3 Phase Power


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3 Phase Power


In the U.S., there are two basic configurations of three-phase power:
An understanding of these two types of power is important for the safe operation of three-phase equipment. Both types are common. Extreme care should be taken to correctly identify how your building is wired.

Three Phase units do not require a start-assist as many Single-Phase units do. The phases are 120 degrees apart, which means it is impossible for the rotor/stator to be exactly aligned magnetically.

Wye Configuration - Three Phase 120/208


Three-phase service is commonly provided to large buildings and facilities to power both three-phase loads and 120 volt single-phase loads. Wye is the most common configuration provided by the electric utility.

Figure 1 is a 120/208V Wye system in North America, which is delivered with four wires composed of one wire for each voltage, plus a neutral.



Single Phase 120/240


Figure 2 is the Residential Standard.



Delta Configuration - Three Phase 120/208/240


Figure 3 provides three voltages - 120/208/240.

The electrician must exercise extreme care to correctly identify the different connections.

Because of the 208V between phase 3 and ground/neutral, this leg is often referred to as the “high leg”.  Single-phase 120V equipment must never be wired in this fashion.

Where does 208V come from?


Using vectors to represent the 3 phases, the diagram becomes:


Note that 208 may be computed by multiplying 120 by 1.73, with 1.73 being the square root of 3.

For those technicians interested, reference the Pythagorean Theorem from Plane Geometry, and the special case where one leg is half the hypotenuse.

Selection of Equipment


Selection of equipment becomes critical with 3-Phase.

Read the documentation and the nameplate to determine the requirements for your system.

The most common 3-Phase voltage is 208V, so verify that your equipment can operate to specification with that voltage.

If 240V 3-Phase is required, the easiest solution is using a Delta configuration, with a more difficult solution of installing an appropriate transformer.

Choose carefully !!!