Transformer Primary/Secondary Current Calculator
Work out the rated primary and secondary line currents of a transformer from its kVA rating and the two voltages, for single- or three-phase units. Handy for sizing HV protection, the LV main cable and the main switch on a distribution transformer.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the transformer rating in kVA (the nameplate rating).
- Enter the primary and secondary voltages — line-to-line values for a three-phase transformer (e.g. 11 000 V and 415 V).
- Enter 1 in the single-phase box for a single-phase transformer, or leave it blank/0 for three-phase.
- Read the primary current, secondary current and voltage ratio; the low-voltage side always carries the larger current.
How it works
Both windings of an ideal transformer handle the same apparent power S (kVA). For three-phase, line current I = S·1000 / (√3 · V_line); for single-phase, I = S·1000 / V. Because S is fixed, the currents scale inversely with voltage, so the primary:secondary current ratio is the inverse of the voltage (turns) ratio Vp:Vs.
Worked example
Worked example. A 100 kVA three-phase transformer stepping 11 kV down to 415 V: Ip = 100 000 / (√3 × 11 000) = 5.25 A on the HV side, and Is = 100 000 / (√3 × 415) = 139.12 A on the LV side. The voltage ratio is 26.51 : 1.
Common mistakes
- Forgetting the √3 for three-phase transformers, which overstates the current by about 73%.
- Entering the phase voltage instead of the line-to-line voltage for a three-phase unit.
- Confusing rated full-load current with fault current — short-circuit current is far higher and depends on the transformer's %impedance, not this calculation.
Frequently asked questions
Which side carries more current?
The low-voltage side. Since both windings pass the same kVA, current is inversely proportional to voltage, so stepping voltage down steps current up by the same ratio.
Is this the full-load current or the actual current?
It is the rated full-load current at the nameplate kVA. The actual current is set by the connected load and is lower when the transformer is not fully loaded.
How do I size the secondary main cable and protection?
Start from the rated secondary current shown here, then apply the wiring-rules derating and protection-coordination requirements. The secondary current also sets the minimum rating of the main switch and LV busbar.
Does this give the fault (short-circuit) current?
No. Prospective fault current is much larger and is estimated from the rated current divided by the per-unit impedance (roughly I_fault ≈ I_rated / %Z × 100). Use the transformer's impedance data for that.
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