Cold room mains and isolator correct sizing

Hi, im pricing a customer to build a cold room but abit unsure how to calculate the exact size cable required for the mains so it is safe. I know the unit requires a 20A circuit breaker which will have to be motor rated for the compressor but not sure what cable will be best for the job....also is the amp of the rotary isolator based on the cable ampage or the circuit breaker ampage?

thanks