Box fill is 8th-grade math: every conductor gets a volume allowance by wire size, add device and ground allowances, and the total can't exceed the box's cubic inches. Metal boxes are stamped; plastic uses Table 314.16(A).
| Conductor size (AWG) | Volume each (cu in) |
|---|---|
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
Each conductor that enters and stays or is spliced = 1; a conductor passing straight through = 1; a loop ≥ twice the free-length = 2. A pigtail that never leaves the box counts nothing.
Each yoke/strap (device) = a double allowance of the largest conductor on it — a device is a brick.
All grounds together = one largest-conductor allowance; internal cable clamps = one more. Freebies and fractions.
Multiply, add, compare to the box. 12-2 in + 12-2 out + one receptacle = 7 counts × 2.25 = 15.75 cu in — an 18 cu in box passes; smaller fails.
12/2 NM, metal device box + mud ring (marked cu in). Any hand tool = Klein Tools.