When to use: Check that an outlet, switch, or junction box is large enough per NEC 314.16. Every current-carrying conductor counts once at its own gauge's volume allowance from Table 314.16(B). Internal cable clamps count once (as one conductor of the largest gauge present), each device strap/yoke (switch or receptacle) counts twice at the largest gauge, and all equipment grounding conductors together count once at the largest gauge.
This tool calculates the minimum required box volume per NEC 314.16 from the actual conductors, clamps, devices, and grounding conductors that will occupy an outlet, switch, or junction box, then checks it against a standard or custom box volume. Overfilled boxes are a common code violation because heat dissipation and safe conductor bending space both depend on adequate free volume.
NEC Table 314.16(B) assigns a cubic-inch volume allowance to each conductor size: 18 AWG = 1.50 in³, 16 AWG = 1.75 in³, 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³, 8 AWG = 3.00 in³, and 6 AWG = 5.00 in³. Every current-carrying conductor that originates outside the box and terminates or splices inside it counts once at its own gauge — conductors that merely pass through the box unbroken also count once each.
Beyond the conductors themselves, NEC 314.16(B)(1)-(5) add allowances for other items sharing the box's volume: all internal cable clamps together count as one additional conductor, sized at the largest gauge conductor present in the box. Each strap or yoke supporting a device (a switch, receptacle, or similar) counts as two additional conductors, also sized at the largest gauge present. All equipment grounding conductors (EGCs) present, regardless of how many there are, count together as a single additional conductor at the largest gauge — with an additional allowance if an isolated-ground EGC is also present.
Clamps, device yokes, and grounding conductors do not each get sized individually — NEC 314.16(B) intentionally simplifies this by requiring the allowance for these items to be based on the single largest conductor gauge present anywhere in the box. This means adding one small 14 AWG lighting circuit to a box that already contains 10 AWG conductors will size the clamp/device/EGC allowances at the larger 10 AWG volume, not the smaller 14 AWG volume — a detail that is easy to miss when manually tallying box fill.
Enter the number of current-carrying conductors of each gauge that will occupy the box (count each conductor per the NEC pass-through/originate-terminate rule). Toggle on internal cable clamps and equipment grounding conductors if present, and enter the number of device straps/yokes (switches or receptacles) mounted in the box. Select a standard box size from the dropdown, or fine-tune/override the volume directly if you have the manufacturer's stamped cubic-inch rating for your specific box. The tool reports the required volume, the fill percentage, and a pass/fail check.
Yes — every current-carrying conductor, including neutrals, counts once at its own gauge per NEC 314.16(B), with the same pass-through/originate-terminate counting rule applied to any conductor entering the box.
NEC 314.16(B)(4) requires each strap containing a device (a switch or receptacle yoke) to be counted as two conductors of the largest size connected to that device, reflecting the additional physical volume the device body and its terminations occupy inside the box.
No — NEC 314.16(B)(5) treats all equipment grounding conductors in the box together as a single additional conductor volume allowance, sized at the largest EGC gauge present, unless an isolated-ground receptacle is also installed, which adds one more allowance for the isolated-ground conductor.
Either use a larger box (or box extension/box with a deeper listed volume), split conductors across multiple boxes with a compliant raceway between them, or reduce the conductor/device count in the box. Overfilling a box is a code violation that risks insulation damage, overheating, and difficulty making safe terminations.
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