I think that I misinterpreted your question and I now realize that you were referring to the single phase standards which reference 120/240V.
The IEEE standards are written for standard commercial and industrial products. The typical oil-filled padmount single phase transformer is used in URD (Underground Residential Distribution). Your application is not a standard one, in fact the only thing that I can think of is that you have some type of generation at 480V single phase (solar?) and must get it back on the grid. IEEE is only going to write standards for more typical applications not very unusual ones (I found this out when I tried to find a standard for liquid rheostats for starting wound rotor motors).
You can reference the single phase transformer standards (IEEE C57.12.(00),(21),(23),(25),(26)) etc. and make the changes for 480V (I doubt that there will be much difference between 240V and 480V) or you can reference the three phase standards ((00), (22), (24), (25), (26)) and change them to single phase. If you are up in the 500kVA range I would do the latter but if you were at the 200 kVA range I would consider the former. The kVA limit on the single phase transformers is 167 kVA so the manufacturer would probably have a hard time squeezing the coil into a small single phase tank.