Working on a project where I am planning a 3-gang switch box. The box will be a plastic carlton super blue (53 in³). My fill-math works out fine, but I'm wondering about the practical reality of such a crowded box.

Box will contain:

  • 14/2 inbound power (4 in³)
  • 14/2 outbound switched-power to fixtures (4 in³)
  • 14/2 outbound switched-power to fixtures (4 in³)
  • 12/3 outbound to another switch box (6.75 in³)
  • 12/3 inbound from another switch box (6.75 in³)
  • 2 dimmer switches 14AWG (8 in³)
  • 1 4 way switch 12AWG (9 in³)

Total: 42.5 in³

If I did the math right - it seems to add up - but that's a lot of stuff - plus it seem odd to me that a dimmer that takes up a lot of volume is the same fill as a basic pole switch.

Am I going to have a hard time getting that all in and should I consider breaking up the switch boxes.

Note - that the 12 AWG is putting a 4 way into an existing branch that was wired with 12 AWG. 14 AWG is on a new circuit.

  • 5
    Dimmers and GFCI's (and timers, and...) are bigger than "usual" but the box fill allows for them - but being anywhere near max fill tends to be tight. It's often worth a few bucks to use a larger-than-minimum box for your own sanity. You don't appear to have counted your grounds. They won't put you over max fill, though. (And the usual opinion that steel boxes are better in every way...)
    – Ecnerwal
    May 18 at 10:30
  • 3
    I just did my first (very small) wiring project using Wago lever-lock connectors. Not only were they so simple that they were a joy to use, but they're physically smaller than a wire nut and will buy you back some space in the box. They won't impact the fill calculation (any more than your dimmers do), but they help with the reality.
    – FreeMan
    May 18 at 12:50
  • 2
    Neatness counts for a lot in cases like this. For example, running a tidy backbone of grounds that stays back and low will help. May 18 at 13:19
  • @Ecnerwal - the deep Carlton was the only 3 gang I could find with 53in capacity. Can I match that capacity in metal with some combination of box plus extension rings? I couldn’t find metal boxes that deep at my LHS. Correct on missing grounds - will add that in. May 18 at 20:46

1 Answer 1


Dimmers do tend to be bigger than simple single pole switches. But if the fill calc is correct then it should be manageable— 53 cu in is a lot.

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