16 mm to 18 mm
To calculate the wire size, a system voltage is needed.
5 mm
To answer this question the voltage of the system is needed.
The National Electrical Code calls for #4 copper or #2 aluminum.
Assuming you are working with 240v, you need 4/0 copper conductors.
10 gauge will suffice.
In North America it takes two "hot" wires to obtain 240 volts.
To answer this question the voltage and load amperage is needed.
AWG 16
Wire size is based of the amperage of the device. To answer this question the amperage is needed or the wattage and voltage of the device.
A #14 wire will do fine for grounding a 20 amp device. That is the size of the ground wire in a 2 conductor # 12 wire building cable.
Don't!