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 up to four grounds together count once. Then give each count its volume from the table in 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 up to four grounds together as one.
Give each count its volume
Use the volume table in 314.16(B): 14 AWG is 2.0, 12 AWG is 2.25, 10 AWG is 2.5, 8 AWG is 3.0 cubic inches. Each conductor goes at its own size, a device yoke at the largest conductor connected to it, and the clamp group at the largest conductor in the box.
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, up to four grounds count one, and each count at its own size.
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