Enter the load power and the source voltage into the calculator to estimate the circuit current. Then select a circuit breaker rating at or above that current (often the next standard size). For continuous loads, common practice is to size the circuit and overcurrent protection at 125% of the continuous load current—always follow your applicable electrical code and manufacturer instructions.

Circuit Breaker Size Calculator

Enter any 2 values to calculate the missing variable. Assumes single-phase/DC with power factor (PF) = 1. For AC loads with PF < 1, use apparent power (VA) or divide watts by PF before using this calculator. The hp option uses 1 mechanical hp ≈ 745.7 W and does not account for motor efficiency or PF.

Circuit Breaker (Current) Formula

The following formula is used to calculate the circuit current from power and voltage (and is often used as a starting point when selecting a circuit breaker).

A = P / V
  • Where A is the current in amperes (amps)
  • P is the power. For DC or resistive AC loads, this can be watts (W). For many AC loads (especially motors), current depends on power factor; you may need apparent power in volt-amperes (VA) or use I = P / (V × PF).
  • V is the source voltage (volts). For example, common receptacle voltages are about 120 V in the US/Canada and about 230 V in many other countries; 240 V is commonly used in the US/Canada for larger appliances.

To calculate the current, divide the power by the voltage. In practice, selecting a “proper” breaker can require additional factors (for example continuous-load sizing at 125%, conductor ampacity, ambient temperature, and local code requirements).

Circuit Breaker Definition

What is a circuit breaker?

A circuit breaker is a device that stops the flow of current in an electrical circuit by opening the circuit when an overcurrent condition occurs (such as an overload or short circuit).

For example, a breaker rated at 1000 amps typically will not trip instantly at a small overload like 1005 amps; breakers follow time-current trip curves, meaning slight overloads may be allowed for some time while larger overcurrents trip faster (and short circuits can trigger near-instantaneous trip depending on the breaker type).

Example Problem

How to calculate circuit breaker size?

First, determine the power needed for the application. For this example, the power is found to be 100 watts (assuming a resistive load or PF = 1).

Next, determine the source voltage. In this case, the source voltage is found to be 50 volts.

Finally, calculate the current using the formula above:

A = P / V

A = 100/50

A = 2 amps.

The calculated load current is 2 amps. In practice, you would typically choose a standard breaker rating at or above this value (and apply any required code factors, such as 125% for continuous loads: 2 × 1.25 = 2.5 amps).