That is a really vague question. Will the load be balanced? Will the load be inductive or resistive? What will the demand factor be? High, medium, or low voltage? I see power system implies facility to me as opposed to a building, MCC, or dedicated branch circuit. Multiple or single point of use? I am also assuming that your cable descrioption is meant to convey a 4 conductor, copper, SWA type of cable. You can get SWA cable good for at least 35KV. What will your harmonic content be from the load? I could ask for a lot more information and most if not all would be pertinent to the choice of conductors sized to meet the ampacity of the load to be served. The breaker will be sized to protect the conductors. Assuming your equipment is properly sized. Is this a transformer feed?
A non engineers( NEC) answer would be ""FOR LESS THAN 600v"""""
< Feeder conductors shall have an ampacity not less than required to supply the load. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125% of the continuous load> Now don't forget to consider voltage drop at the farthest outlet. Will the cable cable be direct burial, cable tray, a lone run or adjacent to others, in conduit, or in free air. These are all little tidbits of a large equation that is liike a jigsaw puzzle. Do not forget the ever influential ambient temp. and his pesky cousin the temp rating of the equipment terminations. 75, 90, 105 degrees C terminations influences the ampacity selection of the conductors. Architecht or engineer submittals would help. At least the above variables should be known, or mopst of them and then those can be used to calculate the remainder. Sorry, not a simple task or maybe a very simple task. But no single generic answer would serve your question properly. Hope my sentence and paragraph structure is more acceptable.
