The five things that take up volume

314.16(B) counts more than wire. In order:

  • Each conductor that enters the box and does not leave gets one allowance. A conductor that runs through without being spliced also gets one. The size of the allowance comes from Table 314.16(B), and it is the conductor's own size.
  • All internal cable clamps together get one allowance, sized on the largest conductor in the box. Not one per clamp. One, total.
  • Each fixture stud or hickey gets one allowance, again on the largest conductor.
  • Each yoke or strap holding a device gets two allowances on the largest conductor connected to that device. Two, for the strap, not for the conductors landing on it. A duplex receptacle on one strap is two allowances, not four.
  • All equipment grounding conductors together get one allowance, sized on the largest EGC. Four grounds in a box is still one allowance.

Pigtails are free

This is the one worth knowing. A conductor that originates inside the box and terminates inside the box counts for nothing. Pigtail a receptacle off a splice and the pigtail is not in the calculation.

That is sometimes the difference between a box that passes and a box that does not, and it also happens to be the better install, because a pigtailed receptacle can be pulled out without opening the circuit downstream.

There is no percentage here

Conduit fill has a percentage. Box fill does not. You add up the allowances and the total either fits inside the box volume or it does not. A box at 99 percent is compliant and a box at 101 percent is a violation.

And the volume marked on the box beats the table every time. Table 314.16(A) covers standard metal boxes. Plastic boxes, mud rings, extension rings and anything with a durable volume marking use the marked number, and listed extension rings add their marked volume to the box.

Where this actually bites

Three-gang switch boxes on a 12 AWG circuit, and the four-square in the basement where somebody has landed six cables because it was convenient. On a three-gang with three switches, three travelers and a ground you are carrying six volume allowances for the straps alone before you count a single conductor.

The fix is almost always cheaper before drywall. A deeper box or an extension ring is a few dollars and five minutes at rough-in. After the fact it is a trip, a patch, and a conversation about why the inspection failed.