Box Fill Calculator
Calculate Electrical Box Fill
About Box Fill Calculations
Box fill calculations ensure electrical boxes have adequate space for all conductors and devices without overcrowding.
NEC Box Fill Rules:
- Each conductor counts as one volume unit
- Ground wires count as one volume unit
- Devices count as two volume units
- Clamps count as one volume unit each
Wire Volume Units:
- #14 AWG: 2.0 cubic inches
- #12 AWG: 2.25 cubic inches
- #10 AWG: 2.5 cubic inches
- #8 AWG: 3.0 cubic inches
Device Volume Units:
- Single Device: 2.0 cubic inches
- Duplex Device: 2.0 cubic inches
- GFCI Device: 2.0 cubic inches
Common Box Capacities:
- 4" × 1.25": 8.0 cubic inches
- 4" × 1.5": 10.0 cubic inches
- 4" × 2.125": 14.0 cubic inches
- 4" × 2.5": 18.0 cubic inches
Safety Requirements:
• Never exceed box capacity
• Allow for wire bending
• Consider future additions
• Follow NEC guidelines
• Use proper box sizing
Proper box fill calculations prevent overheating, ensure code compliance, and maintain electrical safety in residential and commercial installations.