Junction Box Sizing Calculator
Calculate minimum junction box volume per NEC 314.16(B) based on conductors, devices, and clamps.
Formula
Total in³ = Σ(n_conductors × vol/conductor) + devices × 2 × largest + clamp × largest (NEC 314.16B)
- NEC volumes per conductor: #14=2.0, #12=2.25, #10=2.5, #8=3.0 in³.
- Device fill (receptacle/switch): each device counts as 2× the largest wire in the box.
- Cable clamp fill (internal clamps): 1× largest wire volume, regardless of clamp count.
- Ground wires: all grounds together count as 1 conductor of the largest ground size.
4×#14, 2×#12, 1 outlet, 1 clamp
Inputs
- #14 AWG Conductors: 4 wires
- #12 AWG Conductors: 2 wires
- #10 AWG Conductors: 0 wires
- #8 AWG Conductors: 0 wires
- Devices (outlets, switches): 1 devices
- Internal Cable Clamps: 1 clamps
Conductors: 4×2.0 + 2×2.25 = 12.5 in³. Device: 2×2.25 = 4.5 in³. Clamp: 2.25 in³. Total: 19.25 in³.
Frequently asked questions
What size box provides ~20 in³?
A standard 4×4×1.5 in square box provides 21 in³. A 4×4×2.125 in deep box provides ~30.3 in³. Always select a box that meets or exceeds the calculated volume.