Volume Allowance
Per-conductor cubic inch volume requirements used for calculating non-standard box fill.
Professional Usage Disclaimer
This data is provided for informational purposes only. Always verify with the latest edition of the National Electrical Code (NFPA 70) and consult with a licensed electrical professional or AHJ (Authority Having Jurisdiction) before performing any electrical work. Zing² is a reference tool, not a substitute for qualified engineering.
Professional Field Notes & Gotchas
- ▸Device Counts: Each yoke or strap (outlet/switch) counts as TWO volumes based on the largest wire connected to it. A GFCI counts the same as a standard outlet.
- ▸Internal Clamps: One or more internal cable clamps count as ONE volume based on the largest wire. External connectors on the box knockout do NOT count.
- ▸Grounding Rule: All equipment grounding conductors (green/bare) in the box count as only ONE volume together, regardless of how many there are.
- ▸Fill Ratio: If the box is jammed so tight you have to use a hammer to close the faceplate, you likely have a heat dissipation issue, even if the math 'technically' passes.
NEC 2023 vs 2026: What changed?
NEC 2023 vs 2026: Clarifications were made regarding 'large device' allowances. If a device is wider than a standard 2-inch yoke, the manufacturer must now provide specific volume allowance data.
Technical FAQs
How much volume does an outlet take?
Per NEC 314.16(B)(4), a standard duplex outlet (or any yoke/strap) counts as a double volume allowance of the conductor connected to it.
What is the volume for #12 wire?
Each #12 AWG conductor requires 2.25 cubic inches of free space within the box.
Do wire nuts count toward box fill?
No. Components like wire nuts, pigtails (under 12 inches), and external connectors are not calculated as part of the volume allowance.