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Section 220.5 addresses calculation requirements and establishes the use of nominal voltages, which are the approximate standardized voltages at which equipment is designed to operate. Because actual voltage is likely to be slightly higher or lower, nominal voltages are not precise. (Voltage, Nominal is defined in Article 100.)
The primary value of this standardized practice is that everyone using the NEC would use the same voltages in calculations. Section 220.5(A) lists the nominal system voltages as 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts.
Section 220.5 also talks about how current is calculated and states that a fraction of an ampere that is less than 0.5 may be dropped, and a fraction of 0.5 or larger should be rounded up. Rounding to whole numbers is a common practice, but it is actually established as a rule in the National Electrical Code.
Using nominal voltages means in practice that, rather than trying to determine precisely the voltage in a typical single-phase system – which might be 115 or 118 or 122 volts – the stated nominal voltage of 120 volts would be used for calculations. Nominal 480 volt systems may actually be measured at voltages ranging from 440 to 480, and motors for use on such a system are likely to be rated at 460 volts. This eliminates needing to know the precise measured voltage, which might actually change from time to time anyway.
© Hughes Associates,Inc. 2012