Your wiring should be rated to your maximum continuous amperage. So, #8AWG (American Wire Gauge) for a 40A circuit, #6AWG for 50A circuit, etc. Unusually warm charging conditions (batteries inclusive) and/or long run length will demand even larger gauge wiring.
Please refer to the wire ampacities table (link below) for minimums. We suggest the 60˚C column for ampacity ratings which allows for anticipated heat loss and inefficiencies: Wire Ampacities