I have a 1500kva 120/208 volt 3 phase 4 wire pad mount transformer. Using EDSA (a commercial software package)and IEEE std 399-1990 the derating required for the secondary feeder running in an underground duct bank will require 30 - 500kcm cables per pahse due to the insulating properties of the ground. Though the number of cables required seems excessive it is correct. The secondary current of the transformer is 4164 amps and have it terminated at a 5000 amp fusible switch in a switchboard. I am trying to get some confirmation or other ideas from others who have had a similar application.