Don't take this answer as gospel, because there are really a lot of correct answers. I'm just going to give you a reasonable answer.
Given a typical delta-wye transformer, you will have a 30 degree phase shift between the primary and secondary. Don't ask me why...I could never quite get my head around it. Maybe I need to go back and study it a little more.
Given that 30 degree shift, if you take a delta feed from a 480Y/277V system, feed a delta-wye transformer down to 208Y/120V, you would have about 183 volts between 277V phase A and 120V phase A. Not that I would ever consider hooking anything up between them.