Example
A PVC box has two 14/2 cables, one 14/3 cable, and one receptacle. The box has no internal clamps. What's the minimum box volume needed?
Step 1. Count the conductors:
2 conductors per 14/2 cable x 2 cables = 4
3 conductors in the 14/3 cable = 3
All grounds together = 1
No internal clamps = 0
Receptacle outlet = 2
Total conductor equivalents = 10
Step 2. The volume factor for 14-ga. wire is 2 cu. in., so:
10 conductor equivalents
x 2 cu. in.
= 20 cu. in. box
Single-gang boxes come in three sizes, 18 cu. in., 20.4 cu. in. and 22.5 cu. in. Although smaller is cheaper, larger may avoid box-fill violations.
A 4-in. junction box solves most box-fill problems. Throw on a single- or double-gang plaster ring that comes flush with the drywall, and you've got a box that's hard to overfill.
