What is 'Nominal' in Voltage

[jar546]

Since the majority of the NEC revolves around AC voltage, I want to stick with that. The NEC references the word nominal when describing voltage, but does not define what nominal is. How do I know that we are not talking about peak-to-peak voltage? We all assume that nominal voltage would be the true RMS voltage, but where does the NEC actually define the word or meaning of nominal?

 

[Gregg Harris]

Since the majority of the NEC revolves around AC voltage, I want to stick with that. The NEC references the word nominal when describing voltage, but does not define what nominal is. How do I know that we are not talking about peak-to-peak voltage? We all assume that nominal voltage would be the true RMS voltage, but where does the NEC actually define the word or meaning of nominal?

NEC 220.5aNEC® section 220.5(A), unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

 

[wwhitney]

Since the majority of the NEC revolves around AC voltage, I want to stick with that. The NEC references the word nominal when describing voltage, but does not define what nominal is.

Sure it does, see Article 100. From the 2017 NEC (last version available from the publisher in PDF, as far as I known):

Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts). (CMP-1)

Informational Note No. 1: The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.
Informational Note No. 2: See ANSI C84.1-2011, Voltage Ratings for Electric Power Systems and Equipment (60 Hz).
Informational Note No. 3: Certain battery units may be considered to be rated at nominal 48 volts dc, but may have a charging float voltage up to 58 volts. In dc applications, 60 volts is used to cover the entire range of float voltages.

Presumably ANSI C84.1-2011 will tell us that nominal voltage ratings are RMS based for AC voltages.

Cheers, Wayne

 

[jar546]

Presumably ANSI C84.1-2011 will tell us that nominal voltage ratings are RMS based for AC voltages

I believe they’re up to the 2020 version at this point.

 

[socaldano]

Since the majority of the NEC revolves around AC voltage, I want to stick with that. The NEC references the word nominal when describing voltage, but does not define what nominal is. How do I know that we are not talking about peak-to-peak voltage? We all assume that nominal voltage would be the true RMS voltage, but where does the NEC actually define the word or meaning of nominal?

If I understand the question...As I remember "nominal voltage" would be "120v" is 120v +/- 10 volts, or +/- 15 volts without a lot of unnecessary explanation, and understood to be waveform AC.
220 might be 215 to 225 volts.

 

[Gregg Harris]

If I understand the question...As I remember "nominal voltage" would be "120v" is 120v +/- 10 volts, or +/- 15 volts without a lot of unnecessary explanation, and understood to be waveform AC.
220 might be 215 to 225 volts.

Nominal voltage does not take into consideration a plus or minus 10%. Nominal is used for load design calculations. NEMA design criteria for motor design can use a plus or minus value for run performance.