Calculate junction box fill per NEC 314.16(A). Enter your conductors, devices, and clamps to size a compliant box, or use the box picker to find one.
Box fill is the total volume the conductors, devices, clamps, and fittings occupy inside a junction or device box, compared against the box’s marked volume. Each item is assigned a volume allowance based on the largest conductor involved.
You count each current-carrying conductor once, add allowances for internal clamps, support fittings, and each yoke or strap that holds a device, then add for equipment bonding conductors as a group. The total must not exceed the box volume.
A device box with three #12 conductors entering, a receptacle, and internal cable clamps: you count three conductor volumes at the #12 allowance, one clamp allowance also at #12, and two device allowances for the yoke. Total those volumes and compare against the box marking. If it exceeds the box, you move to a deeper box or a larger trade size.
The NEC governs box fill in Article 314.16, with volume allowances per conductor size in Table 314.16(B). The CEC addresses box fill through Rule 12-3036 and the box capacity tables in Section 12. The calculator applies the NEC 314.16 method with a box picker. Confirm allowances against the current code edition.
Add a volume allowance for each conductor, for internal clamps as a group, for support fittings, and two allowances for each device yoke, plus one for all bonding conductors together. The total must not exceed the box’s marked volume.
Yes. Each device mounted on a yoke or strap counts as two conductor volumes, based on the largest conductor connected to that device.
All equipment bonding conductors together count as a single conductor volume, sized to the largest bonding conductor present, regardless of how many enter the box.
You need a larger or deeper box, or a box with a higher marked volume. Overfilling stresses conductors and violates the code.