Transformer Full Load Current Calculation – Formula and kVA Current Tables

Full load current is the first number needed when specifying cables, switchgear, protection settings and busbar ratings for a transformer installation. This page gives the calculation method, worked examples, and ready-reference tables for standard power ratings on both the low-voltage and high-voltage sides.

The Formula

For a three-phase transformer:

For a single-phase transformer the √3 factor is removed:

Note that rated power is expressed in kVA, not kW. Apparent power is used because the transformer's thermal limit is set by current, regardless of the power factor of the connected load. A 1000 kVA transformer carries the same winding current at 0.8 power factor as it does at unity — only the useful power delivered changes.

Worked Example — LV Side

A 630 kVA transformer with a 400 V secondary:

Worked Example — HV Side

The same 630 kVA transformer with a 33 kV primary:

The ratio between the two currents equals the inverse of the voltage ratio — 909 A at 400 V corresponds to 11.02 A at 33 kV. This relationship provides a quick sanity check on any calculation.

LV Current Table — 400 V Secondary

HV Current Table

What Full Load Current Does Not Tell You

Full load current is a steady-state figure. Three other currents matter in the same installation and are considerably larger:

• Inrush current — magnetising inrush on energisation can reach 8 to 12 times full load current for the first few cycles. Protection settings must accommodate this without tripping.
• Short-circuit current — determined by the transformer's impedance voltage. A unit with 4 % impedance passes roughly 25 times full load current into a bolted secondary fault. See Transformer Short-Circuit Current Calculation for the method and fault level tables.
• Overload current — transformers can carry short-duration overloads within the limits of IEC 60076-7, depending on ambient temperature and prior loading.

Practical Notes

• Cable and busbar sizing should be based on full load current with the applicable derating factors for grouping, ambient temperature and installation method.
• Current transformer ratios are normally selected at or just above the transformer's full load current — for example a 1000/5 A CT on the LV side of a 630 kVA, 400 V unit.
• Where a transformer has multiple tap positions, full load current on the HV side varies with the selected tap. The rating plate current corresponds to the principal tap.
• For single-phase units the same table can be used by dividing the three-phase current by √3 and applying the single-phase formula.