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Transformer Calculator – Rating Current

Online calculator for transformer primary and secondary rating current .

Primary Voltage
Secondary Voltage

Primary Rating Current
Secondary Rating Current

How to determine transformer current rating

Transformer primary and secondary current rating calculated by below formulas

1. Three Phase Transformer:

2. Single Phase Transformer:


  • S : Transformer Apparent kVA
  • V : Line voltage of primary or secondary winding in kV
  • I : Ampere rating of primary or secondary respectively in A

Transformer inrush current

When a transformer is energized, primary side will experience a inrush current prior to reach transformer stability operating current. Inrush current is due to a certain amount of residual flux remains in the iron core due to the properties of the magnetic core material.

Inrush current in a dry and liquid filled transformers is equivalent to about 12, or even as high as 18, times rated current with a duration of 0.1s (Source: IEEE 242 Buff Book).

Example of a transformer nameplate with current rating

Transformer primary current rating from nameplate is 54.1A which is same with result from above calculator.