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This introduction to three-phase power and power measurement is intended to provide only a brief overview of the
subject. The professional meter engineer or meter technician should refer to more advanced documents such as the
EEI Handbook for Electricity Metering and the application standardsfor more in-depth and technical coverage of
the subject.
1.1: Three-Phase System Configurations
Three-phase power is most commonly used in situations where large amounts of power will be used because it is
a more effective way to transmit the power and because it provides a smoother delivery of power to the end load.
There are two commonly used connections for three-phase power, a Wye connection or a Delta connection. Each
connection has several different manifestations in actual use. When attempting to determine the type of connec-
tion in use, it is a good practice to follow the circuit back to the transformer that is serving the circuit. It is often
not possible to conclusively determine the correct circuit connection simply by counting the wires in the service or
checking voltages. Checking the transformer connection will provide conclusive evidence of the circuit
connection and the relationships between the phase voltages and ground.
1.1.1: Wye Connection
The Wye connection is so called because when you look at the phase relationships and the winding relationships
between the phases it looks like a Y (wye). Fig. 1.1 depicts the winding relationships for a Wye-connected service.
In a Wye service the neutral (or center point of the wye) is typically grounded. This leads to common voltages of
208/120 and 480/277 (where the first number represents the phase-to-phase voltage and the second number repre-
sents the phase-to-ground voltage).
The three voltages are separated by 120 degrees electrically. Under balanced load conditions with unity power fac-
tor the currents are also separated by 120 degrees. However, unbalanced loads and other conditions can cause the
currents to depart from the ideal 120 degree separation.
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Electro Industries/Gauge Tech
Chapter 1
Three-Phase Power Measurement
Figure 1.1: Three-Phase Wye Winding
Relationship
Figure 1.1: Three-Phase Wye Winding
Doc# E149701
Phase B
Phase A
Phase C
1-1
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