How to size circuit breaker for motor

Sizing a circuit breaker for a motor requires careful consideration of several factors to ensure safe and reliable operation. Here's a step-by-step guide to help you size a circuit breaker for a motor:

  1. Determine the motor's full-load current: The full-load current (FLA) is the maximum current the motor draws when it's operating at its rated capacity. You can find the FLA in the motor's nameplate or manufacturer's documentation. Typical values for FLA are 1-10 times the motor's rated horsepower (HP).
  2. Calculate the motor's locked-rotor current: The locked-rotor current (LRA) is the maximum current the motor draws when it's starting or stalled. This value is usually 3-6 times the FLA. You can use the following formula to estimate LRA:

LRA (A) = 1.5 * FLA (A) * (HP / 746)

  1. Choose a circuit breaker rating: Select a circuit breaker with a rating that exceeds the motor's LRA. A common practice is to choose a breaker with a rating that is 125% to 150% of the motor's LRA.

For example, if the motor's LRA is 30 A, you would choose a circuit breaker with a rating of 37.5 A (125% of 30 A) or 45 A (150% of 30 A).

  1. Consider the motor's voltage and frequency: Ensure the circuit breaker is rated for the same voltage and frequency as the motor. Typical values are 120/240 VAC, 50/60 Hz.
  2. Check the circuit breaker's interrupting capacity: The interrupting capacity (IC) is the maximum current the breaker can interrupt safely. Ensure the breaker's IC rating is greater than the motor's LRA.
  3. Consider the motor's starting method: If the motor uses a soft-start or variable-frequency drive (VFD), you may need to derate the circuit breaker to account for the reduced starting current.
  4. Consult the motor's manufacturer documentation: The motor's manufacturer may provide specific recommendations for circuit breaker sizing. Always consult the documentation to ensure compliance with the motor's design and safety requirements.

Example calculation:

Motor: 10 HP, 230 VAC, 60 Hz FLA: 15 A LRA: 45 A (estimated using the formula above)

Choose a circuit breaker with a rating of 56.25 A (125% of 45 A) or 67.5 A (150% of 45 A).

Remember to always follow the manufacturer's recommendations and local electrical codes when sizing a circuit breaker for a motor.