Motor Breaker Sizing Calculator

Proper motor breaker sizing is critical for electrical safety and system efficiency. This calculator helps determine the appropriate breaker size for electric motors based on their specifications and load characteristics.

How to Use This Calculator

To determine the proper breaker size for your electric motor:

Enter the motor's full load current in amperes (A)
Select the motor's voltage rating (typically 120V, 208V, 230V, or 460V)
Choose the motor's service factor (typically 1.15 for continuous duty or 1.25 for intermittent duty)
Click "Calculate" to determine the recommended breaker size

The calculator will display the recommended breaker size based on the National Electrical Code (NEC) standards and provide additional information about the calculation.

Formula Used

The recommended breaker size is calculated using the following formula:

Breaker Size (A) = (Motor Current × Service Factor) + 10%

Where:

Motor Current = Full load current of the motor in amperes
Service Factor = 1.15 for continuous duty, 1.25 for intermittent duty
The additional 10% accounts for inrush current and voltage drop

For example, a 20A motor with a service factor of 1.15 would require a breaker size of:

(20 × 1.15) + 10% = 23 + 2.3 = 25.3A

Rounding to standard breaker sizes, this would typically be a 25A breaker.

Worked Example

Let's calculate the breaker size for a 30A motor with a 208V rating and continuous duty service factor:

Motor Current = 30A
Service Factor = 1.15
Calculation: (30 × 1.15) + 10% = 34.5 + 3.45 = 37.95A
Standard breaker sizes are typically 30A, 40A, 50A, etc.
Recommended breaker size: 40A

This ensures the motor receives adequate protection while accounting for inrush current and voltage drop.

Important Considerations

Motor Type

Different motor types have different starting currents:

Induction motors typically have higher starting currents than synchronous motors
Explosion-proof motors may require additional protection

Voltage Drop

The calculated breaker size accounts for a 3% voltage drop in the circuit. For longer runs, you may need to select a larger breaker to compensate for additional voltage drop.

Motor Protection

In addition to the breaker, consider installing:

Motor overload relays
Ground fault circuit interrupters (GFCIs)
Thermal overload protection

Local Codes

Always consult your local electrical code requirements, as they may vary from the NEC standards used in this calculator.

Frequently Asked Questions

What is the difference between continuous and intermittent duty?

Continuous duty motors run at full load continuously, while intermittent duty motors operate in cycles with periods of rest. Continuous duty motors typically use a service factor of 1.15, while intermittent duty motors use 1.25.

Why is the calculated breaker size larger than the motor's rated current?

The calculated size accounts for inrush current (the initial surge of current when the motor starts) and voltage drop in the circuit. This ensures the motor receives adequate protection while operating efficiently.

Can I use a smaller breaker than the calculated size?

No, using a smaller breaker than calculated could result in overheating, tripping, or damage to the motor. Always use a breaker size that is equal to or larger than the calculated size.

What if my motor has a different service factor?

If your motor has a different service factor than the standard 1.15 or 1.25, you should use that specific value in the calculator. Always refer to the motor's nameplate or manufacturer specifications for the correct service factor.