Free box fill calculator · NEC 314.16 · No signup
Count your conductors, grounds, devices, clamps, and fittings — get the required volume per NEC 314.16(B) and whether your box is legal, with the volume remaining.
NEC 314.16(B) protects conductors from damage by limiting how full an outlet or device box can be. You add a volume allowance for each item and compare the total to the box's volume: 14 AWG = 2.00 in³, 12 = 2.25, 10 = 2.50, 8 = 3.00, 6 = 5.00.
The counted items: each conductor (one allowance of its size; a wire running through counts once); all internal cable clamps together (one allowance of the largest conductor); each support fitting like a fixture stud or hickey (one of the largest); each device yoke/strap (two of the largest conductor on it); and all equipment grounds together (one of the largest ground). Pigtails that stay in the box aren't counted.
Overfilled boxes are one of the most common rough-in inspection failures. Field PM keeps your QA/QC checklists and inspection logs in the field app so the QA/QC walk catches it before the inspector does. Box volumes come from NEC Table 314.16(A) or the marking inside nonmetallic boxes.
Per NEC 314.16(B), add a volume allowance for each item: each conductor = 1 volume of its size; all internal clamps together = 1 volume of the largest conductor; each support fitting (fixture stud/hickey) = 1 of the largest; each device yoke/strap = 2 of the largest conductor connected to it; and all equipment grounds together = 1 of the largest ground. The total required volume must be ≤ the box volume.
NEC Table 314.16(B): 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³, 8 AWG = 3.00 in³, 6 AWG = 5.00 in³.
A conductor running unbroken through the box counts as one volume allowance. A conductor that enters and terminates also counts as one. Pigtails and jumpers that don't leave the box are not counted.
All equipment grounding conductors together count as a single volume of the largest ground (a separate isolated ground adds one more). Each device yoke (a receptacle or switch strap) counts as two volumes of the largest conductor connected to that yoke.
Standard metal boxes are listed in NEC Table 314.16(A); nonmetallic boxes are marked with their volume inside. Use the marked or table value, or add ganged volumes for multi-gang assemblies.
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