If your client has no purpose for the 3-phase delta and plans to be in the building a while you might want to see what options the utility might give you to change it to a 208/120 volt service, this will lessen the cost of the transformer and the cost of magnetizing its core when the loads are less, 150kva is just over 400 amps at 208 3-phase, and if your client don't wind up loading this transformer close to its full amps, the efficiency starts going down hill, thats why it's always better to keep the transformers on the utility's side of the meter when possible.
Here we use 230.2(D) and install a new service then have the owner just leave the old service turned off if we don't change it all together, sometimes when we have a rental building with a single phase service and the tenet needs a 3-phase service then we install a new service just for the 3-phase loads.
Also if the pots on the pole are three bushing pots then the secondaries can be reconfigured for a WYE with very little work from the utility, change the meter socket from a 5 jaw to a seven jaw 320 with double load lugs, new 4-wire drop, two new 200 amp MB panels (more breaker spaces for less cost then a single 400 amp panel), and you have your 400 amp 208/120 service without the added cost of the transformer, but you will need to check with the utility, but it would be worth it for the cost savings for your client.
Is this a rental building or does you client own it?
If the transformer is a must then yes 250.66 does apply to the required GECs on the secondary side of the transformer as required in 250.30
