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I thought I got the box fill calculations right but there must be an issue, still. Because, assuming my calculations are correct, no single box on the market would be large enough for a simple light switch:

  1. Line: NM 12-2
  2. Load (Light): NM 12-2
  3. Yoke: Normal light switch (smart switch)
  4. Carlon old work plastic box; 14cu and 20cu version

Normal wall boxes are 14cu or even smaller and the 20cu is the extra deep version. Yet, according to my breakdown, not even the largest one would fit:

enter image description here

Until now, it did not even cross my mind this simple configuration would not fit any of the available boxes on the market. The reason why I started the box fill calculations is because I want to add another "load wire" for a receptacle downstream. In this case, my breakdown looks like:

enter image description here

So I'd need 27cu, about TWICE as much as a conventional box.

Where is the issue?

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  • By my understanding, you have one power cable coming into the box, where the hot wire is connected to a switch, and another cable exiting the box, hot wire connected to switch, with the two neutrals nutted together. Presumably you have two grounds and a pigtail nutted together, with the pigtail connected to the switch. That's two #12 spliced the box, and two #12 terminating in the box, with #12 grounds and a 1-gang device. Your minimum box is 15.8 CI. constructionmonkey.com/calculations/misc/boxfill (I'm not sure where the "receptacle" mentioned fits into matters.)
    – Huesmann
    Nov 14, 2023 at 14:06

2 Answers 2

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Clamps add 1 wire count, not one count per clamp. I'm not sure the cheesy excuses for "cable clamps" built in to "blue boxes" even count at all, volume-wise, as they are built in regardless, not added.

Many switches are on 15A circuits with 14Ga wires, and are dead-ended at the switch, so only (modern era) 3 wires + 1 ground.

I like steel boxes, personally, and I use external clamps if not using conduit, so zero clamp count. And it's trivially easy to get 30+ inches when it is needed.

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  • If I reduce the clamps to 0, I am still at 7 allowances (=15.75cu) for the simple light switch (so can't use a regular box) and 9 allowances (=20.25cu) for the light switch+additional wire for receptacle (so can't use any box). Is this right?
    – divB
    Nov 14, 2023 at 0:17
  • Wrt to metal: I prefer them too but I need "old work". Metal needs some stud/wood closeby. It's lath and plaster and I want to minimize the holes I create.
    – divB
    Nov 14, 2023 at 0:18
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    Metal old work boxes exist! Perhaps you should ask a new question about those, or let's see what's already been asked... Tiny boxes have severe limitations, yes, so a 14 CI box isn't much use with many 12 AWG configurations.
    – Ecnerwal
    Nov 14, 2023 at 0:43
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Yes, assuming no clamps, you are correct with your calculation of 7 conductors (2 hot, 2 neutral, 1 for grounds, and 2 for the switch). Adding another outbound wire from the box results in 9 conductors (1 more hot and 1 more neutral). This would seem to be incompatible with the available old work single gang boxes using 12ga wire: 20.25 cu in needed. A 2-gang old work box could be a solution.

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