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Does this meet box fill requirements?

Two, three-gang boxes stacked vertically using a 3/4" x 1-1/2" conduit nipple.

Total of seven cables and 6 devices plus a pair of wires through the nipple.

The seven cables are:

  • One cable feeds power to 4 switches and one outlet spread across the two boxes, via the nipple.
  • Four cables deliver power from the four switches to three lighting circuits and an exhaust fan.
  • The sixth cable brings power from another circuit to a GFCI outlet in the one remaining position, and the seventh takes that load-side GF protected power to some other outlets.

1 Answer 1

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Well, you have to compute box fill individually on a box-by-box basis.

Unfortunately the 2 wires of pass-through must count as 2 wire counts in each box. They're conceptually pigtails, but each box computes separately.

I hope you're using steel boxes so you can reduce your ground wire count by 4 in each box. (let mounting screws and the nipple carry grounds between boxes and to each device).

I strongly recommend to have the "unrelated circuit" wires enter and exit the far end of the box, I'd even suggest a divider in the box to make it more clear to "the next guy" that these neutrals are NOT to be interconnected.

To compute box fill, you calculate

  • the number of hot and neutral wires
  • do not count pigtails which start and end inside the same box (they are free)
  • Ground wires are "4 for the price of 1"
  • 2 wire counts for each "yoke" (switch or receptacle)
  • 1 wire count for all cable clamps.

You multiply this wire count by the cubic inches based on wire size. #14 wires need 2.00 cubic inches. #12 wires need 2.25 cubic inches each.

Then you know how big a box you need. Purchase boxes with that many cubic inches or more.

If you have a mix of wire sizes, count wires as what they are, but count everything else as the largest wire involved.

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  • Various questions deleted about how to count grounds and clamps when there is a mix of #12 and 14 wires and the grounds are a little smaller. I just count everything as 2.25 and I end up in the low to mid 30s in both boxes but a typical 3-gang metal box is in the high 40s. So I don't have to be too precise in my calculations unless I specially buy a shallow box, which I won't. Am I way off?
    – jay613
    May 5, 2022 at 22:23
  • 1
    @jay613 no, sounds right. May 5, 2022 at 23:17

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