Circuit Design

Please could any one shed a bit more light on this subject,

I have a supply 3ph 400amps per phase at the in take I need to run a sub main 275 meters at this end its going to be 42 caravans each one will be fused at 20 amps.

now do i apply only as single phase system as there is no 3 phase demand, this effects the results in cable size and vd and with 42 divided up 14 per phase and does diversity come in to it.

regards Monkee