A 208Y/120 secondary is multi-voltage regardless if the load is all three wire.
The tap rules for multi-voltage vs single-voltage transformers are about the transformer connections not the load. The transformer connections dictate how fault current on the secondary will be reflected back into the primary.
A solidly grounded wye secondary can have a line to ground fault that is not reflected back to the delta primary simply by the turns ratio of transformer.
Do the math. Given a single L-G fault of 2.0PU on a wye secondary, what is the corresponding Line current on the delta primary. Will a primary side protective device clear this L-G fault in the required time frame (assume a NEMA AB-1 characteristic)?
