NEC Table 314.16(A)
How Many Wires Fit in a 4-Inch Square Box?
The maximum conductors a 4-inch square box (commonly called a 4-square box) holds by wire size at the NEC 314.16 fill limit, the realistic count once a device is in the box, and the interactive calculator.
Max Conductors in a 4-Inch Square Box (NEC 314.16(A))
Maximum same-size conductors by box depth, with nothing else in the box. Each is the box volume divided by the per-conductor allowance from NEC Table 314.16(B).
| Box Depth | Volume | #14 | #12 | #10 | #8 |
|---|---|---|---|---|---|
| 4" x 1-1/4" deep | 18.0 | 9 | 8 | 7 | 6 |
| 4" x 1-1/2" deep | 21.0 | 10 | 9 | 8 | 7 |
| 4" x 2-1/8" deep | 30.3 | 15 | 13 | 12 | 10 |
Volumes in cubic inches. Per-conductor allowance (Table 314.16(B)): #14 = 2.00, #12 = 2.25, #10 = 2.50, #8 = 3.00 cu in. Counts assume one wire size and an otherwise empty box.
The Real Count: With a Receptacle in the Box
The bare maximum almost never applies. A real device box holds a receptacle or switch, cable clamps, and a ground, and each takes space before the conductors. In a 4" x 1-1/2" deep 4-inch square box (21 cu in), the yoke counts as two conductor allowances (NEC 314.16(B)(4)), all clamps together count as one (B)(2), and the grounds count as one (B)(5).
| Wire Size | Bare Max | With Device, Clamps, Ground |
|---|---|---|
| #14 AWG | 10 | 6 |
| #12 AWG | 9 | 5 |
| #10 AWG | 8 | 4 |
| #8 AWG | 7 | 3 |
Device (2) + clamps (1) + ground (1) = four conductor allowances used before any wires. Assumes the device and fittings are sized to the same conductor. Two 14/2 cables plus a receptacle is a common real load for this box.
Your Exact Box? Calculate It
The tables cover one wire size at a time. For a real box with mixed wire sizes, multiple devices, or an extension ring, enter the exact contents for the precise NEC 314.16 fill and a pass or fail verdict.
Frequently Asked Questions
How many wires fit in a 4-inch square box?
A 4" x 1-1/2" deep 4-inch square box (21 cu in, NEC Table 314.16(A)) holds 10 #14, 9 #12, or 8 #10 conductors when nothing else is inside. Each conductor takes its volume allowance from NEC Table 314.16(B): 2.00 cu in for #14, 2.25 for #12, 2.50 for #10. So 21 cu in / 2.00 = 10 #14 conductors. Add a device, clamps, or ground and the count drops.
How many wires fit in a 4-inch square box with a receptacle?
About 6 #14 or 5 #12 conductors, once you subtract for the device and fittings. In a 4" x 1-1/2" deep 4-inch square box (21 cu in), one receptacle yoke counts as two conductor allowances (NEC 314.16(B)(4)), all internal clamps together count as one (B)(2), and the grounds count as one (B)(5). That is four #14 allowances (8.00 cu in) gone before any conductors, leaving room for 6 #14. This is the number that matters on a real rough-in, not the bare maximum.
How many #12 THHN wires fit in a 4-inch square box?
9 #12 conductors fit in a 4" x 1-1/2" deep 4-inch square box at the NEC 314.16(A) limit. Each #12 counts as 2.25 cu in (Table 314.16(B)), so 21 cu in / 2.25 = 9. With one receptacle, clamps, and a ground, that drops to about 5 #12 conductors.
What is the volume of a 4-inch square box?
A 4-inch square box comes in a few depths (NEC Table 314.16(A)): 4" x 1-1/4" deep is 18 cu in, 4" x 1-1/2" deep is 21 cu in, 4" x 2-1/8" deep is 30.3 cu in. Box fill uses the listed volume, or the value stamped on the box if the manufacturer marks a larger one.
Do devices and grounds count against the wire count in a 4-inch square box?
Yes. NEC 314.16(B) counts more than the wires. Each device yoke (a switch or receptacle) counts as two conductors at the largest connected size, all internal clamps together count as one, and one to four grounds count as one. That is why the 10-wire bare maximum for a 4" x 1-1/2" deep 4-inch square box becomes roughly 6 once the box holds a real device.
Wires in Other Boxes
Wires in a 4-11/16 Inch Square Box
21 #14 in the 42 cu in size, plus the full count.
Wires in a Single-Gang Device Box
9 #14 in the 18 cu in size, plus the full count.
Wires in a Handy Box
6 #14 in the 13 cu in size, plus the full count.
Wires in a Masonry Box
10 #14 in the 21 cu in size, plus the full count.
How Many Wires in a Box (all types)
Max conductors for every standard box in one chart, with the device and ground rules.
Conduit Fill Calculator
NEC Chapter 9 fill for any mix of conductors in EMT, IMC, RMC, or PVC.
Box passes. What about the pipe feeding it?
Box fill and conduit fill get cited together on inspection. Run the same conductors through the conduit fill calculator before you pull.