Worked example
315 kVA, 11 kV / 400 V, three-phase:
The currents on the two sides are in the inverse ratio of the voltages, because the same kVA passes through the transformer (losses neglected).
Single-phase equivalent
A single-phase transformer of the same 315 kVA at 230 V would carry 315 × 1000 / 230 = 1,370 A. In the three-phase unit each phase winding handles one third of the rating, 105 kVA; on a star-connected 400 V secondary the phase voltage is 231 V and the phase current equals the line current in the table.
What the full-load current is used for
- Cables or busbar trunking from the LV terminals to the main switchboard are sized for at least 454.7 A after applying the correction factors of their installation; the cable size calculator handles the ambient and grouping factors.
- The LV main breaker is chosen with a rated current at or above the full-load current; the smallest of the common MCCB ratings listed on this site that reaches 454.7 A is 500 A. The final choice also depends on the cable, discrimination and the ambient temperature inside the switchboard.
- Current transformers for metering and protection have standard primary ratings in IEC 61869-2 (10, 12.5, 15, 20, 25, 30, 40, 50, 60 and 75 A and their decimal multiples). A common practice is to take the first one at or above the full-load current, here 500 A on the 400 V side; the metering accuracy class and the protection requirements decide the final ratio.
- On the primary side, HV fuses and relay settings start from the primary full-load current but must also ride through the magnetising inrush when the transformer is energised; they are selected from the fuse or relay maker's tables.
- Measured current divided by full-load current gives the transformer loading; the transformer loading calculator adds losses and efficiency.
Example: fault current at the LV terminals
Assume, for illustration only, an impedance voltage of uk = 4 %. IEC 60076-5 lists minimum short-circuit impedances by rating, and the actual value is on the nameplate; it governs. With an infinite upstream network:
A real source impedance lowers this figure and the voltage factor of IEC 60909-0 raises it slightly; the short-circuit calculator takes both into account and continues to the end of a cable. Transformer Short-Circuit Current Calculator