Electrical Box Fill Chart — NEC 314.16 Box Volumes & Conductor Counts
NEC 314.16 box fill tables: standard metal box volumes, per-conductor volume allowances by AWG, and computed maximum conductor counts per box.
Updated August 20, 2026
NEC 314.16 sizes outlet, device, and junction boxes by volume arithmetic: every conductor, clamp, fitting, device yoke, and grounding-conductor group in the box claims a volume allowance, and the box must have at least that much room. The first table below lists the standard boxes the box fill calculator suggests — metal boxes carry the minimum volumes printed in Table 314.16(A), while nonmetallic (plastic) boxes are marked with their cubic-inch volume by the manufacturer per 314.16(A)(2). The second table is Table 314.16(B): the volume one conductor of each AWG size claims, from 2.00 in³ at 14 AWG to 5.00 in³ at 6 AWG.
The third table divides the two — maximum same-size conductors = box volume ÷ per-conductor allowance, rounded down — and for every metal box row it reproduces exactly the conductor counts printed in Table 314.16(A). Those printed counts assume nothing else in the box: no internal clamps, no support fittings, no devices, and no equipment grounding conductors. Each of those claims one or more additional allowances under 314.16(B)(2)–(B)(5) — a duplex receptacle alone takes two allowances of the largest conductor on its yoke — so a real switch or receptacle box holds fewer wires than the table maximum. The box fill calculator applies all five count rules.
Standard Box Volumes — NEC Table 314.16(A) & Marked Plastic Boxes
| Box | Volume (in³) | Volume (cm³) |
|---|---|---|
| Device box 3 x 2 x 2-1/2 in | 12.5 | 204.8 |
| Octagon 4 x 1-1/2 in | 15.5 | 254.0 |
| Device box 3 x 2 x 3-1/2 in | 18.0 | 295.0 |
| Single-gang plastic (18.0 in3 marked) | 18.0 | 295.0 |
| Single-gang plastic (20.0 in3 marked) | 20.0 | 327.7 |
| Square 4 x 1-1/2 in | 21.0 | 344.1 |
| Octagon 4 x 2-1/8 in | 21.5 | 352.3 |
| Single-gang plastic (22.5 in3 marked) | 22.5 | 368.7 |
| Square 4 x 2-1/8 in | 30.3 | 496.5 |
| Square 4-11/16 x 2-1/8 in | 42.0 | 688.3 |
Metal-box volumes are the Table 314.16(A) minimums; single-gang plastic boxes are marked-volume boxes per 314.16(A)(2) (volumes as marked by the manufacturer). cm³ computed as in³ × 16.387064.
Volume Allowance per Conductor — NEC Table 314.16(B)
| Conductor size | Allowance (in³) | Allowance (cm³) |
|---|---|---|
| 14 AWG | 2.00 | 32.8 |
| 12 AWG | 2.25 | 36.9 |
| 10 AWG | 2.50 | 41.0 |
| 8 AWG | 3.00 | 49.2 |
| 6 AWG | 5.00 | 81.9 |
One allowance per insulated conductor entering or leaving the box, 314.16(B)(1). The full table also prints 18 AWG (1.50 in³) and 16 AWG (1.75 in³), outside the sizes tabulated here.
Maximum Same-Size Conductors per Box (Computed, No Devices or Clamps)
| Box | Volume (in³) | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|---|
| Device box 3 x 2 x 2-1/2 in | 12.5 | 6 | 5 | 5 | 4 | 2 |
| Octagon 4 x 1-1/2 in | 15.5 | 7 | 6 | 6 | 5 | 3 |
| Device box 3 x 2 x 3-1/2 in | 18.0 | 9 | 8 | 7 | 6 | 3 |
| Single-gang plastic (18.0 in3 marked) | 18.0 | 9 | 8 | 7 | 6 | 3 |
| Single-gang plastic (20.0 in3 marked) | 20.0 | 10 | 8 | 8 | 6 | 4 |
| Square 4 x 1-1/2 in | 21.0 | 10 | 9 | 8 | 7 | 4 |
| Octagon 4 x 2-1/8 in | 21.5 | 10 | 9 | 8 | 7 | 4 |
| Single-gang plastic (22.5 in3 marked) | 22.5 | 11 | 10 | 9 | 7 | 4 |
| Square 4 x 2-1/8 in | 30.3 | 15 | 13 | 12 | 10 | 6 |
| Square 4-11/16 x 2-1/8 in | 42.0 | 21 | 18 | 16 | 14 | 8 |
Computed as floor(volume ÷ allowance); for the metal boxes these equal the counts printed in Table 314.16(A). Counts assume no clamps, fittings, devices, or EGCs — each of those subtracts allowances per 314.16(B)(2)–(B)(5).
Sources & Further Reading
- NEC (NFPA 70) 2023, Table 314.16(A) — Metal Boxes, and Table 314.16(B) — Volume Allowance Required per Conductor, as encoded and verified in the box fill calculator
- Zing² NEC Table 314.16(A) reproduction and the electrical-inspector NEC 314.16 reproduction — two independent published cross-checks; every computed max-count cell matches the printed 314.16(A) count
- cm³ column and conductor counts: computed (in³ × 16.387064; floor(volume ÷ allowance)), never transcribed
Frequently Asked Questions
How many 12 AWG wires fit in a 4x1-1/2 square box?
Nine, per Table 314.16(A): the box has 21.0 in³ and a 12 AWG conductor claims 2.25 in³, so floor(21.0 ÷ 2.25) = 9. That assumes bare conductor fill only — add a receptacle (2 allowances = 4.5 in³) and a set of internal clamps (1 allowance = 2.25 in³) and the practical count drops to six.
Why does my plastic box show a different volume than Table 314.16(A)?
Nonmetallic boxes are not in Table 314.16(A) at all. Per 314.16(A)(2) they must be durably marked with their volume by the manufacturer — common single-gang nail-on boxes are marked 18.0, 20.0, or 22.5 in³ — and the fill arithmetic then uses the marked volume with the same Table 314.16(B) allowances.
How do devices, clamps, and ground wires count toward box fill?
Beyond one allowance per insulated conductor, 314.16(B) adds: one allowance of the largest conductor where the box has internal cable clamps; one per support fitting (stud or hickey); two allowances of the largest conductor on the yoke for each device strap; and one allowance of the largest EGC for all equipment grounding conductors together, plus a quarter allowance for each EGC beyond four (NEC 2023).
Do pigtails and wires that never leave the box count?
No. The 314.16(B)(1) count is one allowance per insulated conductor that enters or leaves the box; a pigtail made up entirely inside the box is not counted. A conductor that passes through unbroken counts once — unless it is looped long enough to count twice under the 314.16(B)(1) loop rule (twice the minimum free-conductor length of 300.14).
Try the Calculators
Electrical Box Fill Chart — NEC 314.16 Box Volumes & Conductor Counts — reuven.tools/reference/box-fill-volumes — verified against: NEC (NFPA 70) 2023, Table 314.16(A) — Metal Boxes, and Table 314.16(B) — Volume Allowance Required per Conductor, as encoded and verified in the box fill calculator; Zing² NEC Table 314.16(A) reproduction and the electrical-inspector NEC 314.16 reproduction — two independent published cross-checks; every computed max-count cell matches the printed 314.16(A) count; cm³ column and conductor counts: computed (in³ × 16.387064; floor(volume ÷ allowance)), never transcribed