NOMINAL VOLTAGE: The stated or objective value of a given voltage, which may not be the actual value measured.
Nominal System Voltage is a nominal value assigned to designate a system of a given voltage class. For example, in the 15 kV voltage class, a Nominal System
Voltage may be 13.8 kV, or 12.47 kV, or other.
Rated Voltage: The maximum voltage at which an electric component can operate for extended periods without undue degradation or safety hazard.
An oft-referenced root document published by NEMA is ANSI C84.1-1995 AMERICAN NATIONAL STANDARD for Electric Power Systems and Equipment—Voltage Ratings (60 Hertz). In the US, it seems to have tremendous legal significance with respect to electrical systems and related commerce.
