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2–2 EPM 6000 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE
CHAPTER 2: ELECTRICAL BACKGROUND
2.2 Three-Phase System Configurations
2.2.1 Description
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 connection 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.
2.2.2 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 wye (Y). The following figure
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 represents the phase-to-ground voltage).
FIGURE 2–1: Three-Phase Wye Winding
The three voltages are electrically separated by 120°. Under balanced load conditions with
unity power factor, the currents are also separated by 120°. However, unbalanced loads
and other conditions can cause the currents to depart from the ideal 120° separation.
Three-phase voltages and currents are usually represented with a phasor diagram. A
phasor diagram for the typical connected voltages and currents is shown below.
Ia
Vbn
A
B
C
Van
Vcn
N