Sounds to me that you are describing the typical "Customer Owned SDS" [Separately Derived System].
The Customer is supplied with a 480Y/277 VAC 3 Phase 4 wire system of -X- KVA [or MVA].
If the load calced was higher than what can be derived from a 480 VAC service, then this would have [or should have] been supplied by a 2400 VAC system, or a 4160 VAC system.
The Utility supplied Power System will be in respect to the Calculated Load. That's where the System Voltage becomes a factor.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!