Load Analysis Layout

Your question requires a little bit more analysis than just a load calculation for each panel. The calcualted loads for each panel will come into relation due to the fact that the 480/277VAC panel wil be used to feed power to the primary side of the step-down transformer. The size of the transformer will have to be able to carry the entire load calculated for the 120/208VAC panel, therefore the size of the feeder circuit for the primary size of the transformer will be affected by how much load is expected to be demanded from the secondary of it (were the 120/208VAC panel will be connected).

The total load calculations for both panels will have to be performed per the latest National Electrical Code enforced at your location. After you have performed the calculations for all your single and 3-phase loads in the 120/208VAC panel, then you will have to fine the size of transformer that will be able to supply the entire load for this panel and size the conductors and disconnect unit for the correct capacity.

Then you will need to calculate the amount of amps required to feed the primary side of the transformer (as per the transformer size you selected) and add this load to the rest of the loads that will be powered from the 480/277VAC panel.

Remeber that for 3-phase loads you will need to use the 1.73 multiplier for your voltage and once calculated, the total 3-phase load in VA will be devided equally between the three phases (this concept applies to both panels).

This is just a basic explanation, there are a lot more issues that need to be consider like AIC rating of equipment, available fault currents at all point for equipment, size of feeder conductors and distances, etc...