He created a 'periodic table' for plugs himself, though there's also a handy chart/illustration at:

* https://en.wikipedia.org/wiki/NEMA_connector#Nomenclature

Perhaps also worth noting that while the US generally uses 277/480V for distribution (before stepping down to 120/240-208), Canada (also) uses 347/600V in many places: more power for the same copper cable thickness (or less copper for a given power, due to less amperage).