You must select the conductor to cope with the current it will carry. If you are running multiple devices off the one outlet, you must sum the current for all devices to come up with your maximum current rating, then you can size the wire that supplies the outlet. If you are running DC motors, you should add 30% to the conductor size to cope with an inrush current. And use SAE cable.
The Redarc website has a handy cable size calculator.
https://www.redarc.com.au/handy-hint...ze_calculator/