Box Fill Calculator - NEC Junction Box Capacity

NEC box fill requirements prevent overcrowding of conductors and devices within electrical boxes, ensuring safe installations with adequate space for wire connections and heat dissipation. Article 314.16 establishes volume allowances based on conductor size, with each conductor, device, clamp, and fitting consuming specific amounts of box volume. Proper box fill calculations prevent overheating, facilitate future modifications, and ensure code compliance during inspections.

Each conductor entering the box counts as one fill unit based on its size. Devices (switches, receptacles) count as two conductors of the largest wire connected to them. Cable clamps count as one conductor of the largest size, and fixture studs or hickeys each count as one conductor. Ground wires collectively count as one conductor of the largest ground wire in the box, regardless of quantity.