Determining the thickness of the power cables for EV applications: The German technical norm VDE 0298-4/2003 gives the answer. Please see the summary in the table. Simply said: 2.5 mm2 can handle 30A, 10 mm2 can handle 60A, 25 mm2 can handle 120A, 50 mm2 is good up to 170A, and for 300A currents you should use 120 mm2 cables.