Transformer full-load current and kVA calculator
Full-load current on both windings, the LV short-circuit current from the impedance, and the next common kVA rating for your load with power factor, growth and loading margin. A ready table of full-load currents for common ratings sits below.
Basis
Three-phase relations I = S / (√3 × V) and Isc = I × 100 / Z %; infinite source assumed for fault level.
Reference values for planning and comparison. Confirm against the current standard, the purchaser’s specification and the manufacturer’s datasheet before ordering.
Transformer full-load current table (11 kV / 433 V)
| Rating | HV A (11 kV) | LV A (433 V) |
|---|---|---|
| 16 kVA | 0.84 | 21.3 |
| 25 kVA | 1.31 | 33.3 |
| 63 kVA | 3.31 | 84 |
| 100 kVA | 5.25 | 133.3 |
| 160 kVA | 8.4 | 213.3 |
| 200 kVA | 10.5 | 266.7 |
| 250 kVA | 13.12 | 333.3 |
| 315 kVA | 16.53 | 420 |
| 400 kVA | 20.99 | 533.3 |
| 500 kVA | 26.24 | 666.7 |
| 630 kVA | 33.07 | 840 |
| 800 kVA | 41.99 | 1,067 |
| 1000 kVA | 52.49 | 1,333 |
| 1250 kVA | 65.61 | 1,667 |
| 1600 kVA | 83.98 | 2,133 |
| 2000 kVA | 105 | 2,667 |
| 2500 kVA | 131.2 | 3,333 |
| 3150 kVA | 165.3 | 4,200 |
| 5000 kVA | 262.4 | 6,667 |
Worked example: 2000 kVA, 33 kV / 433 V, 6.25 % impedance
- LV full-load current = 2,000,000 ÷ (√3 × 433) = 2,667 A.
- HV full-load current = 2,000,000 ÷ (√3 × 33,000) = 35 A.
- LV fault current ≈ 2,667 × 100 ÷ 6.25 = 42.7 kA with a stiff source.
What is the full-load current of a 1000 kVA transformer?
At 433 V the LV full-load current is about 1333 A; on an 11 kV primary it is about 52.5 A.
How is transformer fault current calculated?
With an infinite source, Isc = full-load current × 100 / %Z. A 1000 kVA, 433 V transformer at 5 % impedance gives about 26.7 kA on the LV side. Source impedance lowers this.
How do I choose the kVA of a transformer?
Divide the maximum demand in kW by the power factor, add load growth, then allow for the loading you want to run at. The calculator rounds up to the next common rating.