depends purely on your charger.
look at the ampere rating it needs.
cables available in the market are already normally rated to 600 volts, so you dont have to worry about that.
you could get away with a twin core. but do it properly and add a proper earth wire.
if your charger is 3 phase, you will need a 5 core wire.
7/0.044 (equal to 6mm wire) is good for upto 32 amps.