Sounds like a Typ-o (Typed Error) on the nameplate... being that there is an X0 terminal on the "OUTPUT" Windings' sideUNLESS...
- The "X" Terminals are "X0", "X1" and "X2" - with a total of Three (3) "X" side Terminals,
- "X0" is physically bonded to one of the "X" Terminals - as in X0 is bonded to X1,
- "X0" is bonded to the common starpoint of the Secondary Windings, and all three Windings are connected in a Star (Wye) fashion,
- "X0" is derived from a center tap to one Secondary Winding,
- "X0" is un used for this setup, and only exists because the Transformer manufacturer builds their equipment with an X0 Terminal by default,
- It really is a 480V Closed Delta Primary x 208Y/120V 3Ø 4 Wire Wye Secondary - just with a modified nameplate, indicating reverse connection scheme,
- The Transformer is intended to be 208V Delta Primary x 480Y/277 3Ø 4 Wire Wye Secondary,
- The Transformer is 25 KVA or less, and is not a "Real" Closed Delta Primary x "Real" Wye Secondary - it is in fact an Open Delta "Tee" on both sides,
- You have entered the 'Twilight Zone"...
First off, what schematics are shown on the nameplate?
Does the nameplate's schematic show a Delta x Delta arrangement, or a Delta x Wye arrangement?
Can you visually trace where the "X0" terminal's lead goes to on the Windings?
If this is a Wye Secondary, the X0 lead will terminate to a bus (or bar) which connects to the ends of all three windings - usually at the bottom of the coils.
If the X0 lead terminates to a single coil, then you may have a center tapped Delta.
If the X0 lead terminates to one of the other "X" leads, then you may have provisions for a corner grounded Delta.
If X0 is a lead from what is indicated on the nameplate's schematic as the 'Lower Voltage Windings" - the 208 Volt side, chances are this is a Delta / Wye Isolated Transformer, with the wye side voltage = 208; and therefore the X0 terminal is not used...*** PLEASE BE ABSOLUTELY SURE, DO NOT AUTOMATICALLY TAKE THIS OPTION WITHOUT VERIFICATION!!! ***
If the Transformer is small - like 25KVA or less, then you likely have an Open Delta "Tee" arrangemet, which will come - by default - with an "X0" lead on the "Lower Voltage Side".
That lead is the "86% Tap" of the "Kicker Winding".
We really need to get some more information from you, in order to get a solid answer on your Transformer.
Please record as much data from the Transformer - information on nameplate, plus what is visually inside; and if available, any documentation - then post this information to this thread.
One last question: where did you get this Transformer? Can you get some information from the place of purchase?