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For some terms, the precise context or phrase in which the term is used is critical. For example, there are several related definitions of voltage in Article 100, including nominal voltage (essentially a convenient label for a voltage system), voltage to ground and voltage of a circuit. Depending on the context of a particular rule, it may be important to know which one is being discussed.
Each of these types of voltage may be referenced in Section 210.6 where voltage limits are placed on circuits based on their usage.
To distinguish nominal system voltages, Section 220.5 establishes the standard voltages for the purpose of load calculations. Actual voltage may be a little bit different than the nominal voltage or the standard voltage, but these values are established for use in NEC rules and to provide consistency in calculations.The nominal circuit voltage is defined in Article 100 as “A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (such as 120/240 volts, 480Y/277 volts, 600 volts). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment." Voltage to ground is defined in Article 100 as follows: “For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.” In an ungrounded system, this is not necessarily the actual voltage to ground that could be measured, but simply a definition for the purposes of applying NEC requirements. For example, a 480Y/277 volt system or circuit is typically grounded, and the voltage to ground is 277 volts. If, in some situation, the same system was operated ungrounded, the voltage to ground would be considered to be 480 volts, even though the actual voltage to ground could be greater than or less than 480 volts and could vary from time to time.
© Hughes Associates,Inc. 2012