Electrical Distribution Systems
WIRING SCHEMES AND GROUNDING
The first scheme (Figure 15A) provides for the connection of the load from a phase leg to any
Figure 15 Three-Phase To Single-Phase Connections
ground point and is referred to as a phase-to-ground scheme. The remaining scheme (Figure
15B) connects the single-phase load between any two legs of the three-phase source and is
referred to as a phase-to-phase connection. The choice of schemes, phase-to phase or phase-to-
ground, allows several voltage options depending on whether the source three-phase system is
a delta or wye configuration. This will be discussed in the three-phase segment of this chapter.
The only approved method of wiring single-phase power is the scheme commonly referred to as
the 3-wire, single-phase Edison system. The illustration in Figure 16 depicts the use of a center-
tapped transformer, with the center tap grounded, providing half voltage (120 V) connections on
either side or full voltage (240 V) across both sides.