Figure 4-17.Single-phase transformer connected to give
120/240-volt three-wire single-phase service. Transformer
secondary coils are connected in series.
ungrounded delta primary voltage system to obtain
120/240-volt single-phase three-wire service.
Normally the wire connected to the center low-voltage
bushing will be connected to ground. Grounding the
wire connecting to the center bushing limits the
voltage aboveground to 120 volts, even though the
wires connecting to the outside secondary bushings
have 240 volts between them.
Figure 4-18 schematically shows the single-phase
distribution transformer connections to a three-phase
four-wire wye grounded neutral primary system rated
4,160Y/2,400 volts to obtain 120/240-volt single-
phase secondary service. The three-phase four-wire
wye grounded neutral system has voltage between
phases equal to the phase or line to neutral voltage
multiplied by 1.73. In figure 4-18, the primary system
line to neutral voltage is 2,400 volts, and the voltage
between phases is 1.73 times 2,400, or 4,160 volts.
This system is designated as a
system. Other standard three-phase four-wire wye
grounded neutral primary system voltages are
a n d
The transformer connections to obtain 120,
120/240-volt secondary service are normally completed
inside the tank of the transformer (fig. 4-19). The
Figure 4-19.Shows connections for 120/240-volt three-wire,
240-volt three-wire, and 120-volt two-wire services.
Figure 4-18.Single-phase transformer connected to provide 120/240-volt three-wire single-phase service. Primary winding is
connected to neutral or ground.