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 4,160Y/2,400-volt system. Other standard three-phase four-wire wye grounded neutral primary system voltages are 8,320Y/4,800, 12,470Y/7,200, 13,200Y/7,620 and 13,800Y/7,970.

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.