18 AWG Wire Box Fill
18 AWG conductors are typically used for low-voltage applications and control wiring.
1.5
cu.in. per conductor
24.6
mm³ per conductor
7A
Max Ampacity
Common Applications
Box Capacity for 18 AWG
Maximum number of 18 AWG conductors that fit in common electrical boxes (conductors only, not accounting for devices, clamps, or grounds).
| Box Type | Volume | Max 18 AWG* |
|---|---|---|
| 4" Square x 1-1/4" Deep | 18 cu.in. | 12 |
| 4" Square x 1-1/2" Deep | 21 cu.in. | 14 |
| 4" Square x 2-1/8" Deep | 30.3 cu.in. | 20 |
| 4-11/16" Square x 1-1/4" Deep | 25.5 cu.in. | 17 |
| 4-11/16" Square x 1-1/2" Deep | 29.5 cu.in. | 19 |
| 4-11/16" Square x 2-1/8" Deep | 42 cu.in. | 28 |
| Octagon Box 1-1/2" Deep | 15.5 cu.in. | 10 |
| 4" Octagon Box 1-1/2" Deep | 15.5 cu.in. | 10 |
| Octagon Box 2-1/8" Deep | 21.5 cu.in. | 14 |
| Single Gang Device Box 2-3/4" Deep | 14 cu.in. | 9 |
| Single Gang Device Box 18 cu.in. | 18 cu.in. | 12 |
| Single Gang Device Box 3-1/2" Deep | 18 cu.in. | 12 |
*Theoretical maximum. Use our box fill calculator for actual installation capacity including devices and other fill factors.
Calculate Box Fill with 18 AWG
Enter your 18 AWG conductor count and other components for instant NEC compliance verification.
Open Box Fill Calculator18 AWG Conductor Volume Allowance
Per NEC Table 314.16(B), 18 AWG conductors require 1.5 cubic inches (24.6 mm³) of box volume each. This volume allowance applies to:
- Each conductor originating outside the box and terminating or splicing inside
- Conductors passing through without splice (counted once per conductor)
- Device fill calculations (based on largest conductor connected)
- Equipment grounding conductor fill (based on largest ground)
When to Use 18 AWG Wire
18 AWG conductors are rated for 7 amps and commonly used for thermostat wiring, doorbell circuits, low-voltage controls. Always verify conductor sizing meets circuit requirements per NEC Article 310.
Box Fill Calculation Example
For a receptacle installation with two 18/2 NM cables:
- 4 × 18 AWG conductors = 4 × 1.5 = 6.00 cu.in.
- 2 × grounds (count as 1) = 1 × 1.5 = 1.5 cu.in.
- Internal clamps = 1 × 1.5 = 1.5 cu.in.
- 1 × receptacle = 2 × 1.5 = 3.00 cu.in.
- Total: 12.00 cu.in. minimum box volume required
Recommended Box Sizes for 18 AWG
Based on typical installations, consider these box sizes for 18 AWG wiring:
- 4" Square × 1-1/2" (21 cu.in.) - Good for single device with 2 cables
- 4" Square × 2-1/8" (30.3 cu.in.) - Better for 3+ cables
- 4-11/16" Square × 2-1/8" (42 cu.in.) - Best for multiple devices