs to be mounted in accordance with its UL listing, otherwise it violates 110.3 – ThreePhaseEel Mar 20 '15 at 2:19
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There is no problem using the metal boxes, it is done all the time.

All of the ground wires count as only 1 wire for box fill calculations

I will Never be caught saying this it the best idea - 3 gang plastic 1.5" extender, UPC 018997489678

3 gang mud ring for the metal box http://www.homedepot.com/p/Raco-3-Gang-Raised-3-4-in-Mud-Ring-For-RACO-942-and-952-Gang-Box-822/100539345

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This will work as long as you can mount the box securely, and are not violating the box's listing.

As Some Guy pointed out, the whole lot of EGCs (ground wires) only counts as 1 wire for box fill purposes.

And yes, an extension ring on a stud-type (old work or new work) box can work as well, if you don't mind the resulting box sticking out from the wall.

  • Thoughts that come into my head sometimes. Is there a limit on the number of extension rings, that can be used, say a 1900 box, final opening flush to the wall, because the extension boxes hold together and do not make the hole used any bigger, or not flush and just 3 sides against a corner. I know I can just google it or pull out the code book, but I was also wondering what evil things you have come across. I guess the real limit is the sense of humor of the AHJ – Some Guy Mar 18 '15 at 15:15
  • @SomeGuy -- see diy.stackexchange.com/a/58723/27099 – ThreePhaseEel Mar 18 '15 at 22:29

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