Device Fill Calculations: Switches, Receptacles & More
Device fill is a critical component of box fill calculations. NEC 314.16(B)(4) requires a double volume allowance for each device yoke, which can quickly use up available box space.
What Counts as a Device?
Per NEC 314.16(B)(4), devices requiring fill allowance include:
- Receptacles (single, duplex, GFCI, etc.)
- Switches (single pole, 3-way, 4-way, dimmers)
- Combination devices (switch/receptacle)
- Smart switches and dimmers
- USB outlets
- Fan speed controls
The Double Volume Rule
Each device mounted on a yoke or strap requires TWO times the volume allowance of the largest conductor connected to that device.
Formula
Device Fill = 2 × (largest connected conductor volume)
Examples by Conductor Size
| Largest Connected Wire | Single Volume | Device Fill (2×) |
|---|---|---|
| 14 AWG | 2.00 cu.in. | 4.00 cu.in. |
| 12 AWG | 2.25 cu.in. | 4.50 cu.in. |
| 10 AWG | 2.50 cu.in. | 5.00 cu.in. |
Multiple Devices in One Box
Each device requires its own double volume allowance. For a double-gang box with two receptacles:
- Receptacle 1 (with 12 AWG): 2 × 2.25 = 4.50 cu.in.
- Receptacle 2 (with 12 AWG): 2 × 2.25 = 4.50 cu.in.
- Total device fill: 9.00 cu.in.
Special Cases
Multi-Gang Devices
Devices that occupy more than one yoke position (like some dimmer switches) require additional fill. Per NEC 314.16(B)(4) Exception: For each additional gang the device covers, add another double volume allowance.
Example: 2-Gang Dimmer
A dimmer that takes up 2 gang positions with 14 AWG wiring:
- Base fill: 2 × 2.00 = 4.00 cu.in.
- Additional gang: 2 × 2.00 = 4.00 cu.in.
- Total: 8.00 cu.in.
GFCI Receptacles
GFCI receptacles count the same as standard receptacles—they fit on a standard single-gang yoke. Just ensure you account for the additional load-side conductors.
Combination Devices
A device with both a switch and receptacle on one yoke counts as ONE device (one double volume), not two separate devices.
What Doesn't Count as Device Fill
- Wire nuts and splices (no fill required)
- Wire connectors (push-in, Wago, etc.)
- Blank cover plates
- Boxes without devices (junction boxes)
Recommended Box Sizes for Common Configurations
| Configuration | Minimum Box |
|---|---|
| Single receptacle, 2 cables | 4" sq. × 1-1/2" or 18 cu.in. device box |
| Switch + receptacle, 2 cables | 4" sq. × 2-1/8" |
| Double receptacle, 3 cables | 4-11/16" sq. or deeper |
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