Is a 12-2 NM cable counted as "3" (THREE) wires/cables/conductors since it has 3 wires in it or "1" (ONE) wires/cables/conductors?


According to 370-16 with this 3 X 3.5 X 2 box, I can put "8" (EIGHT) wires in the box (actually it's stamped 20.3 in^3), whatever a wire is defined as.

But, as pictured below, I have 9 (as per NEC I'm not counting the pigtails as they never leave the box). The outlet adds "2" (TWO), and how many do I add for each twist-cap?

Seems to me even the biggest box I can buy won't accommodate a simple 3-NM connection.

The picture below is just something I slapped together in the shop so I could visualize, IT IS NOT INTENDED FOR ACTUAL INSTALLATION! As you can see, I stripped all of the NM.

2 Answers 2


Each current carrying conductor (hot, neutral) that enters the box, counts as one (1) box fill unit. All the grounding conductors together, count as one (1) box fill unit. Pigtails do not count. The device counts as two (2) box fill units.

Total all the box fill units, and multiply by the multiplier listed for the conductor size.

Three 12/2 with ground is seven (7) box fill units, plus two (2) box fill units for the device. That's 20.25 cu.in.

  • And, for our Canadian friends, things get even more complicated. The device counts for nothing and the grounds count for nothing, but the number of caps do count, so pigtails matter. There are various combinations of conductors (of specific size) and caps, so if you're filling the box with #12 wire it counts for more fill than for #14 wire, etc. Each box size will have a fill limit dictated by the number of wires, the size of the wires, and the total number of caps. If you're not meeting code, however, you'll probably have a really hard time getting everything in the box at all, so...
    – J...
    Nov 7, 2016 at 1:10

Also to add to that, 2020 NEC 314.16(B)(5) states that each additional equipment grounding conductor over 4 counts as .25 in the box fill calculation.

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