Breaker Size Calculator

This breaker size calculator finds the circuit breaker rating for a load. Enter the load current and the continuous load factor, and the breaker size calculator applies the 125 percent rule and rounds up to the next standard breaker size. It is a quick circuit breaker sizing calculator for single-phase and three-phase circuits once you know the current.

Load Current

Continuous Load Factor

Results

Enter your values and press Calculate to see results.

The Breaker Size Calculator is part of our Cable & Protection collection. Browse more cable & protection to size, convert, and check your work alongside this tool.

What the breaker size calculator shows

A breaker protects the wire in a circuit by tripping before it overheats. The breaker size calculator takes the load current, scales it by the continuous load factor, and picks the next standard breaker rating. That is the core of breaker sizing for most building circuits.

Determining circuit breaker size

Determining circuit breaker size is a two-step process:

Minimum rating = Load current × Factor ÷ 100

Breaker size = next standard size ≥ minimum rating

Standard sizes run 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and upward. The wire must then be rated at least as high as the breaker.

Circuit breaker sizing example

A continuous load drawing 32 A. The breaker size calculator gives:

Minimum = 32 × 1.25 = 40 A

Breaker size = 40 A

Use wire rated for at least 40 A with this breaker.

How to use the circuit breaker sizing calculator

Enter the load current in amps and the continuous load factor. For loads that run three hours or more, codes such as the NEC require 125 percent. The circuit breaker sizing calculator returns the breaker size, the minimum rating before rounding, and the load current. If you only know the power, convert it to amps first.

Reading the breaker sizing result

The breaker size is the smallest standard rating that covers the load with the required margin. Never install a larger breaker than the wire can carry, since the breaker protects the conductor. Motor circuits follow separate rules that allow larger breakers for starting current, so check the code for motors and HVAC equipment.

When to determine breaker size

Adding a circuit to a panel and needing to determine breaker size
Checking an existing breaker against its load
Sizing a feeder for a subpanel
Using a 3 phase breaker size calculator for industrial loads

Common breaker size calculator mistakes

Oversizing the breaker. It must not exceed the wire rating.
Skipping the 125 percent factor. Continuous loads need it.
Using single phase current for 3 phase loads. Use the line current formula.

Tips for accurate results

Use nameplate current, not a guess.
Leave spare capacity in the panel for future loads.
Have a licensed electrician confirm any new circuit.

Related to the breaker size calculator

Breaker and wire go together. The wire size calculator picks a conductor that matches the breaker, and the kW to amps calculator turns a power rating into the current you need here.

3 phase breaker sizing

For 3 phase breaker sizing, first find the line current: I = P ÷ (√3 × V × PF). A 15 kW load at 400 V and power factor 0.9 draws about 24 A, so with 125 percent the minimum rating is 30 A and a 30 A breaker fits. Enter that current here as you would for single phase. The wires for that circuit then run in conduit sized by the conduit fill calculator.

MCB size calculator vs motor protection

A miniature circuit breaker (MCB) for lighting and sockets follows the simple rule above, and an mcb size calculator applies it directly. Motors are different: their starting current can be six times the running current, so codes allow larger breakers or motor protection devices sized for the inrush.