Count every unit — conductors, devices, grounds, and clamps — then check your box volume. Know before you close it up.
| Insulated conductors | — |
| Devices / yokes (× 2) | — |
| Grounding conductors | — |
| Internal clamps | — |
| Total units | — |
NEC 314.16 requires that every outlet box be large enough to contain the conductors and devices installed in it without damaging the insulation or creating dangerous conditions. Section 314.16(B) assigns a volume allowance to each item in the box, and the total required volume must not exceed the rated box volume.
The allowance per conductor depends on the largest AWG in the box — from 1.50 in³ for 18 AWG to 5.00 in³ for 6 AWG. Every insulated conductor counts as one unit. All equipment grounding conductors together count as one unit. A device (yoke) counts as two units. Internal cable clamps all together count as one unit — external clamps don't count at all.
| Item | Count as | NEC Ref |
|---|---|---|
| Each insulated conductor entering box | 1 conductor | 314.16(B)(1) |
| All EGCs together | 1 conductor | 314.16(B)(5) |
| Each device / yoke | 2 conductors | 314.16(B)(4) |
| All internal clamps together | 1 conductor | 314.16(B)(2) |
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