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Home / Technical Articles / Connections and working principles of three-phase distribution transformers

Three-Phase Transformers

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.

Connections and working principles of three-phase distribution transformers
Connections and working principles of three-phase distribution transformers

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.

1000kVA 3-phase padmounted transformer
Figure 1 – 1000kVA 3-phase padmounted transformer (photo credit:

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.

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

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