What cable size is needed for 415VAC 3 Phase 400kVA generator to a load 50meters away?

As example, at 380V the generator produces 685A per phase. Therefore you will require 2 x 120mm2 cables per phase. (1 x 120mm2 cable will support 365A per phase).

In total you will require approximately 1/2km of cable to cover the 50m run between the generator and the ATS panel/Load. i.e. 5 phases in total - 2 x 50m cables per phase.