Breaker Size for Motor Calculator

Determining the correct breaker size for a motor is crucial for electrical safety and proper motor operation. This calculator helps you select the appropriate breaker size based on motor horsepower and voltage, following NEC (National Electrical Code) guidelines.

How to Use This Calculator

To use the breaker size calculator:

Enter the motor horsepower (HP) in the first field
Select the motor voltage (typically 120V, 208V, or 240V)
Click "Calculate" to determine the recommended breaker size
Review the result and any additional recommendations

The calculator will display the recommended breaker size in amperes and provide additional information about the calculation.

Formula Used

The breaker size calculation is based on the following formula:

Breaker Size (Amps) = (Motor Horsepower × 746) / (Voltage × Power Factor × 1.25)

Where:

Motor Horsepower - The power rating of the motor in horsepower (HP)
Voltage - The operating voltage of the motor (120V, 208V, or 240V)
Power Factor - Typically 0.85 for general-purpose motors (adjust if known)
1.25 - A safety factor to account for voltage drops and motor starting current

The result is rounded up to the nearest standard breaker size (e.g., 15A, 20A, 30A, etc.).

Worked Examples

Example 1: 5 HP Motor at 208V

Calculation:

Breaker Size = (5 × 746) / (208 × 0.85 × 1.25) Breaker Size = 3730 / 221.75 Breaker Size ≈ 16.8A

The recommended breaker size is 20A (rounded up from 16.8A).

Example 2: 10 HP Motor at 240V

Calculation:

Breaker Size = (10 × 746) / (240 × 0.85 × 1.25) Breaker Size = 7460 / 253.5 Breaker Size ≈ 29.4A

The recommended breaker size is 30A (rounded up from 29.4A).

Frequently Asked Questions

Why is the breaker size larger than the calculated value?

The calculation includes a safety factor (1.25) to account for voltage drops and motor starting current. The result is always rounded up to the nearest standard breaker size.

What if my motor has a different power factor?

The calculator uses a standard power factor of 0.85. If you know the specific power factor for your motor, you can adjust the calculation accordingly.

Can I use a smaller breaker than recommended?

No, using a smaller breaker than recommended can cause overheating and damage to the motor. Always use a breaker size equal to or larger than the calculated value.

What are the standard breaker sizes?

Standard breaker sizes are typically 15A, 20A, 30A, 40A, 50A, 60A, etc. The calculator rounds up to the nearest standard size.