The contractor would definitely prefer cables as the costs are cheaper and there's more flexibility in the event if the design changes and site adjustments. Busducts have an inherent problem. They are rigid, as in the switchgear and transformer terminations must be as accurately decided during...
You may like to note that there is a difference in testing standards between a standby set and a continuous one. The requirements for a standby set is not as stringent, in terms of stability and response, as compared to a continuous genset. So, that'll alleviate you from some of your concerns on...
Only reason for proposing single core is the ease of installation, large multicore cables are rigid and cannot be installed if too many bends in ducts.
However, multicore is advantageous in less voltage drop and cancellation of the magnetic fields.
I assume you want to know the leakage current when the transformer is energised, you should add the two charging currents separately, HV and LV charging current and ignore the HV/LV capacitance since it's not to ground. But be careful not to take the capacitance as they are, if the capacitance...
