What size of a circuit breaker for a 400v 64 amps?

To select a circuit breaker for a 400V system with a load of 64 amps, you typically choose a breaker rated for at least 125% of the continuous load. Therefore, the minimum rating would be 64 amps x 1.25 = 80 amps. A circuit breaker rated at 80 amps would be appropriate, ensuring it can safely handle the load without tripping under normal conditions. Additionally, consider the type of load and application, as well as any relevant local electrical codes.