Transformer full load current calculator

Transformer full load current calculator

The transformer full load current calculator is designed to calculate the full load current of the transformer by entering primary and secondary side voltage. Other things that have to select are the type of transformer (single-phase or three-phase transformer) and kva rating of the transformer.

What is the full load current of transformer

Rated current is the current for which the transformer has been designed. For example, if the transformer is rated for 500 Amperes, it means that its transformer conductor size and its oil cooling have been kept sufficient enough for the transformer to deliver 500 amps continuously.

If the current exceeds this rated value, there will be overheating and, in extreme cases, it will cause insulation failure and a short circuit. This is because, out of all the materials used in the transformers, the temperature withstand limit is the lowest for insulation. If the current far exceeds the rated value, copper may even melt.

Transformer primary and secondary current calculation

For Single-phase transformer :

Single-phase transformer KVA = Volts x Amps / 1000

Hence, the formula for full load current will be :

Single Phase Transformer Full-Load Current (Amps)= kVA × 1000 / V

For Three-Phase transformer :

In the case of, Three-Phase transformer, KVA = Volts x Amps x 1.732 / 1000

Hence formula for full load current will be :

Three Phase Transformer Full-Load Current (Amps) = kVA × 1000 / (1.732 × V)

3 phase transformer calculation formulas

Let us take two examples for calculating transformer full load current.

100 kva transformer current carrying capacity

3 phase transformer primary and secondary current calculation or current capacity of the distribution transformer on the primary side and secondary side can be calculated as follow-

let, Primary side voltage is 11 kV and the Secondary side voltage is 415 v.

current on the secondary (side 415 v) side will be calculated as follow:

 100 kva=√3.x 415v x I

 I= 100×1000 / 1.732 X 415v= 139 amp

similarly current capacity on the primary side (11 kv side) shall be-

 I= 100×1000 / 1.732 X 11000 v= 5.25 amp

400 kva maximum current carrying capacity

Let, Primary side voltage is 11 kV and the Secondary side voltage is 415 v.

Secondary side full load current:

400 kva= √3.x 415v x I

I= 400 x 1000/√3.x 415v = 556 Amp


  • Transformers are sized by determining the total load required (in amps).
  • Transformer capacity is rated in KVA (kilo-volt-amperes).
  • The load voltage and load amps must be known to calculate KVA rating.
  • NOTE: We do not recommend loading a transformer above 80% of its KVA rating. When the KVA rating has been calculated, divide that number by 0.8 to get the minimum KVA rating needed. All values should be reviewed and confirmed by an electrician or electrical engineer.

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