Just to add to resqcapt19's post, a wye secondary could be bonded to ground at an end, such as X1, and then it would behave much like a corner grounded closed delta. That could be a suitable replacement transformer for an existing corner grounded delta system.
Now a question is, if the supply really is 480Y/277 at the source (X0 bonded), and the conductors brought to the panel are only X1,X2,X3 and G (not neutral), can you still safely and legally use 480/277 ratings? One thing you obviously cannot do is connect any 1-pole 277 volt loads. But 2-pole and 3-pole breakers (such as EDB series for Square-D NF panels) would be rated at 480/277 volts, so I would think they can be used even though the neutral is absent.
If this is an ungrounded or impedance grounded system, then it has to be treated as a pure 480 volt system since under a single-fault condition, it actually will be. And 240.85 limits 480/277 breakers to only solidy grounded systems with (not more than) 277 volts to ground. I assume Yoopersup's "straight 480 plus ground" could be a corner grounded delta where you cannot use 480/277 rated breakers (unless you know for sure otherwise, assume the worst). But can you use them if the system is definitely a 480Y/277 even if the neutral is not present?