Box Fill Calculator | NEC 314.16 Junction Box Volume
Free online tool • Calculated per NEC Table 314.16(A) and 314.16(B) • 2026 NEC
Use this free box fill calculator to verify compliance with NEC Section 314.16. Select your box type, add all conductors, devices, and clamps, and instantly see whether your electrical box is within code. Prevents dangerous overfilling that can cause overheating and insulation damage.
Conductors (entering or leaving box)
All EGCs count as 1 × the largest
Each device = 2× largest wire on that yoke
Results
PASSWhat Is NEC Box Fill (NEC 314.16)?
NEC Section 314.16 governs the maximum number of conductors and other items allowed inside a metal or nonmetallic outlet or device box. Overfilling a box creates dangerous conditions: insulation can chafe against the sharp edges of conductors or box knockouts, leading to short circuits, arc faults, and fires. The NEC assigns each wire gauge a specific cubic-inch volume allowance, and all items inside the box must collectively stay within the box's stamped or listed cubic-inch rating.
NEC Table 314.16(B) Volume Allowances
Each conductor entering or leaving the box counts toward the fill. Conductors that originate and terminate within the box (like pigtails under 4 inches) are not counted. Use these volume allowances per wire:
| Wire Size | Volume per Conductor |
|---|---|
| 18 AWG | 1.50 in³ |
| 16 AWG | 1.75 in³ |
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
Device and Clamp Volume Allowances
Wiring Devices (NEC 314.16(B)(4)): Each switch, outlet, or GFCI receptacle on a single strap counts as twice the volume of the largest conductor connected to it. Two switches on a two-gang plate still get individual yoke calculations per device.
Equipment Grounding Conductors (NEC 314.16(B)(5)): All equipment grounding conductors combined count as just one allowance equal to the largest grounding conductor. This is a commonly missed rule that reduces box fill calculations significantly.
Internal Cable Clamps (NEC 314.16(B)(2)): One or more internal cable clamps inside the box adds a single volume allowance of the largest conductor. External connectors on the outside of knockouts are not counted.
Common Standard Box Volumes (NEC Table 314.16(A))
| Box Type & Dimensions | Volume (in³) |
|---|---|
| 3×2×1½" Device Box | 7.5 in³ |
| 3×2×2" Device Box | 10.0 in³ |
| 3×2×2½" Device Box | 12.5 in³ |
| 3×2×3½" Device Box | 18.0 in³ |
| 4" Square, 1½" deep | 21.0 in³ |
| 4" Square, 2⅛" deep | 30.3 in³ |
| 4 11/16" Square, 2⅛" deep | 42.0 in³ |
| 4" Round, 2⅛" deep | 21.5 in³ |
Frequently Asked Questions
What is the NEC box fill calculation?
NEC Section 314.16 requires that the total volume of conductors, devices, clamps, and grounding wires inside a junction or device box does not exceed the box's rated cubic-inch capacity. Each wire gauge has a specific volume allowance per NEC Table 314.16(B), and each wiring device counts as twice the volume of the largest wire connected to it.
How many wires can fit in an electrical box?
The number of wires that fit in an electrical box depends on the box's cubic-inch volume and the wire gauges used. For 14 AWG wire, each conductor counts as 2.00 in³. A common 3×2×2½" box (12.5 in³) can hold up to 6 conductors of 14 AWG — but you must also account for devices, clamps, and ground wires that consume additional volume.
What is the volume allowance for 12 AWG wire in a box?
Per NEC Table 314.16(B), 12 AWG wire has a volume allowance of 2.25 cubic inches per conductor. This means each 12 AWG conductor entering the box uses 2.25 in³ of the box's rated capacity.
Do all ground wires count separately in box fill?
No. NEC 314.16(B)(5) states that all equipment grounding conductors combined count as a single conductor of the largest grounding conductor size present. So whether you have one or five ground wires, they count as just one volume allowance.
Does a switch or outlet add to box fill?
Yes. Each wiring device (switch, receptacle, GFCI) on a single strap or yoke counts as two times the volume of the largest conductor connected to it, per NEC 314.16(B)(4). A switch connected to 14 AWG wire adds 2 × 2.00 = 4.00 in³ to the total.
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