It depends on if it is single phase or three phase.
Originally Posted by stakata
One document to look at is the 'Red Book' IEEE STD 141 which is based on ANSI C84.1.
The simple methodology is:
For single phase systems use LV/HV, such as 120/240
For three phase systems use HV/LV, including the symbol Y when applicable such as 240/120 and 208Y/120.
Industry standards are really just suggestions, so it is likely you will find exceptions to this methodology.
Just because you can, doesn't mean you should.