Transformer primary and secondary current calculator

Preliminary estimate

Inputs

Calculation note

Current is calculated from rated kVA on the primary and secondary sides using each selected voltage and phase.

Method

1-phase A = kVA x 1000 / V

3-phase A = kVA x 1000 / (sqrt(3) x V)

Voltage ratio = secondary V / primary V

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Formula variables

  • kVA is the transformer apparent-power rating.
  • Primary voltage is the supply-side nominal voltage.
  • Secondary voltage is the load-side nominal voltage.
  • Three-phase calculations use line-to-line voltage.

Assumptions

  • The same transformer kVA is used on both sides for full-load current arithmetic.
  • Three-phase current uses line-to-line voltage.
  • The result does not size overcurrent protection, conductors, grounding, bonding, inrush allowance, or available fault current.
  • Transformer nameplate data, impedance, temperature rise, taps, manufacturer instructions, and applicable code review still govern equipment use.

Examples

600 V to 208 V three-phase transformer. A 75 kVA transformer estimates about 72.17 A on the 600 V primary and 208.18 A on the 208 V secondary.

Nameplate workflow. Use the actual transformer nameplate voltages and phase before using current results in downstream planning.