How to Size a Transformer

  • Transformer size is determined by the kVA of the load.
  • Load voltage, or secondary voltage, is the voltage needed to operate the load.
  • Line voltage, or primary voltage, is the voltage from the source.
  • Single-Phase has two lines of AC power.
  • Three-Phase has three lines of AC power, with each line 120 degrees out of phase with the other two.
  • kVA is kilovolt ampere or thousand volt ampere. This is how transformers are rated.

To determine the size of the transformer you need, determine the following requirements for your transformer:

  1. Load Voltage
  2. Load Current/Amps
  3. Line Voltage

Next, determine if your application is single-phase or three-phase, and use this corresponding formula:


The kVA of the transformer should be equal to or greater than the kVA of the load to handle present requirements and to account for future expansion.

As a service to you, we offer a transformer specifier to help you find your transformers quickly and easily.

Calculate your kVA

Use the calculating forms below if you know your Load Voltage and Load Current.

Single phase


Three phase


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