Technical Article

Transformer Selection Criteria

December 20, 2021 by Alex Roderick

There are many considerations involved in selecting a transformer. These include the requirements for the primary and secondary voltage; whether it is single-phase or 3-phase; the present and future loads considerations; whether it is to be used for step up, step down, or isolation; whether it is to be a two-winding transformer, autotransformer, or buck-boost; the permitted noise level; and the required and available cooling and ventilation.


When selecting a transformer, the first decision that needs to be made concerns the required voltages at the primary and secondary. A facility may have several bus voltages present, and a transformer must be selected according to where and how it will be used. Once the bus voltage is known, the primary and secondary coil voltage can be selected to give the desired load voltage.



Once the voltage has been determined, there are usually several options for the transformer primary. For example, if the supply is 3-phase, 4160 V, there are several options for the primary coil voltage. See Figure 1. This source is 4160 V phase-to-phase and 2400 V phase-to-ground.


Figure 1. There are several options for the transformer's primary wiring.


Single-wound 4160 V primary coils can be wired in a delta connection across the lines. In addition, dual-wound 2400 V primary coils can be wired in parallel and connected in a wye from phase to neutral.

A transformer with dual-wound 1200 V primary coils can also be selected. In this case, the two coils need to be wired in series and connected in a wye from phase to neutral in order to create the desired voltage. Having these three options available for one source can make the purchase of a transformer less expensive and delivery much faster.



The transformer secondary offers many options for the different types of loads and voltage requirements of the equipment to be powered. For example, a common 3-phase transformer is rated at 480 V/277 V on the secondary and is used for power and lighting in many plants. See Figure 2. This type of transformer is equipped with three single-wound 277 V secondary coils connected in a wye with the center tap used as a neutral for the 277 V single-phase loads. The three-phase conductors give 480 V phase-to-phase for equipment power. The center tap is grounded, making it possible for overload current protection devices to function under ground fault conditions.


Figure 2. There are several options for the transformer secondary wiring.


If the needed secondary is a 3-wire, 240 V system, then a unit with dual-wound 240 V secondary coils can be used. The coils can be wired in parallel and connected in a delta. This system will usually have the B phase grounded for the operation of the overcurrent devices in case of a fault to ground in the system. This same transformer can be used for a 480 V, 3-wire system, with the 240 V coils wired in series and connected in a delta. Again, we usually find the B phase grounded.

Another common secondary connection is a 4-wire, 120 V/240 V service-connected in a delta, with dual-wound 120 V coils connected in series. See Figure 3. The center connection between the A and C phases is grounded. This arrangement gives 120 V phase-to-ground between each of these two phases and ground. This connection also gives 208 V phase-to-ground between the B phase and ground.


Figure 3. A common secondary connection is a 4-wire, 120 V/208 V/240 V service.


This same unit can be used for a 208Y/120 V service with the coils in parallel and connected in a wye. The center connection is used as a neutral and connected to the ground. This connection gives 120 V phase-to-neutral or phase-to-ground and 208 V phase-to-phase.



The type of electrical system determines the type of transformer required. Single-phase transformers are used to transform single-phase power, and 3-phase transformers are used to transform 3-phase power. In addition, there are a few other types of phasing used in special applications.



For single-phase transformers, almost any unit can be selected as long as the kVA rating matches the load requirements. A transformer should be sized so that it is equal to or larger than the load calculation. Single-phase transformers can be connected together into a 3-phase system.



Individual 3-phase units with all the coils on a common core are very common. In addition, three single-phase transformers are often used to connect to a 3-phase circuit. See Figure 4. An advantage of using three single-phase units is that, in case of failure, it is much less expensive to replace one single-phase unit than a common-core 3-phase unit.


Figure 4. Three single-phase transformers can be connected together to connect to a 3-phase circuit.


Other Phasing

In a few areas, 2-phase systems can be found. Synchronous converters use interconnected units to provide 6-phase as the input for the conversion to DC. These systems often use standard single-phase transformers with special connections to create these phase configurations.



When sizing a transformer, present loads and expected future loads should be considered. Large power transformers are expensive, and future expansion should be taken into account when selecting a transformer. It may be more cost-effective to use an oversized transformer from the beginning than to have to replace a transformer in the near future. Power factor correction can be installed at the time of transformer installation to rectify the poor power factor of a large, unloaded transformer.


Open Delta

An open delta transformer configuration can be used when selecting a transformer to allow for future expansion. This connection allows the use of two transformers for the present load with the option of later ordering the third unit when the loads are increased. See Figure 5. This connection offers the user 58% of the capacity of a standard closed delta.


Figure 5. An open delta configuration can be used when selecting a transformer to allow for future expansion.


It is also possible to create an open delta with two transformers of different sizes. The larger unit can be used to provide power to the single-phase loads. For example, a 25 kVA transformer can be connected to a 15 kVA transformer in an open delta. The 25 kVA transformer is used for the single-phase loads, while both transformers supply the 3-phase loads. When selecting the two units, they must be sized to operate at 86% or less of their rated capacity. The impedance of the two transformers should be matched in this installation. When a third unit is added to close the delta, each transformer can deliver 100% of its rating.