No personal offense intended, but there’s a slight terminology issue here. By ANSI definition, a lower-voltage winding is designated as having “X” terminals and the higher-voltage winding is identified by “H” terminals. For the described transformer, there would be no X0 terminal—a 480V wye-point connection would be labeled H0.
Connecting the 240V “neutral” to a winding midpoint would serve no purpose. [Small aside: based on IEEE C57.12.70, the 240V delta midcoil tap would be labeled X4 or X6.]
If there are no 277V loads to be served, then a 480Y/277V winding is unnecessary, and may have a longer leadtime and higher price. In this case, the 480V system is so small that system grounding gains very little. A delta-delta transformer may be more easily obtained.
An economical approach could be http://www.jeffersonelectric.com/pdf/3-phaseencap.pdf
page 7 for a 240-to-480V autotransformer configuration, using two 5kVA 1ø transformers to serve up to a 17.3kVA 480V 3ø 3-wire load.
[This message has been edited by Bjarney (edited 06-02-2003).]