Wire size for amperage is based on acceptable heat generated per unit length. House wiring standards for amperage is based on about 1 to 2 watts per foot maximum heat generation. It is less amperage for wire that has heavy insulation or placed in a plastic conduit which reduces ability to dissipate heat. Example of this is SOOW heavy extension cord wire. Power dissipation is I^2*R where R is the resistance of the wire.
Voltage is not a factor other then the insulation placed on the wire.