# Circuit breaker calculation for 30 horsepower three phase electric motor

A great question!

To calculate the circuit breaker rating for a 30 horsepower three-phase electric motor, we'll need to consider a few factors:

- Motor power rating: 30 horsepower (HP) = 22.4 kW (kilowatts)
- Motor voltage: Typically, three-phase motors operate at 230V, 460V, or 575V. Let's assume 460V for this example.
- Motor current: We can estimate the motor current using the motor power rating and voltage. A common formula is:

Motor current (I) = Motor power (P) / Motor voltage (V)

I = 22.4 kW / 460V ≈ 48.7 amps

- Motor inrush current: Motors often have a high inrush current when starting, which can be 2-5 times the normal operating current. Let's assume a 3-4 times inrush current, which is a reasonable estimate.

Inrush current (Iin) = 3-4 × 48.7 amps ≈ 146-193 amps

- Circuit breaker rating: The circuit breaker rating should be able to handle the motor's inrush current and normal operating current. A common practice is to select a circuit breaker with a rating that is 125% of the motor's full-load current.

Circuit breaker rating (Icb) = 1.25 × 48.7 amps ≈ 61.1 amps

Considering the inrush current, it's recommended to select a circuit breaker with a higher rating to ensure it can handle the initial surge. A common practice is to add 10-20% to the calculated rating.

Circuit breaker rating (Icb) = 61.1 amps × 1.15 (10% increase) ≈ 70.3 amps

Based on these calculations, a suitable circuit breaker rating for a 30 horsepower three-phase electric motor operating at 460V would be:

- 70.3 amps (or the next available standard rating, such as 70 amps or 75 amps)

Please note that this calculation is an estimate and should be verified by a licensed electrician or a qualified engineer. Additionally, other factors such as the motor's efficiency, power factor, and the electrical system's design should also be considered when selecting a circuit breaker.