Transformer Full Load Current Calculator

By: Patrick Myers Patrick Myers is the CEO and Founder of Calculator Academy. He has over eight years of experience developing online calculators and holds a B.S. in Mechanical Engineering. His work focuses on formula implementation, calculator accuracy, unit conversions, and creating practical tools that are easy to use.

Last Updated: July 6, 2026
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About the Transformer Full Load Current Calculator

Use this tool to estimate the full-load line current on both sides of a transformer from its kVA rating and voltages. It is useful for electricians, engineers, students, and anyone checking expected transformer current for single-phase or three-phase systems.

How to use this calculator

  1. Enter the transformer rating in kVA from the nameplate.
  2. Select Three-phase or Single-phase.
  3. Enter the primary voltage in volts.
  4. Enter the secondary voltage in volts.
  5. Select Calculate Current to view the primary and secondary full-load current.
  6. Use Reset to restore the default 75 kVA, three-phase, 480 V to 208 V example.

How it works

The calculator converts the transformer apparent power rating from kVA to VA by multiplying by 1,000. It then divides that apparent power by the appropriate voltage term to estimate full-load current.

For a single-phase transformer, the formula is I = kVA × 1000 ÷ V. For a three-phase transformer, the formula is I = kVA × 1000 ÷ (√3 × VLL), where VLL is the line-to-line voltage and the result is line current.

The same method is applied separately to the primary voltage and the secondary voltage, so lower voltage sides generally show higher current for the same kVA rating. The calculation uses apparent power only, so power factor is not required.

Results are educational estimates. Electrical design, conductor sizing, overcurrent protection, and code compliance should be verified by a qualified professional.

Example calculation

For a 75 kVA three-phase transformer with a 480 V primary and 208 V secondary, primary current is 75 × 1000 ÷ (√3 × 480) = 90.2 A. Secondary current is 75 × 1000 ÷ (√3 × 208) = 208.2 A.

Frequently asked questions

What voltage should I enter for a three-phase transformer?

Enter the line-to-line voltage. The calculator returns the three-phase line current.

Does transformer full-load current use kW or kVA?

It uses kVA, or apparent power. Power factor is not needed for this full-load current estimate.

Why is the secondary current higher than the primary current?

For the same kVA rating, current increases when voltage decreases. A step-down transformer therefore has higher current on the lower-voltage side.

Is this the same as sizing a breaker or wire?

No. It estimates full-load current only. Breaker, conductor, and protection sizing must account for codes, temperature, installation conditions, and transformer rules.