System Voltages in Transmission Lines
Table shown is the standard system voltages from
ANSI standards C84 and C92.2
According to ANSI standards C84 and C92.2, system voltages are recommend to be within the table shown below. 345kV, 500kV and 765kV are considered to be in the Extra High Voltage (EHV) level.
The choice of system voltage is in the decision of the utility. However, some points needs to be considered in choosing such, like voltage economics, conductors, distances, equipments, etc.
Standard handbook for Electrical Engineers by Fink & Beaty