What are the steps to calculate box fill?
Count five things, then multiply. Each conductor entering the box counts once, each clamp group counts once, each support fitting counts once, each device yoke counts twice, and all grounds together count once. Multiply that total count by the volume for the largest conductor present from Table 314.16(B), then compare to the box volume.
Last reviewed June 2026 · Answered by JourneymanIQ staff
Count the five categories
Conductors entering and terminating count one each, clamps as one group, support fittings as one, each device yoke as two, and all grounds together as one.
Multiply by the largest volume
Use Table 314.16(B) with the largest conductor present: 14 AWG is 2.0, 12 AWG is 2.25, 10 AWG is 2.5, 8 AWG is 3.0 cubic inches.
Compare to the box
Add the volumes and check the total against the box capacity. If the required volume is larger than the box, you need a bigger box.
Five counts, device yoke counts two, all grounds count one, times the largest-conductor volume.
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