Properly sizing a circuit breaker for an electric motor is critical for both operational reliability and safety. Unlike standard resistive loads, motors experience high "inrush" current when starting. This calculator helps you determine the correct breaker size based on NEC (National Electrical Code) guidelines.
Motor Breaker Sizing Tool
*Calculations based on NEC Table 430.248/430.250 approximations.
Understanding Motor Breaker Sizing
Electric motors are unique electrical loads. When you first flip the switch, a motor can pull 600% to 800% of its rated current for a fraction of a second. If you sized your breaker exactly at the motor's Full Load Amps (FLA), the breaker would trip every single time you tried to start the machine.
The NEC Article 430 Rule
The National Electrical Code (NEC) provides specific guidelines in Article 430 for "Motor Branch-Circuit Short-Circuit and Ground-Fault Protection." The goal is to provide a breaker large enough to handle the starting spike but small enough to protect the circuit from a catastrophic short circuit.
- Inverse Time Breakers: These are your standard residential and commercial breakers. The NEC typically allows these to be sized at 250% of the motor's FLA.
- Dual-Element Fuses: Because these have a built-in time delay, they are usually sized at 175% of the FLA.
- Overload Protection: It is important to remember that the circuit breaker is NOT there to protect the motor from overheating due to a slight overload. That is the job of the overload heater or motor starter. The breaker is there to protect against short circuits.
How to Use This Calculator
To get an accurate result, follow these steps:
- Enter the Horsepower rating found on the motor's nameplate.
- Select the operating voltage. Note that 3-phase motors require significantly less current than single-phase motors for the same amount of work.
- Choose the protection type. In 90% of modern applications, you will be using an Inverse Time Breaker.
- The calculator will provide the estimated FLA and the next standard breaker size.
Standard Breaker Sizes
According to NEC 240.6, standard ampere ratings for fuses and inverse time circuit breakers are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600.
If your calculated value falls between two standard sizes, the NEC generally allows you to "round up" to the next standard size, provided it does not exceed the maximum percentage allowed by Table 430.52.