I used a box to splice wires when moving conduit during basement renovations. I'm in Chicagoland area and use 1/2" EMT. I spliced 4 circuits back together in a 4 x 4 x 2-1/8 box with cover. The circuits are 2 15A and 2 20A circuits, which are 4 12 gauge and 4 14 gauge wires.

My box fill calculations are: No continuous wires, so there are 8 12 gauge and 8 14 gauge wires entering box. 8 x 2.25 + 8 x 2 = 34 cu. in. The box capacity is 30.3 cu. in. with flat cover plate

Is this correct? If so, can this be remedied with a 7.3 cu in mud ring and blank plastic cover plate?

2 Answers 2


The mud ring sounds like it would work as long as the cu inch is listed, but now you're stuck using a plastic cover. You could use a box extender with a regular 4x4 cover -

box extender

The one above is a Garvin BEX-450, a 1/2" extender for a 4" box, but there are many many variants available.


Yes, you can use a mud ring and blank steel cover. They make those, even if your local purveyor does not have them.

If a receptacle might be useful there, you can use a 1-gang domed mud ring intended for a receptacle, and fit one. You'll pay the cubic-inch cost of a yoke: 2 wires (4.5 ci).

They also make extension boxes in a variety of styles, my stock in trade is one that simply gives you another 1.5" of 4x4 box. I use them all the time, as they have the same cubic inches as a straight 4x4x1.5" box - easy to remember.

Although when I expect to do that many splices, I aim for 4-11/16 (aka 120mm) square boxes, typically 2-1/8" deep by default for 42 ci, though they also make deeper ones. These are much cheaper at real electrical supply houses than the big-box stores (which is ironic, since it is a big box).

  • Thanks, this solution would be cheaper and quicker, however, the 1/2" solution works better for me in this case as it's lower profile.
    – Mark_B
    Oct 30, 2018 at 19:38

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