Junction Box Sizing Calculator
### National Electrical Code (NEC) Article 314 Box Fill Principles NEC Article 314.16 governs the maximum number of conductors allowed in standard outlet, device, and junction boxes to prevent.
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📐 Formula
💡 Practical Example
Wiring a commercial circuit with 4 x 14 AWG conductors, 4 x 12 AWG conductors, 1 clamp, and 1 ground wire: Requires 21.50 cubic inches total volume, fitting inside a standard 4" x 4" x 2-1/8" square junction box.
📖 About Junction Box Sizing Calculator
National Electrical Code (NEC) Article 314 Box Fill Principles
NEC Article 314.16 governs the maximum number of conductors allowed in standard outlet, device, and junction boxes to prevent electrical fires and thermal overload.
Volume Allowances per Conductor (NEC Table 314.16B)
- 14 AWG Wire: 2.00 in^3 per conductor
- 12 AWG Wire: 2.25 in^3 per conductor
- 10 AWG Wire: 2.50 in^3 per conductor
- Internal Cable Clamps: 1 conductor volume allowance based on the largest conductor present
- Equipment Grounding Conductors: 1 conductor volume allowance based on the largest grounding wire present
- Switches / Receptacles: 2 conductor volume allowances based on the largest conductor connected to the device
Common Standard Metal Box Capacities
- 4" × 4" × 1-1/2" Square Box: 21.0 in^3
- 4" × 4" × 2-1/8" Deep Square Box: 30.3 in^3
- 4-11/16" × 4-11/16" × 2-1/8" Deep Box: 42.0 in^3
How to Use This Calculator
Enter your 14 AWG Conductor Wires Count (e.g. 4 in wires), 12 AWG Conductor Wires Count (e.g. 4 in wires), 10 AWG Conductor Wires Count (e.g. 0 in wires), Internal Clamps & Device Yokes Count (e.g. 1 in yokes), and Equipment Grounding Wires Present (e.g. 1 in groups). Results update instantly as you type — adjust any value to compare scenarios.
The Formula
Internal clamps = 1 conductor allowance of largest gauge present
All grounding conductors = 1 conductor allowance of largest gauge present
Worked Example
Wiring a commercial circuit with 4 x 14 AWG conductors, 4 x 12 AWG conductors, 1 clamp, and 1 ground wire: Requires 21.50 cubic inches total volume, fitting inside a standard 4" x 4" x 2-1/8" square junction box.
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