Electrical Box Fill Calculator
Calculate electrical box fill requirements per NEC 2020 Article 314.16(B)
Calculate Electrical Box Fill Requirements
Count each conducting wire entering the box (regardless of size)
The largest wire gauge among all conducting wires entering the box
Internal clamps require 1 volume allowance. External clamps do not count.
One or more luminaire studs or hickeys require 1 volume allowance
Each device (switch, outlet, fixture) counts as 2 volume allowances
NEC 2020: 1-4 grounding wires = 1 allowance, 5+ wires = additional ¼ allowance each
The largest grounding wire gauge entering the box
Box Fill Requirements
NEC 2020 Reference: Article 314.16(B) Box Volume Calculations
Largest Conductor Volume: 12 AWG = 2.25 in³ (36.9 cm³) per allowance
Recommended Standard Box Sizes
...and 10 more options
⚠️ Electrical Safety Warning
• This calculator is for estimation purposes only
• Always consult a licensed electrician for electrical installations
• Follow local electrical codes and NEC requirements
• Overfilled boxes create fire hazards and code violations
• Verify box capacity markings before final installation
Example Calculation
Kitchen Outlet Box
Application: Kitchen GFCI outlet installation
Conducting Wires: 6 × 12 AWG + 2 × 16 AWG = 8 wires total
Devices: 2 convenience outlets
Grounding Wires: 5 × 12 AWG grounding conductors
Internal Clamps: Yes
Support Fittings: No
NEC 2020 Calculation
• Conducting Wires: 8 allowances × 2.25 in³ = 18.0 in³
• Internal Clamps: 1 allowance × 2.25 in³ = 2.25 in³
• Devices: 4 allowances × 2.25 in³ = 9.0 in³
• Grounding (5 wires): 2 allowances × 2.25 in³ = 4.5 in³
Total: 14.25 allowances = 32.06 in³
Recommended: 4¹¹⁄₁₆×4¹¹⁄₁₆×2⅛ Square (42.0 in³)
NEC 2020 Wire Volumes
Per Conductor (Table 314.16B)
Volume Allowance Rules
Common Box Applications
Understanding Electrical Box Fill Calculations
Why Box Fill Matters
Proper electrical box fill calculations ensure safe installations that comply with the National Electrical Code (NEC). Overfilled boxes can cause overheating, difficult installations, and potential fire hazards.
NEC 2020 Updates
- •Grounding wire calculations simplified: 1-4 wires = 1 allowance
- •Additional grounding wires count as ¼ allowance each
- •Based on largest grounding conductor, not largest overall conductor
- •Pigtail wires that remain in box don't count
Calculation Components
Vtotal = Vw + Vc + Vs + Vd + Vg
- Vw: Conductor fill volume
- Vc: Clamp fill volume
- Vs: Support fittings volume
- Vd: Device fill volume
- Vg: Grounding conductor volume
Professional Installation Required: This calculator provides estimates only. Always consult a licensed electrician for actual installations and follow local electrical codes.