Box Fill Calculator — NEC 314.16
Free · works offline · no account · NEC 2023 tables
Count it the way 314.16(B) does: every conductor, one allowance for internal clamps, two per device yoke, and one for all the grounds together. OhmBase totals the cubic inches and recommends a standard box that fits.
Open the free calculator →How the math works
Each conductor counts once at its Table 314.16(B) volume (2.25 in³ for 12 AWG, 2.00 for 14 AWG…). Internal cable clamps add one allowance at the largest conductor size. Each strap/yoke with a device adds two allowances at the largest conductor connected to it. All equipment grounding conductors together add one allowance at the largest EGC. Compare the total against the box’s marked volume.
Worked example
12 AWG circuit · 5 conductors · 1 clamp allowance · 1 device · grounds present
- Conductors: 5 × 2.25 = 11.25 in³
- Clamp 2.25 + device 4.50 + grounds 2.25 = 9.00 in³
- Total 20.25 in³ → use a 21.0 in³ box or larger.
FAQ
- Do pigtails count as conductors?
- No — conductors that originate and terminate inside the box (pigtails, equipment bonding jumpers) are not counted per 314.16(B)(1).
- How do mixed wire sizes work?
- Each conductor counts at its own gauge’s volume; clamps and devices count at the largest conductor they touch; the ground allowance uses the largest EGC in the box.
- What about pull boxes for larger raceways?
- Boxes with conductors 4 AWG and larger are sized by 314.28 (8× trade size for straight pulls, 6× plus extras for angle pulls) — OhmBase has a separate tab for that.