Breaker Size Calculator

How to Calculate Circuit Breaker Size

A circuit breaker has one job: to open (trip) before the wire in the circuit overheats dangerously. It protects the wire, not the appliance. Sizing a breaker correctly is critical for safety - undersized breakers trip on normal loads and create pressure to bypass the protection; oversized breakers fail to trip before the wire melts, potentially starting fires inside walls.

This calculator applies NEC 210.20(A) rules for continuous loads (125% factor), non-continuous loads (100% factor), and motor branch circuits (250% factor per NEC 430.52), then rounds up to standard breaker sizes listed in NEC 240.6(A).

Core Formula: Watts to Amps

The fundamental relationship is Ohm's Law adapted for AC power:

For three-phase circuits:

Continuous vs Non-Continuous Loads

The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. This distinction matters because breakers are designed to carry 80% of their rating continuously. Per NEC 210.20(A), the breaker and feeder wire must be sized to 125% of the continuous load plus 100% of the non-continuous load:

Load Type Examples

Motor Branch Circuit Sizing (NEC 430.52)

Motors draw 5-7x their running current during startup (inrush current). A standard breaker sized to running current would trip every time the motor starts. NEC 430.52 solves this by allowing the branch breaker to be 250% of the motor full-load amperage (FLA). The motor itself is protected separately by overload relays or built-in thermal protection.

The conductors, however, are sized at 125% of the FLA per NEC 430.22. This is why motor circuits have small wires but large breakers.

Standard Breaker Sizes (NEC 240.6)

Breakers are manufactured in specific standard sizes. After calculating your adjusted load amps, round UP to the next standard size:

Common Residential Breakers by Circuit

Worked Example: Water Heater

Size a circuit for a 4,500 W electric water heater on 240V:

1. Amps - 4500 / 240 = 18.75 A
2. Type - Continuous (runs more than 3 hours when heating)
3. Factor - 18.75 × 1.25 = 23.4 A
4. Next standard breaker - 25 A (or 30 A for margin)
5. Wire size - 10 AWG copper THHN (30 A ampacity at 75C)

Most installations use a 30 A breaker with 10 AWG for margin, even though 25 A technically works. The cost difference is negligible and startup tolerance is better.

Worked Example: EV Charger

Size a circuit for a Level 2 EV charger rated 48 A at 240 V:

1. Charger rating - 48 A (11.5 kW)
2. Type - Continuous (charging 6+ hours)
3. Factor - 48 × 1.25 = 60 A
4. Breaker - 60 A dedicated 2-pole
5. Wire size - 6 AWG copper THHN (65 A ampacity at 75C)

Power Factor and 3-Phase Systems

Resistive loads (heaters, incandescent bulbs, electric stoves) have a power factor (PF) of 1.0 - volts times amps equals watts exactly. Inductive loads (motors, fluorescent ballasts, transformers) have PF below 1.0 because current lags voltage. The apparent power (VA) is always larger than real power (watts) by the reciprocal of PF.