A three-phase service can be supplied by one three-phase transformer unit or by interconnecting three single-phase transformer units. One three-phase unit is smaller than an equivalent size bank consisting of three single-phase units.
One three-phase transformer tends to be used in underground vaults or as a pad-mount transformer (Figure 1). An illustration of a three-phase transformer is shown in Figure 2.
One three-phase unit is easier to install because the polarity and interconnections between the phases are fixed. The use of three single-phase units is common in overhead distribution.
When single-phase transformers are banked together, they can be interconnected to supply more than one type of service. For example, three transformers with 120/24 V secondaries can supply a 120/208, 240/416, or a 240 V three-wire service.
Fewer specialized emergency spare transformers are needed when single-phase transformers are used.