If 480Y/277V service is needed, the most common transformer configuration will have a 208V-delta winding, served by a 208V 3ø 3-wire source — a neutral conductor is not used.
See “Group A” units at www.acmepowerdist.com/pdf/Page%2014.pdf
See connection 46 at www.acmepowerdist.com/pdf/Page%2055-63.pdf
Grounding of the HO terminal is the same as for other separately-derived systems, such as the more common 480-delta—208Y/120V unit.
Low-voltage wye—wye transformers can have problems with triplen-harmonic currents, so are not too common and should be carefully applied.
[This message has been edited by Bjarney (edited 11-10-2004).]