Circuit Breaker Sizing Calculator (NEC 80% / 125% Rule)
Panel schedule planning, safe breaker selection for EV chargers, heaters, and lighting. Applies NEC 125% continuous load rule (loads operating 3+ hours) to recommend standard standard thermal-magnetic breaker sizes.
Step-by-Step Derivation ▼ Show Steps
Calculations adhere to standard electrical engineering physics and National Electrical Code (NFPA 70) regulations.
Engineering Principles & Calculations
This calculator uses the exact mathematical relationships between Continuous Load (Watts/Amps), Non-Continuous Load (Watts/Amps), Voltage (120V / 240V / 208V / 480V) to determine Calculated Design Amps, Recommended Breaker Size (15A, 20A, 30A, 40A, 50A, 60A, etc.), Minimum Wire Gauge (AWG). In practical installations, factors such as ambient raceway temperature, harmonic distortion, duty cycle continuous operation, and conductor impedance must be accounted for by a certified electrician.
Applies NEC 125% continuous load rule (loads operating 3+ hours) to recommend standard standard thermal-magnetic breaker sizes.
Typical Application & Industry Guidelines
Panel schedule planning, safe breaker selection for EV chargers, heaters, and lighting.
Circuit Breaker Sizing Calculator (NEC 80% / 125% Rule) Benchmark Lookup Table
| Standard Breaker Size | Max 80% Continuous Load | Max Continuous Watts @ 120V | Min Copper Wire Gauge |
|---|---|---|---|
| 15 Amp Breaker | 12.0 Amps | 1,440 Watts | 14 AWG Copper |
| 20 Amp Breaker | 16.0 Amps | 1,920 Watts | 12 AWG Copper |
| 30 Amp Breaker | 24.0 Amps | 2,880 Watts | 10 AWG Copper |
| 40 Amp Breaker | 32.0 Amps | 3,840 Watts | 8 AWG Copper |
| 50 Amp Breaker | 40.0 Amps | 4,800 Watts | 6 AWG Copper |
| 60 Amp Breaker | 48.0 Amps | 5,760 Watts | 4 AWG Copper |