You permit 5% voltage drop of 380V, over a distance of 5m.
You need a table giving specs for wire gauges. See the figures for ohms per 1000 feet, or ohms per kilometer.
Your genset puts out an expected Ampere level. This figure, times your wire resistance (5m length), shall equal your permitted voltage drop. Do calculations for various wire gauges. Choose the gauge that does not drop more than your desired spec.
Since power factor is less than 1, you need to reduce demand on your genset in the same proportion. You may or may not need to increase wire diameter accordingly.