How do you tell low voltage three phase from high voltage three phase?

To understand how voltage standards for motors are set it is important to know the basics of the power systems they operate on. In general, utilities that supply power in the USA, and most other 60 cycle countries, are required to provide power to the incoming point of a facility in multiples of 120 volts. Thus incoming equipment, such as circuit breaker panels, are rated in multiples of 120 volts.

The common voltages are 120, 240, 480, and 600 In addition, utilities are obligated by the regional governing authorities, (usually called Public Utility Commissions) to regulate the voltage within a fairly narrow range such as plus or minus 5% For example, in most single phase residential systems the voltage is 120/240. It is brought to the building with 3 wires, one being a neutral and the other two having voltages 120 volts different from the neutral wire. The voltage difference between the two "hot" wires is 240 volts In 3 phase systems the situation is a bit different.

There are 3 phase, 3 wire, ungrounded systems where the voltage between the three wires is 240 volts. The big brother of that system is the ungrounded 3 phase, 3 wire 480 volt system. Ungrounded systems are usually found in older facilities In newer installations, the two most popular systems are called 4 wire grounded wye systems.

The low voltage version is represented by a 120/208 volt system. The higher voltage version it is a 277/480 volt system. On both of these "grounded wye" systems, the low voltage portion (120 or 277 volts) is only available as single phase.

The high voltage (208 or 480 volts) is available as either single phase or 3 phase. It should be noted that in the 4 wire grounded wye systems the high voltage is 1.73 times (the square root of 3) higher than the low voltage. These grounded wye systems are generally felt to be safer and more flexible than the older ungrounded systems.

The flexibility comes from the ability to handle single phase lighting circuits, that operate at 120 volts or 277 volts, from the same system that feeds the 3 phase circuits for motors, equipment for heating, air conditioning, elevators, and industrial machinery.

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