How much current and how far? What is the load? Is it mostly resistive or is it reactive? My initial guess is regular THHN would be fine unless there is strict requirements as far as losses are concern. My only significant concern would be the insulation voltage rating if the load was reactive and the VSWR (Voltage Standing Wave Ratio)due to mismatched impedances was excessive.
Obviously circuit breakers and transformers are going to be outside their approval rating operating conditions.
Larry C
