Wire Size Calculator

How to Calculate Wire Size

Two Key Factors

When determining wire size, two requirements must be met:

1. Ampacity: The wire must handle the current without overheating (NEC Table 310.16)
2. Voltage Drop: The wire must deliver voltage within acceptable limits (typically 3% for branch circuits)

Ampacity Calculation

Minimum Ampacity = Load Current × 125% (for continuous loads)

Select wire size from NEC Table 310.16 that meets or exceeds this value.

Voltage Drop Calculation

Maximum Allowable Voltage Drop = System Voltage × (Max Drop % / 100)

For single phase: Wire Size based on Vd = (2 × L × I × R) / 1000

Common Wire Sizes and Ampacities

Copper conductors at 75°C:

• 14 AWG: 20A
• 12 AWG: 25A
• 10 AWG: 35A
• 8 AWG: 50A
• 6 AWG: 65A
• 4 AWG: 85A
• 2 AWG: 115A
• 1/0 AWG: 150A
• 4/0 AWG: 230A