What size is that 400V transformer? If it's 25kVA-50kVA, it probably isn't capable of more than 20kAIC or thereabouts. The impedances of the wires and cable and extension cords will drastically reduce the peak fault current from the maximum the transformer is theoretically capable of.
- Start with the transformer impedance. Then, add up all the line resistances between the transformer and your fault point using NEC tables. You'll certainly need to take the complex impedance into account, especially if your fault current is approaching the limit of your breakers.
