Box Fill
Cubic-inch fill by box volume and conductor count.
Example
You enter
- Box volume (in³) 22.5
- Conductors by size {"12":6}
- Device count 1
- Internal clamps true
- Largest AWG for clamp and device 12
You get
- Fill 20.25
- Free (in³) 2.25
- Result Pass
Checked against published worked examples from: NFPA.
Details, formula, and sources
Formula or tableSum of conductor, device-yoke, clamp, and grounding allowances per NEC 314.16(B), volumes from 314.16(B)(1) keyed to the largest conductor in the box.
Source and editionNEC 2023 (NFPA 70). Section 314.16.
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesbox_volume_in3, conductors_by_size, devices, internal_clamps, largest_awg_for_clamp_and_device, fill_in3, free_in3, pass
- Volume allowance per conductor from NEC Table 314.16(B), keyed to the largest conductor enteringNEC 2023
- Yoke allowance 2 × largest-conductor allowance per yokeNEC 314.16(B)(4)