Box fill: how many wires fit in a box

What the code requires

The code requires a box's volume to be at least the total of its fill allowances. Wires, devices and certain fittings each count, and the allowances are added together. One or more internal cable clamps count as one allowance, and each type of luminaire stud or hickey counts once, both based on the largest wire in the box. Small fittings such as locknuts and bushings do not count. If a barrier divides a box, each space is figured separately.

The allowance for each conductor depends on its size, from Table 314.16(B)(1): 18 AWG is 1.50 in³, 16 AWG is 1.75 in³, 14 AWG is 2.00 in³, 12 AWG is 2.25 in³, 10 AWG is 2.50 in³, 8 AWG is 3.00 in³ and 6 AWG is 5.00 in³.

Each switch or receptacle yoke counts double, based on the largest wire connected to it. A device wider than a standard 2-in. device box gets a double allowance for each gang it needs. Up to four equipment grounding conductors share one allowance based on the largest of them. Each additional one adds a quarter allowance, also based on the largest. Bonding jumpers are no longer counted.

New in 2023, a terminal block counts as one allowance for the whole assembly, not one per pole, based on the largest wire landed on it.

On the job

• Overfilled device boxes are a common correction, especially when bulky GFCI or AFCI devices go into shallow boxes.
• Each device yoke counts as two wires of the largest size connected to it, which is easy to miss when adding a circuit to an existing box.
• Box depth is a separate rule in 314.24, so a box can pass the fill count and still be too shallow for a deep device.

What changed in 2023

The 2023 NEC added an allowance for terminal blocks, stopped counting bonding jumpers in the grounding-conductor allowance, and deleted the 314.16(B)(2) paragraph on clamp assemblies with built-in cable terminations for specific nonmetallic boxes.

Related sections

• NEC 314.24
• NEC 314.17
• NEC 300.14
• NEC 314.20: Setting boxes flush with the wall surface
• NEC 250.146(D)

Where this comes from

• NFPA: 2026 NEC First Draft, CMP-8 (shows 2023 text of 314.16)
• Electrical Contractor: Exploring Chapter 3 in 2023, Accepting NEC Change part 9 (M. Earley)
• Electrical Contractor, Avoiding Overfill: splices and splicing devices in box fill (M. Ode)
• EZ-pdh: 2023 NEC Significant Code Changes, Part 2
• Read the official NEC free at NFPA