What size transformer do you need to start a 15 hp motor?

To start a 15 hp motor, you typically need a transformer that can handle the motor's starting current, which can be significantly higher than its running current. Generally, the starting current can be 6 to 8 times the full-load current of the motor. For a 15 hp motor, the full-load current is about 18-20 amps at 230V or 10-12 amps at 460V, so the transformer should be rated to handle at least 100-160 amps. Additionally, it's advisable to consult the motor's specifications and consider factors like starting method and application to determine the exact transformer size.