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I've got my copy of "Electrical Wiring: Residential" 18th ed, complete with all the information, tables, and formulae for choosing electrical box sizes, but I've got to imagine that experienced electricians aren't spending 10 minutes per box calculating the proper size, and they're still meeting code. Are there any industry rules of thumb for looking at a circuit diagram, and quickly choosing the correct box based on what's going in it? I'm mainly interested in straightforward circuits comprised of receptacles and two- and three-way snap switches with standard NM-B cable (14/2, 14/3, 12/2, etc.).

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The tip I got from an electrician was to just buy the big boxes; the cost difference is minor, it's easier to work in bigger boxes, and then you don't have to work about it.

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    The biggest one that will fit, use that one.
    – Mazura
    Feb 12, 2015 at 2:54
  • Sure makes future work easier and a lot more fun.
    – Fiasco Labs
    Feb 12, 2015 at 3:04
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Every 14 ga. wire entering (or exiting) the box needs 2 cubic inches. Every 12 ga. needs 2.25 cubic inches. All grounds together count as 1 wire, as do internal clamps. Every device (such as a switch or a receptacle on a single strap) counts as double the wire size it connects to.

Rough add-up (" standing for cubic inches):

One 14/2 cable in (2x2"), two 14/2 cables out (4x2"), ground wire (1x2"), two single pole switches using 14 ga. wire (2x2") = 18". Virtually any box could handle this.

As more circuits pass through, 3-ways are added in, the numbers add up.

In sum, every 14/2 coming in costs 4", every 12/2, 4.5". Every switch or receptacle counts for the same. You also need to account for ground and clamps.

At some point it becomes somewhat intuitive. It only becomes a problem when you are trying to cram a lot into a little space (like I am in this question)

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  • I was going to mention it's usually not a problem until you try to shove five 3-ways into a triple gang box ;)
    – Mazura
    Feb 12, 2015 at 1:59
  • @Mazura And then you pull out the charts (and your hair).
    – bib
    Feb 12, 2015 at 2:08
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    Are these cubic inches in your calculations?
    – Hank
    Feb 12, 2015 at 2:34
  • @HenryJackson Of course they should be! Thanks, I'll clarify. And you are welcome to edit directly anytime.
    – bib
    Feb 12, 2015 at 13:08
  • This is technically the right answer. However this would not be very quick, would require said electrician to have a very detailed diagram of EXACTLY how he will wire a house, and then he would have to count the wires in each box based on the diagram. If you do this you are truly one step ahead of all the guys I use in my area.
    – DMoore
    Feb 12, 2015 at 15:59

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