Section 314.16/Box Fill Calculations
NEC 314.16 establishes the maximum number of conductors, devices, and fittings in outlet and junction boxes based on box volume.
The total volume of conductors, devices, fittings, and clamps in a box must not exceed the box volume from Table 314.16(A) (for standard metal boxes) or the volume marked on listed nonmetallic boxes. Each conductor counts as one volume allowance based on the largest conductor size: 2.00 cu in for #14, 2.25 cu in for #12, 2.50 cu in for #10. Each device (switch, receptacle) counts as two volume allowances. All equipment grounding conductors together count as one volume allowance. Each cable clamp assembly counts as one volume allowance. The calculation determines the minimum box size required.
Why this section exists
An overfilled electrical box is a fire and shock hazard. Conductors forced into a too-small box can damage insulation, creating short circuits and ground faults. Devices pushed tight against conductors can loosen terminal connections. Heat dissipation is reduced in overfilled boxes, accelerating insulation degradation. The box fill calculation ensures adequate volume for safe installation, splicing, and future maintenance of the conductors and devices.
What plan reviewers look for
Plan reviewers check the electrical drawings for box sizes at locations with multiple circuits, switches, or receptacles. They verify the box fill calculation accounts for all conductors entering the box (including pigtails and equipment grounds), all devices, and all clamps. For switch boxes with multiple switches and a neutral (per 404.2), they verify the box is large enough.