Calculate circuit breaker size, load power, or source voltage from any two values, with W, kW, hp, V, kV, A, and kA units.

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 Size Formula

The calculator uses the basic power relationship for DC or single-phase loads when power factor is 1. For AC loads with a power factor below 1, use apparent power in volt-amperes (VA), or divide watts by power factor before entering the load power.

I = P / V
P = V * I
V = P / I
  • I = current or estimated breaker rating, in amperes (A)
  • P = load power, in watts (W) or volt-amperes (VA)
  • V = source voltage, in volts (V)

If you enter load power and voltage, the calculator finds current using I = P / V. If you enter voltage and current, it finds load power using P = V * I. If you enter load power and current, it finds source voltage using V = P / I.

The unit selectors convert kW to watts, hp to watts, kV to volts, and kA to amps before the formula is applied. The hp option uses 1 hp = 745.7 W and does not include motor efficiency or power factor.

Common Breaker Sizes and Load Current Reference

After calculating current, choose a breaker size that is allowed for the circuit, conductor size, load type, and electrical code that applies. The table below shows common breaker ratings and the approximate maximum continuous load often associated with the 80% loading rule.

Breaker rating Approx. 80% continuous load Typical use note
15 A 12 A Lighting and general small loads
20 A 16 A General receptacle circuits
30 A 24 A Larger dedicated loads
40 A 32 A Higher-power appliances or equipment
50 A 40 A Large dedicated circuits
60 A 48 A Subpanels or large equipment, when permitted

Power, Voltage, and Current Examples

Load power Voltage Calculated current
1,500 W 120 V 12.5 A
2,400 W 240 V 10 A
5,000 W 240 V 20.83 A
10 kW 240 V 41.67 A

Example Calculations

Example 1: Find the current for a 1,500 W load on 120 V

Use the current formula:

I = P / V

Substitute the values:

I = 1500 / 120 = 12.5 A

The load current is 12.5 A. If this is a continuous load, breaker sizing may need additional adjustment, such as using 125% of the continuous current where required.

Example 2: Find the load power on a 240 V circuit with 30 A current

Use the power formula:

P = V * I

Substitute the values:

P = 240 * 30 = 7200 W

The load power is 7,200 W, or 7.2 kW.

FAQs

Is the calculated current the same as the breaker size I should install?

Not always. The calculated current is the expected load current from the power and voltage values you entered. The actual breaker size must also match the conductor ampacity, circuit type, temperature rating, load type, and applicable electrical code. Continuous loads may require sizing at 125% of the load current.

How do I use this calculator for an AC load with power factor below 1?

If you know apparent power in VA, enter that value as the load power. If you only know real power in watts and the power factor, divide watts by power factor first. For example, a 1,000 W load at 0.8 power factor is 1,250 VA, because 1,000 ÷ 0.8 = 1,250.

Does the hp input give an exact motor breaker size?

No. The hp input converts mechanical horsepower to watts using 1 hp = 745.7 W. It does not account for motor efficiency, power factor, starting current, service factor, or motor circuit rules. Motor breaker sizing often requires separate code-based calculations.