Article 314 · device boxes
Box Fill Calculations: Article 314 Without the Headache
Article 314 looks intimidating until you see that the whole calc is a count. Five categories of items each contribute a conductor count. You multiply by the volume from Table 314.16(B). Done.
Last reviewed May 2026
The five count categories
Per 314.16(B), each of the following contributes to the conductor count. Use the count to multiply the box-fill volume.
- Each conductor that originates outside the box and terminates or is spliced inside = 1 count. Conductors that just pass through unbroken = 1 count.
- Each conductor that originates and terminates inside the box = 0 (pigtails don’t count).
- Each cable clamp inside the box = 1 count, total (regardless of how many clamps).
- Each support fitting (e.g., fixture stud, hickey) = 1 count, total.
- Each device or yoke = 2 counts, sized for the largest conductor connected to the device.
- All equipment grounding conductors together = 1 count (or 1 + 0.25 if isolated grounds also present).
Volume per conductor (Table 314.16(B))
- 18 AWG → 1.5 cu in
- 16 AWG → 1.75 cu in
- 14 AWG → 2.0 cu in
- 12 AWG → 2.25 cu in
- 10 AWG → 2.5 cu in
- 8 AWG → 3.0 cu in
- 6 AWG → 5.0 cu in
The five-step calc
- Count the conductors entering the box and terminating or spliced (each = 1).
- Add 1 for cable clamps (any number, total = 1).
- Add 1 for support fittings (any number, total = 1).
- Add 2 for each device/yoke (each device = 2).
- Add 1 for all the EGCs together (or 1.25 if isolated grounds).
- Multiply each count by the volume in Table 314.16(B), using the largest conductor present for clamps, supports, and devices.
- Compare to the box's listed cubic-inch capacity.
A worked example
4-inch square device box, 21.0 cu in capacity. Inside: four 12 AWG conductors (two hot, two neutral) entering and spliced; one cable clamp; one duplex receptacle (12 AWG yoke); two 12 AWG equipment grounding conductors.
- Conductors entering = 4 × 2.25 = 9.0 cu in
- Cable clamps = 1 × 2.25 = 2.25 cu in (largest conductor is 12 AWG)
- Device (yoke) = 2 × 2.25 = 4.5 cu in
- EGCs (counted as 1) = 1 × 2.25 = 2.25 cu in
- Total = 18.0 cu in. Box capacity = 21.0 cu in. Compliant.
Three traps
- Counting pigtails. Conductors that originate AND terminate inside the box do not count.
- Counting clamps individually. All clamps together count once.
- Forgetting the device counts as 2 (not 1).
Drill box fill on real exam questions
The diagnostic includes box fill across the common scenarios: device boxes, junction boxes, and multi-gang assemblies.