Circuit Breaker & Fuse Size Calculator

Enter your electrical load parameters to get recommended circuit breaker size, fuse size, and wire gauge per NEC standards. Supports general, motor, lighting, and heating loads.

Load type

Voltage (V)

Phase

Enter load as

Load amps (A)

Calculated kW

Power factor

Default 0.85. Resistive loads (heating) use 1.0.

Continuous load

Applies 125% multiplier per NEC 210.20

About this calculator

Based on

✓ NEC 210.20 continuous load 125% multiplier
✓ NEC 240.6 standard ampere ratings for breakers and fuses
✓ NEC 310.16 copper conductor ampacities at 75C
✓ NEC 430.52 motor circuit breaker sizing (250% FLC for inverse-time)
✓ Single-phase and three-phase power formulas

Does not replace

✗ A licensed engineer's load calculation and design
✗ Derating for ambient temperature, conduit fill, or bundling
✗ Voltage drop calculations for long runs
✗ Local code amendments or AHJ-specific requirements

Electrical specs buried in datasheets and PDFs?

Rastro's electrical AI agent extracts breaker sizes, wire gauges, and load ratings from product datasheets, then maps them to structured catalog attributes at scale.

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