Box Fill Calculator

Calculate junction box size per NEC 314.16(B).

Conductors

Each conductor entering the box counts (except equipment grounding).

Other Items

All internal clamps count as 1× largest conductor
Fixture studs, hickeys (1× each)
Each device = 2× largest conductor
All grounds count as 1× largest

Conductor Volume Allowances

Wire Size Volume (in³)
#14 AWG2.00
#12 AWG2.25
#10 AWG2.50
#8 AWG3.00
#6 AWG5.00

Per NEC 314.16(B), Table 314.16(B)

NEC 314.16 Box Fill Rules

What Counts

  • Each conductor entering the box (hot, neutral, travelers)
  • Conductors looped with no splice still count once if unbroken
  • Pigtails and splices inside box don't add count
  • Internal clamps (all) = 1× largest conductor
  • Support fittings = 1× largest conductor each
  • Devices = 2× largest conductor each
  • Grounding (all) = 1× largest conductor

What Doesn't Count

  • Conductors that pass through without splice or termination (if box has correct KO size)
  • Conductors less than 6" long inside the box
  • External cable clamps
  • Wire nuts, connectors (already factored into conductor allowance)

Important Notes

  • Use the largest conductor size for clamps, supports, devices, and grounding calculations
  • If mixed wire sizes, count each size separately