Box Fill (314.16) and Conduit Fill (Chapter 9)
Overcrowding a box or a conduit traps heat and makes wire-pulling damage insulation. Two calculations keep things safe: box fill (how much a box must hold) and conduit fill (how many wires fit in a pipe).
1. Box fill — CEC 314.16
You total up "volume allowances" and the box must be at least that big. Each allowance comes from Table 314.16(B), based on conductor size:
| Conductor | Volume allowance |
|---|---|
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
What counts (and how much):
- Each current-carrying conductor entering and terminating/splicing in the box = 1 allowance.
- One or more internal cable clamps = 1 allowance total (based on the largest conductor).
- Each support fitting (fixture stud, hickey) = 1 allowance each.
- Each device yoke/strap (switch, receptacle) = 2 allowances (double), based on the largest conductor connected to that device.
- Equipment grounding conductors: the first four EGCs together = 1 allowance (based on the largest EGC); each EGC beyond four adds ¼ allowance. (For a typical box with ≤4 grounds, that's simply 1 allowance.) (2023 NEC refinement — CEC 314.16(B)(5))