Quick answer
The cable from a DG set is sized on the set's full-load current, not on the kVA badge alone. For a three-phase set the current is kVA multiplied by 1000, divided by 1.732 times the line voltage. At 415 V that works out to about 1.39 A for every kVA, so a 250 kVA set draws roughly 348 A at full load, and the cable and breaker are chosen above that figure with room to spare. Common practice is a 3.5-core armoured cable, usually aluminium in the larger sizes, with an added margin for the high ambient inside a canopy or plant room. Where two or more cables run in parallel per phase, each one is derated for grouping. A long run to a distant panel needs a voltage-drop check as well. See APAR LV XLPE power cables.