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I need help with box fill calculations, please. In the first picture, this is a 4” drawn box that measures 2-1/8” deep. Two sets of 12-2-2 (one feed and one load), so a total of 8 conductors. I am asking you to show me how to do the box fill math for the 8 conductors, two standard 20A outlets with two 3/8” Type NM push-in connectors. Grounding: wire-nut the two 12-2-2 grounds together with 3 pigtails (one to left outlet, right outlet, and one to the box screw). The 12-2-2 romex feed line enters on the left side top - Black/White will connect to the top of the left outlet, and Red/Gray will connect to the top of the right outlet. The 12-2-2 romex load line is on the right and will connect to the bottoms of the outlets the same way and will continue to the next box in the string.
4” x 4” x 2-1/4” Box

Type NM Connectors

In the third photo, I will do the same but that is a 4-11/16” box with a 3/4” Mud Ring and I will install GFCI outlets in the larger box. Again, two 12-2-2 romex wires (one line and one load) - the load line is what will feed the following string of 4” boxes that I described above.

4-11/16” Box with 3/4” Mud Ring

I just want to be sure I am good with my box calculations. If I did my math correctly using a calculator, I am 29.75” in a 32” box. Can anyone please confirm this for me and show me how you did the math? I want to ensure I’m good. Thank you!

riffin-rich
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2 Answers2

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We disregard cables and count individual wires.

  • Eight #12 conductors (hots and neutral): Eight #12
  • Grounds are 4 for the price of 1: One #12
  • All cable clamps together count as one count of the largest wire, so total one #12. However these don't seem to invade the box so I'm willing to call them zero.
  • Receptacles count as 2 of the largest wire on them. So four #12
  • Pigtails are free.

Incoming cables MUST ground to the metal box FIRST. Receptacles automagically pick up ground off the box anyway, no need for ground jumpers. But pigtails are free anyway, so they don't affect wire count.

So I get thirteen #12.

#12 costs 2.25 cubic inches. 13 x 2.25 = 29.25 cubic inches.

Check the box stamping for what size box that is.

Same math on the 4-11/16" box, except those are 42 cubic inches, but you'll get some more cubic inches from the mud ring, so you're fine.

The first box is a 2-gang box, not a 4x4 box. 4x4 boxes have screws on 2 corners, and MUST use a mud ring (in drywall) or domed cover (otherwise). As such, you're not in a good place to finish that job.

  • If you plan to finish the wall with drywall, you need to install that box "proud of the studs" by about the thickness of the drywall. You're not allowed to have a box deeply below the drywall unless you're planning to use a box extension sleeve; only 1/4" short is allowed if the wall material is non-flammable like drywall. 0" allowed in wood. Alternately you could swap that for a true corner-screw 4x4 box, and use a mud ring as you're planning with the larger box; this would help with cubic inches actually.
  • If you plan to leave the wall unfinished, you'll need a cover plate there. The corners of it will stick out and tend to snag clothes. You are better off changing that to a 4x4 box and installing a domed cover which has no corners to snag.
Harper - Reinstate Monica
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  • Thanks Harper. I will be covering the walls with 3/4” OSB sheathing and if I did my math well enough, the 2-gang boxes will be 1/4” behind the face of the sheathing with the intent of sucking the wall plate tight against the wall. The sheathing was probably a bad choice at $84/sheet (covid supply issues when I bought them) but I’ll be hanging stuff off of the walls and need the strength. Given it’s a woodworking shop, I won’t need to worry about dings in the drywall either. Thanks again. – riffin-rich Nov 08 '22 at 00:08
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    @riffin-rich "the boxes will be 1/4" behind the face of the OSB sheathing" now that's exactly what I said you *couldn't* do :) See, OSB is flammable so there can't be any gap at all! Given your aspirations to suck the covers down tight, your best bet is box extension sleeves which telescope somewhat. By the way, never aim for 1/4", aim for the least possible. Can't tell you how many ADA ramps need to be torn out because they see 8% in ADA, aim for 8%, get 8.2% as-built and they shoulda looked closer, 8% is condemning. – Harper - Reinstate Monica Nov 08 '22 at 02:53
  • You helped me again, sir! I’ve moved the boxes. I wish I knew about Adjust-a-boxes before I started this project. – riffin-rich Nov 11 '22 at 02:13
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OK, I'm getting 2-12-2-2 NM cables so 8 #12 @2.25 per cable= 18. Two devices 2 @ 2.25 per device = 9.0 2.25 for ground ( all grounds count 1) = 2.25. I get 29.25 cu in. The plastic clamps don't count. The box and mud ring should have the cu in stamped inside.

JACK
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