2 AWG in Aluminium is required.
Depends on length. The more length, the more resistance.
12 AWG should be fine since resistance of wire is only about .16 ohms per 100 ft.
A #3 copper wire with an insulation factor of 75 and 90 degrees C is rated at 100 and 105 amps respectively.
You size it by the current required by the secondary, and the primary will be smaller wire.
Check the web for sites to calculate the required wire size for the suggested load.
#6 wire stranded
what size wire would be required for an ak012 with a power supply of 208/240-3-60 hz
Service wire for 100 amps requires AWG #3 copper.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.