See attached legend from my former consulting days. I believe this to be industry standard throughout that most engineers or electricians would easily recognize. We used these hash marks for voltages lower than 480V. Typically, single phase loads like 277V, 240V, 208V and 120V. Usually used these on the receptacle and lighting drawings. That being said, obviously a custom legend could differ....but most would agree with the attached nomenclature.
The 480V loads where typically spelled out in this type of fashion for example: 3#8 +1#10 Gnd. No hash marks for the big power load stuff.