How to calculate wires and switches based on the capacity of the motor? - Solutions - Huaqiang Electronic Network

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SMD aluminum electrolytic capacitor

When selecting the appropriate circuit breaker, contactor, thermal element, and wiring for a three-phase squirrel cage induction motor, the following calculation method can be used: 1. If the motor power is N kW, the rated current of the motor is approximately 2N A. This value is usually very close to the rated current listed on the motor’s nameplate. You can double-check this using the motor manual. This formula is highly accurate. For example: a 7.5 kW motor has a rated current of 15A; a 55 kW motor has a rated current of 110A. The rated current is the most important factor when choosing the circuit breaker, contactor, and wire size. 2. To select the circuit breaker, it's generally recommended to choose a rating of about 4N A. For example, if the motor is 7.5 kW with a rated current of 15A, the circuit breaker should be rated at 32A. For a 55 kW motor with a rated current of 110A, the circuit breaker should be rated at 250A. Note that for fans and pumps, which have longer starting times and higher starting torque, it's better to choose a slightly larger circuit breaker. 3. Contactors are typically selected based on 3.5 to 4 times the motor’s rated current. 4. For wire selection, it's generally recommended to use a wire size that can handle 1.5 times the motor’s rated current. However, you should also consider the installation environment, such as whether the wire is in conduit or exposed, and apply an appropriate derating factor accordingly.