Transformer fundamentals

Transformer fundamentals

How to calculate the kVA rating of a transformer

We are going to simply explain how to calculate the kVA rating of a transformer. There are two simple calculations which can be used for single and three phase transformers. Below we will work through both options.

What is a transformer?

You can use transformers for a wide range of applications. Transformers are a simple device which convert AC power at a certain voltage level to AC power at a different voltage.

Standard power transformers consist of two electrically isolated coils and operate on mutual induction. Thus an EMF is induced in the transformer secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding.

Single phase transformers

panel transformer

A single phase transformer consists of two electrical coils of wire; a primary coil and a secondary coil. The primary coil on a single phase transformer is usually on the input power supply side. The secondary winding delivers the output power. In a single phase transformer the primary is usually the side with the higher voltage.

A single phase transformer has two caolis (primary and secondary) that are not in electrical contact with each other but are instead wrapped together around a common closed magnetic iron circuit.

Three-phase transformer

A standard three phase transformer is manufactured with three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core.

On a three phase transformer the primary windings can be connected in 2 common configurations; delta (Δ) or star (Y) and the secondary windings are connected similarly depending on requirements. The reasons for choosing a Y or Δ configuration for transformer winding connections are the same as for any other three-phase application: Y connections provide the opportunity for multiple voltages while Δ connections enjoy a higher level of reliability.

The winding connection options:

Primary – Secondary

Y – Y

Y – Δ

Δ – Y

Δ – Δ

How to calculate the kVA rating of a transformer

Here are two simple equations. Use these equations to work out the kVA rating of a single and three phase transformer.

Rating of single phase transformer

P = V x I

Rating of a single phase transformer in kVA

kVA= (V x I) / 1000

Rating of a three phase transformer

P = √3. V x I

Rating of a three phase transformer in kVA

kVA = (√3. V x I) /1000


It is important to select the correct kVA rating to ensure you are getting the most efficient transformer possible.

If you have any questions regarding the items above or have a special requirement you would like to discuss with us please contact us. Our transformer team are here to help you understand and will guide you to ensure you purchase the transformer which is best equipped for your job.

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