Transformer Short-Circuit Current Calculator
Find the nominal secondary current at a distribution transformer's terminals and the prospective three-phase short-circuit current (kA) assuming an infinite upstream busbar.
Formula used
In = S / (√3 × U) · Ik = In / (uk / 100)
Transformer secondary current and three-phase short-circuit current table (400 V)
Calculated at the transformer LV terminals assuming an infinite upstream busbar (uk = 4% up to 630 kVA, uk = 6% from 800 kVA). Upstream network impedance and cable impedance reduce the fault current along the line; motor contribution and peak factor are assessed per IEC 60909.
| Transformer power (kVA) | uk (%) | Nominal current In (A) | Short-circuit current Ik (kA) | Next breaking capacity step |
|---|---|---|---|---|
| 100 | 4% | 144.34 | 3.61 | 4.5 kA |
| 160 | 4% | 230.94 | 5.77 | 6 kA |
| 250 | 4% | 360.84 | 9.02 | 10 kA |
| 400 | 4% | 577.35 | 14.43 | 15 kA |
| 630 | 4% | 909.33 | 22.73 | 25 kA |
| 800 | 6% | 1154.70 | 19.25 | 25 kA |
| 1000 | 6% | 1443.38 | 24.06 | 25 kA |
| 1250 | 6% | 1804.22 | 30.07 | 36 kA |
| 1600 | 6% | 2309.40 | 38.49 | 50 kA |
| 2000 | 6% | 2886.75 | 48.11 | 50 kA |
| 2500 | 6% | 3608.44 | 60.14 | 70 kA |
Example: 1,000 kVA, 400 V transformer with uk = 6%
- Nominal secondary current: In = 1,000,000 VA / (√3 × 400 V) = 1443.38 A.
- Three-phase short-circuit current at terminals: Ik = In / (0.06 × 1,000) = 24.06 kA.
- Next standard breaking capacity step: 25 kA.
How is transformer short-circuit current calculated?
First, the nominal secondary current is found from In = S / (√3 × U). Ignoring upstream network impedance, the symmetrical three-phase short-circuit current at the transformer terminals is Ik = In / (uk / 100). For a 1,000 kVA, 400 V transformer with uk = 6%, In = 1443.38 A and Ik = 24.06 kA.
How is transformer short-circuit current calculated?
Dividing the nominal secondary current by the per-unit impedance voltage (uk / 100) gives the approximate three-phase symmetrical short-circuit current at the transformer terminals.
Is this enough for selecting breaking capacity (Icu / Ics)?
Upstream grid impedance, cable length, parallel transformers, motor contribution and peak short-circuit current must be calculated per IEC 60909 to verify breaker capacity.