Power distribution
Transformer Full-Load Current Calculator
Calculate the rated line current on both the primary and secondary of a transformer from its kVA rating, the two voltages, and the phase arrangement — the starting point for sizing the incoming and outgoing protection, cables, and CTs.
Formulas used
where S is the apparent power (VA), V the line voltage on the side being calculated, and I the line current on that side. The apparent power is the same on both windings (losses aside), so the low-voltage side always carries the higher current. A 1000 kVA, 11 kV / 400 V transformer draws about 52 A on the primary but delivers about 1443 A on the secondary.
What the currents are used for
- Secondary current sizes the main LV breaker or ACB, the busbar, and the outgoing distribution.
- Primary current sizes the HV protection and the incoming cable or ring-main unit.
- Both feed the CT ratios for metering and protection on each side.
Frequently asked questions
Why is the secondary current so much higher?
Power is conserved across the windings, so as the voltage steps down the current steps up by the same ratio. The low-voltage side therefore carries the larger current and needs the heavier busbar and protection.
Which voltage do I enter for three-phase?
The line-to-line voltage on each side (for example 11000 V and 400 V), because the √3 relationship uses the line voltage.
Does this give the fault current too?
No. This is the rated full-load current. The prospective short-circuit current at the secondary depends on the impedance voltage uk — use a short-circuit calculation for that.
Size the protection next
With both currents known, select the primary and secondary breakers, ATS, and CTs — or ask JUTRION for a suitable set for your market.
Engineering estimate for guidance only. Final protection, cable, and CT selection must account for inrush, the impedance voltage, the fault level, and the applicable standards, verified by a qualified person.