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Transformer full-load current calculator
Rated current on both sides of a three-phase or single-phase transformer from its kVA rating and voltages, and the maximum fault current at the secondary terminals from uk. Free, no signup.
- Primary, secondary and fault current
- Table of standard ratings
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Secondary full-load current
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Primary: —
Check the inputs: positive power and voltages, and uk between 0 and 100 % (or empty).
- Primary full-load current
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- Secondary full-load current
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- Voltage ratio
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- Secondary fault current (infinite source)
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- Fault level at the secondary
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Fault current at the secondary terminals with an infinitely strong upstream network and no voltage factor: the most the transformer lets through. For IEC 60909 with the network impedance, use the short-circuit calculator.
How the calculation works
The full-load (rated) current is the current at rated power and rated voltage. The power is the same on both sides, so the current changes in inverse proportion to the voltage.
- Three-phase transformerIr = Sr / (√3 · Ur)On each side, with that side’s line-to-line voltage.
- Single-phase transformerIr = Sr / Ur
- Maximum fault current at the secondary terminalsIk = Ir / (uk / 100), Sk = Sr / (uk / 100)With an infinitely strong upstream network. A real network adds impedance and lowers the current.
Worked example
A 1000 kVA, 20/0.4 kV three-phase transformer with uk = 6 %:
| Primary full-load current | 28.87 A |
|---|---|
| Secondary full-load current | 1443 A |
| Secondary fault current (infinite source) | 24.06 kA |
| Fault level at the secondary | 16.67 MVA |
Full-load current of standard three-phase transformers
| Rating | 400 V | 480 V | 11 kV | 20 kV | 33 kV |
|---|---|---|---|---|---|
| 25 kVA | 36.08 | 30.07 | 1.312 | 0.7217 | 0.4374 |
| 50 kVA | 72.17 | 60.14 | 2.624 | 1.443 | 0.8748 |
| 63 kVA | 90.93 | 75.78 | 3.307 | 1.819 | 1.102 |
| 100 kVA | 144.3 | 120.3 | 5.249 | 2.887 | 1.750 |
| 160 kVA | 230.9 | 192.5 | 8.398 | 4.619 | 2.799 |
| 200 kVA | 288.7 | 240.6 | 10.50 | 5.774 | 3.499 |
| 250 kVA | 360.8 | 300.7 | 13.12 | 7.217 | 4.374 |
| 315 kVA | 454.7 | 378.9 | 16.53 | 9.093 | 5.511 |
| 400 kVA | 577.4 | 481.1 | 20.99 | 11.55 | 6.998 |
| 500 kVA | 721.7 | 601.4 | 26.24 | 14.43 | 8.748 |
| 630 kVA | 909.3 | 757.8 | 33.07 | 18.19 | 11.02 |
| 800 kVA | 1,155 | 962.3 | 41.99 | 23.09 | 14.00 |
| 1,000 kVA | 1,443 | 1,203 | 52.49 | 28.87 | 17.50 |
| 1,250 kVA | 1,804 | 1,504 | 65.61 | 36.08 | 21.87 |
| 1,600 kVA | 2,309 | 1,925 | 83.98 | 46.19 | 27.99 |
| 2,000 kVA | 2,887 | 2,406 | 105.0 | 57.74 | 34.99 |
| 2,500 kVA | 3,608 | 3,007 | 131.2 | 72.17 | 43.74 |
| 3,150 kVA | 4,547 | 3,789 | 165.3 | 90.93 | 55.11 |
Rule of thumb: at 400 V the full-load current is about 1.44 A per kVA, and at 480 V about 1.2 A per kVA.
Get these formulas and more in a free two-page PDF cheat sheet
Frequently asked questions
What is the full-load current of a 1000 kVA transformer?
At 400 V three-phase it is 1443 A; at 480 V, 1203 A; at 11 kV, 52.5 A; and at 20 kV, 28.9 A. On each side, divide the rated power by √3 times that side’s line-to-line voltage.
Can a transformer carry more than its full-load current?
For a while. Loading guides such as IEC 60076-7 and IEEE C57.91 allow overloads that depend on the ambient temperature, the load before the overload and its duration, because the real limit is the hot-spot temperature of the winding. Running above rated current for long periods shortens the insulation life.
Why is the secondary current higher than the primary current?
The power is almost the same on both sides, so the current changes in inverse proportion to the voltage. A 20/0.4 kV transformer has a ratio of 50, so its secondary current is 50 times the primary current.
How is uk related to the short-circuit current?
uk is the voltage, in percent of rated voltage, that drives rated current through the transformer with the secondary short-circuited. At full voltage the current is therefore the rated current divided by uk: 6 % gives about 16.7 times the rated current when the upstream network is infinitely strong.
Which current should I use to size the secondary cables?
Size them for the full-load current, or for a lower maximum load if it is well defined, corrected for the installation method and grouping. Then check that they withstand the fault current. The cable sizing calculator does both checks.