Special Transformer Configurations

Special Transformer Configurations

A how-to guide for special transformer applications. Banking, frequency rating converns, zig zag autotransformers, calculating 3 phase unbalanced loads, using 3 phase transformers in phase of 1 phase transformers, and more.

Banking of Transformers

Transformer banking

Two or three single phase transformers can be connected to make a three phase bank. The primary windings of the single phase transformers can be connected in delta or wye. The secondary windings can also be connected in either a delta or wye configuration. The equivalent capacity of the bank will be equal to three times the nameplate rating of each single phase transformer. Usually this type of installation is more expensive than using a single three phase transformer.


In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments. Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.

50 Hz Operation

Transformers rated at 60Hz should not be used on a 50Hz supply due to higher losses and core saturation and the resultant higher temperature rise. Transformers rated for 50Hz, however, can be operated on a 60Hz supply.

Reverse Connection

In general, distribution transformers can be reverse connected without derating the nameplate kVA capacity. However, some precautions need to be taken for reverse connection of some smaller transformers. On transformers under 6 kVA three phase and 3 kVA single phase, there is a "turns ratio compensation" on the low voltage winding. When the input voltage, equal to the nameplate rated voltage, is connected to the low voltage winding, the output voltage will be slightly lower than the nameplate rating. When a three phase transformer is reverse connected thus resulting in a Wye-Delta configuration, the neutral terminal must be isolated. Further, the reverse connected transformer may draw a higher inrush current during energization. Hence the sizing of the line fusing or circuit breaker may be affected.

Transformer Reverse Connection

Load Balancing on 1 and 3 Phase Transformers

A single phase transformer with 120/240V secondary has two separate 120V secondary windings and is usually connected into a 3 wire system. Care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total load. Similarly for a three phase transformer, each phase should be considered as a single phase transformer. When distributing single phase loads between the three phases, each of the three windings should be evenly loaded.

Sound Level

Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission. Many transformers are built to meet the latest ANSI, CSA and UL standards. These standards use NEMA-ST20 (see table for outline).

Two Winding kVA
Self Cooled Ventilated
Voltage Line to Line
Self Cooled Non-Venilated
1.2kV dB-A > 1.2kV dB-A dB-A
0-9 40 45 45
10-50 45 50 50
51-150 50 55 55
151-300 55 58 57
301-500 60 60 59
501-700 62 62 61
701-1000 64 64 63