Count each conductor that enters and leaves. A wire that enters and terminates inside the box without leaving does not count.
Each yoke or strap counts as 2 volumes at the largest conductor connected to it.
All grounds together count as 1 volume, however many there are. Enter 1 for yes, 0 for no.
All internal clamps together count as 1 volume. Enter 1 for yes, 0 for no.
Reference data and planning arithmetic. Does not apply ambient or bundling derating, motor rules, or local amendments. Not a substitute for the NEC edition your jurisdiction has adopted, an inspection, or a licensed electrician.
How do you calculate box fill?
Count one volume per conductor entering and leaving, one for all equipment grounds together, two per device yoke, one for all internal clamps together, and one per support fitting — taking the device, ground and clamp allowances at the largest conductor in the box.
A conductor that enters and terminates inside the box without leaving counts as zero. Wire connectors have no allowance at all.
Worked example: a switch box with two 14/2 cables entering, internal clamps and one switch needs 8.00 in³ for the four conductors, 2.00 for the grounds, 2.00 for the clamps and 4.00 for the switch yoke — 16.00 in³ total. A 3 x 2 x 2½ box holds 12.5 in³ and fails; a 3 x 2 x 3½ holds 18.0 in³ and passes.
Per NEC Table 314.16(B) the per-conductor volumes are 2.00 in³ for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, 3.00 for 8 AWG and 5.00 for 6 AWG.
The counting rules
Box fill is not a count of wires. NEC 314.16(B) assigns volume allowances to several different things, and the ones people forget are the devices and the clamps — a single switch counts for twice as much as a conductor:
- Each conductor entering and leaving the box: 1 volume.
- A conductor that enters and terminates inside without leaving: 0. Pigtails are free.
- All equipment grounding conductors together: 1 volume total, however many there are, at the largest ground present.
- Each device yoke or strap (switch, receptacle): 2 volumes, at the largest conductor connected to it.
- All internal cable clamps together: 1 volume, at the largest conductor in the box.
- Each support fitting (fixture stud, hickey): 1 volume each.
| Box | Volume | 14 AWG alone | 12 AWG alone |
|---|---|---|---|
| 3 x 2 x 1-1/2 device box | 7.5 in³ | 3 conductors | 3 conductors |
| 3 x 2 x 2 device box | 10 in³ | 5 conductors | 4 conductors |
| 3 x 2 x 2-1/2 device box | 12.5 in³ | 6 conductors | 5 conductors |
| 3 x 2 x 2-3/4 device box | 14 in³ | 7 conductors | 6 conductors |
| 3 x 2 x 3-1/2 device box | 18 in³ | 9 conductors | 8 conductors |
| 4 x 1-1/4 round/octagon | 12.5 in³ | 6 conductors | 5 conductors |
| 4 x 1-1/2 round/octagon | 15.5 in³ | 7 conductors | 6 conductors |
| 4 x 2-1/8 round/octagon | 21.5 in³ | 10 conductors | 9 conductors |
| 4 x 1-1/4 square | 18 in³ | 9 conductors | 8 conductors |
| 4 x 1-1/2 square | 21 in³ | 10 conductors | 9 conductors |
| 4 x 2-1/8 square | 30.3 in³ | 15 conductors | 13 conductors |
| 4-11/16 x 1-1/4 square | 25.5 in³ | 12 conductors | 11 conductors |
| 4-11/16 x 1-1/2 square | 29.5 in³ | 14 conductors | 13 conductors |
| 4-11/16 x 2-1/8 square | 42 in³ | 21 conductors | 18 conductors |
The largest-conductor rule
The device, ground and clamp allowances are all calculated at the volume for the largest conductor in the box, not at each item's own size. One 12 AWG conductor among a box of 14 AWG raises every one of those allowances from 2.00 to 2.25 in³.
On a box with a device, grounds and clamps that is three allowances, so a single larger conductor adds 0.75 in³ on top of its own volume. It is a small number that turns a marginal box into a failing one.
A worked example
A standard single-switch box with two cables entering, all 14/2 with ground, internal clamps:
- 4 current-carrying conductors (2 hot, 2 neutral) × 2.00 = 8.00 in³
- Equipment grounds, counted once × 2.00 = 2.00 in³
- Internal clamps, counted once × 2.00 = 2.00 in³
- One switch yoke, 2 volumes × 2.00 = 4.00 in³
- Total: 16.00 in³
A 3 × 2 × 2½ box holds 12.5 in³ and fails. A 3 × 2 × 3½ holds 18.0 in³ and passes with 2 in³ spare. This is why so many old shallow boxes are overfilled once someone adds a cable, and why box fill is one of the most commonly cited items on an inspection.
Two practical notes: plaster rings and box extenders have marked volumes that add to the total, and any box larger than 100 in³ is outside Table 314.16(A) and is sized by its marked volume instead.
Common questions
How do I calculate box fill?
Add one volume per conductor entering and leaving, one for all grounds together, two per device yoke, one for all internal clamps together, and one per support fitting — using the largest conductor volume for the device, ground and clamp allowances. Compare the total to the box volume.
Do wire nuts and pigtails count toward box fill?
No. Wire connectors have no volume allowance, and a conductor that enters and terminates inside the box without leaving does not count at all.
How many 14 AWG wires fit in an 18 cubic inch box?
Nine conductors alone. With a switch, grounds and clamps the arithmetic changes: that combination uses 8 in³ of allowances, leaving room for five conductors.
Does a receptacle count as one or two?
Two volumes, based on the largest conductor connected to it. That is per yoke or strap, so a duplex receptacle on one yoke is still two volumes, not four.