NEC Code Compliance calculator
Box Fill Calculator
NEC 314.16 box fill calculator for U.S. electrical rough-in planning. Calculate required volume for conductors, devices, clamps, and grounding conductors in outlet boxes, switch boxes, and junction boxes before closing the installation.
Updated August 17, 2026
Example Calculations
How to Use
NEC 314.16 Box Fill Formula
Required Volume = Conductors + Devices + Clamps + Grounds + Fittings
NEC 314.16(B) Volume Allowances per Conductor
| Wire Size (AWG) | Volume (cu.in.) | Typical Use |
|---|---|---|
| 14 AWG | 2.00 | 15A lighting circuits |
| 12 AWG | 2.25 | 20A receptacle circuits |
| 10 AWG | 2.50 | 30A circuits |
| 8 AWG | 3.00 | 40A circuits |
| 6 AWG | 5.00 | 55A circuits |
Box Fill Counting Rules (NEC 314.16(B))
| Item | Count | Example |
|---|---|---|
| Conductors | 1× each | 6 wires = 6× volume |
| Devices (yoke) | 2× largest | Switch = 2× volume |
| Equipment Grounds | 1× total | All grounds = 1× volume |
| Internal Clamps | 1× total | All clamps = 1× volume |
| Support Fittings | 1× each | Hickey = 1× volume |
Calculation Example: Switch box with 2 cables (6× 14 AWG) + 1 device + grounds + clamps:
Conductors: 6×2.00 = 12.00 + Device: 2×2.00 = 4.00 + Ground: 2.00 + Clamp: 2.00 = 20.00 cu.in. minimum
Use Conduit Fill Calculator for raceway sizing. For wire sizing, use Wire Size Calculator.
Technical notes
Why Box Fill Limits Matter
NEC 314.16 box-fill rules keep conductor volume, device yokes, clamps, fittings, and equipment grounding conductors tied to the cubic-inch capacity of the box. Treat the result as a rough-in planning check: if the required volume exceeds the listed box volume, choose a larger box or reduce the contents before closing the installation.
Decoding the "Two Volume Allowance" Yoke Rule
NEC 314.16(B)(4) counts each yoke or strap containing one or more devices as a double volume allowance based on the largest conductor connected to that device. This matters in retrofits because deep receptacles, switches, and controls can consume more box volume than the visible conductor count suggests. If a 14 AWG device yoke is added, include the required device volume in the calculation before deciding whether the existing box is large enough.
The Ground Wire Exception: NEC 314.16(B)(5)
NEC 314.16(B)(5) treats equipment grounding conductors differently from current-carrying conductors. Count them according to the adopted code rule and the largest equipment grounding conductor present, then verify whether additional equipment grounding conductors change the allowance for the edition and installation path being used.
Common Applications
Frequently Asked Questions
How do I calculate box fill for a switch with multiple cables?
Count each conductor entering the box (hot, neutral, travelers), add 2× for each device yoke, 1× for all grounds combined, and 1× for internal clamps. Multiply by the volume per conductor from NEC 314.16(B). Example: 3-way switch with 3 cables (9 conductors): 9×2.00 + 2.00 (device) + 2.00 (ground) + 2.00 (clamp) = 22 cu.in. minimum.
What size box do I need for a standard 20A duplex receptacle?
A typical 20A receptacle with 2 cables (4 conductors) plus grounds and clamps needs: 4×2.25 (conductors) + 4.50 (device) + 2.25 (ground) + 2.25 (clamp) = 18.00 cu.in. A standard single-gang 18 cu.in. PVC box is adequate. For through-circuit wiring, use a 20+ cu.in. box.
Do all ground wires count separately for box fill?
No. Per NEC 314.16(B)(5), all equipment grounding conductors count as only one conductor volume combined, based on the largest ground wire size. For example, 4 bare 14 AWG grounds = only 2.00 cu.in. total, not 8.00 cu.in.
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