Circuit Breaker Calculator

How to Size a Circuit Breaker

Basic Formula

Current (Amps) = Power (Watts) / Voltage (Volts) Breaker Size = Current × 1.25 (for continuous loads)

Continuous vs. Non-Continuous Loads

Continuous Load (3+ hours): Size breaker at 125% of calculated current. The NEC defines continuous load as operating for 3 hours or more.

Non-Continuous Load: Size breaker at 100% of calculated current. Examples include lighting, receptacles, and most general-purpose circuits.

Standard Breaker Sizes

Standard circuit breaker amperage ratings (NEC 240.6):

• 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A (common sizes)
• 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A (larger sizes)
• 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000A (commercial/industrial)

Breaker and Wire Size Relationship

The minimum wire size must be rated for at least the breaker size:

• 15A breaker: 14 AWG copper minimum
• 20A breaker: 12 AWG copper minimum
• 30A breaker: 10 AWG copper minimum
• 40A breaker: 8 AWG copper minimum
• 50A breaker: 6 AWG copper minimum
• 60A breaker: 6 AWG copper or 4 AWG aluminum
• 100A breaker: 4 AWG copper or 2 AWG aluminum

Common Applications

• Lighting circuits: 15A breaker, 14 AWG wire
• General receptacles: 20A breaker, 12 AWG wire
• Small appliances (kitchen): 20A breaker, 12 AWG wire
• Dryer: 30A breaker, 10 AWG wire (240V)
• Electric range: 40-50A breaker, 6-8 AWG wire (240V)
• AC unit: sized per nameplate, typically 30-60A
• Water heater: 30A breaker (4500W) or 38A breaker (5500W), 10 AWG
• EV charger: sized per manufacturer, typically 40-60A