How to size a breaker for a motor

Sizing a breaker for a motor involves calculating the motor's current draw and selecting a breaker that can safely interrupt the current in the event of an overload or short circuit. Here's a step-by-step guide to help you size a breaker for a motor:

  1. Determine the motor's full-load current: You can find the motor's full-load current in the motor's nameplate or in the manufacturer's documentation. If you can't find this information, you can estimate it using the motor's horsepower (hp) and voltage (V) ratings. A common formula is:

Full-load current (A) = Motor horsepower (hp) x 0.746 / Voltage (V)

For example, a 10 hp motor running at 230V would have a full-load current of:

Full-load current (A) = 10 hp x 0.746 / 230V ≈ 37.5 A 2. Calculate the motor's inrush current: The inrush current is the initial surge of current when the motor starts. This current is typically 2-5 times the full-load current. For our example, the inrush current would be:

Inrush current (A) = 37.5 A x 3 (a common multiplier) ≈ 112.5 A 3. Choose a breaker with a sufficient interrupting rating: The interrupting rating is the maximum current the breaker can safely interrupt in a short circuit. The interrupting rating is usually expressed in amperes (A) or kiloamperes (kA). For our example, we would need a breaker with an interrupting rating of at least 112.5 A. 4. Consider the motor's locked-rotor current: The locked-rotor current is the maximum current the motor draws when it's stalled (i.e., not rotating). This current is typically 2-5 times the full-load current. For our example, the locked-rotor current would be:

Locked-rotor current (A) = 37.5 A x 3 (a common multiplier) ≈ 112.5 A 5. Choose a breaker with a sufficient trip setting: The trip setting is the current at which the breaker will trip (i.e., open). The trip setting should be set slightly higher than the motor's locked-rotor current to ensure the breaker trips before the motor overheats. For our example, we would set the trip setting to around 120 A (a common setting). 6. Check the breaker's thermal rating: The thermal rating is the maximum current the breaker can handle continuously without overheating. The thermal rating should be higher than the motor's full-load current. For our example, the breaker's thermal rating should be at least 37.5 A. 7. Consult the breaker's documentation: Check the breaker's documentation to ensure it meets the above requirements and is suitable for the motor's application.

Some general guidelines to keep in mind:

Remember to always follow the manufacturer's recommendations for the motor and breaker, and consult with a licensed electrician if you're unsure about the sizing or installation of the breaker.