Box Fill Calculator

How it’s calculated. NEC 314.16(B) volume allowances per conductor: 14 AWG = 2.00, 12 = 2.25, 10 = 2.50, 8 = 3.00, 6 = 5.00 in³. Each device/yoke counts as 2× the largest conductor connected to that yoke (314.16(B)(4)) — not the largest in the box — and a device on a yoke or strap wider than 2 in. takes that double allowance for each gang it occupies, so enter the extra gangs those wide devices take in the field above (a 2-gang timer is one device plus one extra gang, 4×); internal cable clamps add 1× the largest conductor (314.16(B)(2)); each support fitting — a luminaire stud or a hickey — adds a further 1× the largest conductor under 314.16(B)(3), one allowance per type; and all equipment-grounding conductors together count as 1× the largest ground, plus ¼ allowance for each grounding conductor beyond four (314.16(B)(5)).

Counting conductors — 314.16(B)(1). A conductor that originates outside the box and terminates inside counts as one. A conductor that passes through unspliced counts as one — but if it is looped with at least 12 in. of free conductor, it counts as two. A conductor that both originates and terminates inside the box (a pigtail) counts as zero.

Conductors that both originate and terminate inside the box (pigtails) are not counted. Compare the required volume to the box’s marked cubic-inch capacity. This is an estimate — confirm against the box listing and the full design.

Worked example. A duplex receptacle fed by one 12 AWG MC cable and feeding another downstream: four 12 AWG circuit conductors enter the box (2.25 in³ each = 9.00), the receptacle yoke counts twice the largest conductor landed on it (2 × 2.25 = 4.50), and the two 12 AWG grounds together count once (2.25) — 15.75 in³. The MC connectors clamp outside the box, so no clamp allowance applies. A 3×2×2¾ device box (14.0 in³) is over fill; a 3×2×3½ (18.0) or a 4×1½ square (21.0) works, and a plaster ring marked with its volume adds to the total. Add a third cable — a 12/3 MC bringing a second circuit through — and the count becomes seven conductors (15.75) plus the yoke (4.50) plus the grounds (2.25) = 22.50 in³, so the 4×1½ square is now over and a 4×2⅛ square (30.3 in³) is the answer. A deeper box is cheap; a failed rough-in inspection is not.

Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.