How Many Wires Fit in a Single-Gang Box?
A single-gang box fills up faster than the wire count suggests, because the device, the internal clamps, and the equipment ground take their cut of the National Electrical Code (NEC) 314.16(B) volume before the first circuit conductor is counted. In a deep 18.0 cubic-inch single-gang metal device box with one receptacle, internal clamps, and a ground, that overhead is 9.00 cubic inches, leaving room for exactly two 12/2 cables, or two 14/2 cables plus a little slack. Anything past that needs a deeper box or a bigger one. The full derivation and a capacity table by box depth are below.
Skip the longhand and check any box on the Box Fill Calculator: enter the box volume, the conductor counts, the device yokes, and whether clamps and grounds are present, and it returns the required volume and whether the box is adequate.
The overhead eats the box before the wires do
NEC 314.16(B) sizes a box by adding up volume allowances, not by counting wires against a fixed maximum. Each allowance comes from Table 314.16(B): 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG. A single-gang box that holds a receptacle or switch pays four of those allowances before a single circuit conductor is added:
- Device yoke: a double allowance for the strap that holds the receptacle or switch, based on the largest conductor connected to that yoke (314.16(B)(4)).
- Internal clamps: one allowance for all cable clamps together, again at the largest conductor (314.16(B)(2)).
- Equipment ground: one allowance for every ground in the box combined, based on the largest one (314.16(B)(5)).
On a 12 AWG circuit that overhead is 2 × 2.25 (device) + 2.25 (clamps) + 2.25 (ground) = 9.00 cubic inches. On 14 AWG it is 2 × 2.00 + 2.00 + 2.00 = 8.00 cubic inches. That is before you count the conductors that actually carry the circuit, and it is why a single-gang box runs out of room quickly.
Worked example: two 12/2 cables and a receptacle
Take the deepest common single-gang metal device box, the 3 × 2 × 3½ at 18.0 cubic inches (Table 314.16(A)). It feeds a duplex receptacle with two 12/2 NM cables landing in it, and the box has internal cable clamps.
Conductors: two blacks + two whites = 4 × 12 AWG × 2.25 = 9.00 in³ (314.16(B)(1)).
Clamps: one allowance at 12 AWG = 2.25 in³ (314.16(B)(2)).
Receptacle yoke: double allowance = 2 × 2.25 = 4.50 in³ (314.16(B)(4)).
Grounds: the two bare 12 AWG grounds count once = 2.25 in³ (314.16(B)(5)).
Required: 9.00 + 2.25 + 4.50 + 2.25 = 18.00 in³.
The box holds 18.0 cubic inches, so it is filled to the line: adequate, with nothing to spare. A third 12/2 cable would add two more conductors (4.50 in³) and push the requirement to 22.50 cubic inches, well past what the box can hold. Two cables and a device is the practical ceiling for an 18 cubic-inch single-gang box on 12 AWG.
Capacity by box depth
The table below counts how many same-size conductors a single-gang metal device box can hold once you subtract the device, clamp, and ground overhead (2 ×a + a + a = 4a, where a is the Table 314.16(B) allowance). Divide the conductor count by two to get the number of 2-wire cables. Box volumes are the Table 314.16(A) minimums for standard 3 ×2 device boxes.
| Device box (Table 314.16(A)) | Volume | 14 AWG conductors | 12 AWG conductors |
|---|---|---|---|
| 3×2×2 | 10.0 in³ | 1 | 0 |
| 3×2×2¼ | 10.5 in³ | 1 | 0 |
| 3×2×2½ | 12.5 in³ | 2 | 1 |
| 3×2×2¾ | 14.0 in³ | 3 | 2 |
| 3×2×3½ | 18.0 in³ | 5 | 4 |
The math on the last row: for 14 AWG, (18.0 − 8.00) ÷ 2.00 = 5 conductors; for 12 AWG, (18.0 − 9.00) ÷ 2.25 = 4 conductors. Both stop at two cables. The shallow 10.0 cubic-inch box cannot take even one 12/2 cable once a device is on the yoke, which is why those bodies are usually reserved for a single switch loop on 14 AWG or a pass-through with no device.
Plastic boxes: read the stamp, drop the clamp
Table 314.16(A) lists metal boxes only. NEC 314.16(A) requires a nonmetallic box to be marked with its volume, so for a plastic single-gang box you use the stamped figure instead of the table. Most nonmetallic device boxes have no internal clamps either (a single-gang nonmetallic box with the cable fastened within 8 inches of the box does not require an internal clamp, the Exception to 314.17(C)), so the overhead drops to 3a (device + ground). A box stamped 22.5 cubic inches with a receptacle on 14 AWG and no clamps leaves 22.5 − 6.00 = 16.5 cubic inches, or eight 14 AWG conductors (16.5 ÷ 2.00 = 8.25, rounded down), room for four 14/2 cables. Always confirm the marked volume; it is the only reliable number for a nonmetallic box.
When a single gang will not do it
When the conductor count outgrows the box, the fix is depth or footprint, not creative counting. A deeper single-gang body buys a little; a 4-inch square box with a single-gang mud ring buys a lot more and is the usual next step for a device fed by three or more cables. Working the full method through on a 4-square is covered in this box fill calculation example. One rule that trips people up in tight boxes: a conductor that both begins and ends inside the box, such as a pigtail between two receptacle screws, is not counted (314.16(B)(1)); only conductors that leave the box or pass through it are.
NEC reference
NEC 2020. Volume allowances from Table 314.16(B); the conductor, clamp, device-yoke, and grounding counting rules from 314.16(B)(1) through (B)(5); standard metal box volumes from Table 314.16(A); nonmetallic-box marking from 314.16(A). The Table 314.16(B) allowances are unchanged across the 2017, 2020, and 2023 editions. One note on the ground allowance: NEC 2020 counts all equipment grounding conductors as a single allowance based on the largest; later editions have refined how additional grounding conductors are counted, so confirm the wording in your adopted edition.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
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NEC Code Quickstart
This guide covers one calculation. NEC Code Quickstart walks all twelve exam calculations start to finish, each with a worked example and the exact code reference: ampacity and derating, breaker sizing, voltage drop, box and conduit fill, grounding, dwelling load, range and dryer demand, motor circuits, and transformer current. Written for the 2023 NEC with notes for the 2026 edition.
Related
- Box Fill Calculator: add up the 314.16(B) allowances and check any box against its volume.
- Box Fill Calculation Example: 4-Inch Square Box: the full method worked on a 4-square, including the step-up decision.
- Conduit Fill Calculator: conductor fill for EMT, RMC, and PVC per NEC Chapter 9.