Box Fill Calculator

This adds up electrical box fill the way NEC 314.16 does it: a volume for each conductor at its own wire size, a single shared allowance for all the equipment grounding conductors, one for internal cable clamps, one for each luminaire stud, and a double allowance for every device yoke. Pick a box from Table 314.16(A) or type the volume marked on the one you have, and the result shows each allowance separately, the total, and whether it fits. A second mode runs the other way and says how many conductors of one size the box still holds. It does not cover conduit fill, which is a different table entirely.

Calculator Type

Add up what is going in the box and compare it with the volume the box has.

Only used when the box above is set to "Another box". Plastic boxes are stamped with their volume inside; a plaster ring or extension ring adds its own marked volume on top.
Two 12-2 cables landing in the box is four: two hots and two neutrals. The grounds go in their own box below.
However many there are, they share one single allowance, taken at the size of the largest of them.
One per yoke, not per device: a duplex receptacle on one strap is one yoke. Each takes a double allowance.
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Take the box, the devices and the clamps off first, then see how many conductors of one size the rest of the volume holds.

Only used when the box above is set to "Another box". Plastic boxes are stamped with their volume inside; a plaster ring or extension ring adds its own marked volume on top.
However many there are, they share one single allowance, taken at the size of the largest of them.
One per yoke, not per device: a duplex receptacle on one strap is one yoke. Each takes a double allowance.
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