I'm splitting a room by adding a wall into it, and I would like to put receptacles into the new wall. I'm trying to do this making as few holes in the existing walls as possible, so I'm wondering if the following layout is safe.

layout for electrical showing existing and new setup

The main change is replacing a single gang rheostat switch with a junction box like this, covered by a wall plate like this that would split the source into three cables going downstream. The junction box will be behind where a door opens, which is why I need to move the switch, and I'm not too concerned about the look of it.

The combination switch would be single gang as well since it is going into a narrow space (an area of wall between a door frame and a chimney).

I have done some reading, but I'm new to electrical work and just wanted to make sure this basic idea is safe / see if there's a more sensible way to do it.

I plan on using 12/2 Romex with ground cable, which I think is what is used in the rest of the circuit. All walls are drywall.

  • How many wires go up to the ceiling fan/light? Aug 20, 2020 at 4:28
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    Each wire and device has a specific cubic inch requirement. I suggest you use an online calculator so you can know how alterations can change the required size. constructionmonkey.com/calculations/electrical/boxfill Aug 20, 2020 at 5:11
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    If the WHOLE circuit (not just the parts you have altered) is 12Ga you could consider going to a 20A breaker. If there is ANY 14Ga it must remain 15A, and your box fill would be easier to manage if your new wires were also 14Ga (2.0 cubic inches for each 14Ga wire "count" .vs. 2.25 cubic inches for each 12Ga wire "count") - it adds up.
    – Ecnerwal
    Aug 20, 2020 at 11:44

1 Answer 1


box fill for 12 awg wire has a multiplier of 2.25. the ground wire is only counted once so with 3 cables plus the feed 1x2.25 for the ground 8x 2.25 for the 4 hot and 4 neutral. There is no switch or device in this box so we could skip that but if there was 4.5 adder would be needed because 2x of the largest wire connecting to it. The integrated clamps in that type of box do not count but if internal clamps in the box that would be a 2.25 adder.

so right now with 9x 2.25 The required volume would be 20.25 cubic inches

The box you listed is a little undersized. When I have this many cables I usually go to a 4 square box they do make 4 square old work boxes but those usually require clamps and a 2 gang face plate because a mud ring (single gang adapter) kinda messes up the old work idea. A 4 square x 2-1/8 deep would provide 30.3 cu inch if you wanted to put the device in that box it would be large enough for that and the clamps.

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