ANSI C84.1 (2020 is the latest version) establishes voltage classes, ratings, and tolerances in the United States for 60 Hz power systems. Both standard nominal and preferred standard nominal voltages are listed. Modern systems will typically use one of the preferred standard nominal voltages listed below.
Use of another voltage is permissible though equipment selection may be limited, particularly at low voltages.
Important to note is that the nomenclature of these voltage classes is only used for engineering classification purposes. A low voltage system is not necessarily safer to operate than a medium voltage system.
Low Voltage Class (LV), Nominal
Low Voltage systems operate at less than or equal to 1000 V. Utilization equipment operates at these voltages.
Preferred 3-wire, single-phase system:
- 120/240 (120 V from leg 1 or 2 to neutral and 240 V from leg 1 to leg 2).
Preferred 3-wire, three-phase system (V phase-to-phase):
Preferred 4-wire, three-phase systems are:
- 208Y/120 (208 V phase-to-phase and 120 V phase-to-ground from a wye wound transformer or generator.)
- 240/120 (240 V phase-to-phase and 240/120 V from a mid tap of one winding that is connected to neutral from a delta wound transformer or generator.)
- 480Y/277 (480 V phase-to-phase and 277 V phase-to-ground from a wye wound transformer or generator.)
Medium Voltage Class (MV), Nominal
Medium Voltage systems operate at greater than 1000 V and less than 100 kV. The primary distribution system within a plant or campus will use medium voltage. Large motors will also operate at medium voltage. Preferred 3-wire, three-phase systems (phase-to-phase):
Preferred 4-wire, three-phase systems (phase-to-phase and phase-to-ground as derived from wye wound transformer or generator):
High Voltage Class (HV), Nominal
High Voltage systems operate at greater than or equal to 100 kV and less than or equal to 230 kV. Systems that operate at high voltage are used for transferring large amounts of power at scale. These systems are typically operated by electric utilities. Preferred 3-wire, three-phase systems (phase-to-phase):