Transformer Current and Fault Level Calculator
Full-load amps and fault level for a distribution transformer.
Your numbers
Your results
Transformer Current and Fault Level Calculator by TheCalcTools
How the Transformer Current and Fault Level Calculator works
Enter the transformer’s kVA, primary and secondary voltages and impedance (%Z) from its nameplate.
Full-load current = kVA ÷ (√3 × V).
The secondary fault current assumes an infinite source: I_sc = I_full-load ÷ Z.
The formula
Worked example
Using the default values shown in the calculator:
| Transformer rating (kVA) | 400 |
|---|---|
| Primary voltage (V) | 11000 |
| Secondary voltage (V) | 400 |
| Impedance voltage (%) | 4 % |
- Primary: 400,000 ÷ (√3 × 11,000) = 21.0 A. Secondary: 577.4 A.
- Fault current at 4% = 14.43 kA (10.00 MVA).
Result:
| Primary full-load current (A) | 21.0 |
|---|---|
| Secondary full-load current (A) | 577.4 |
| Secondary fault current (kA) | 14.43 |
| Fault level (MVA) | 10.00 |
Frequently asked questions
Is the fault current this high in practice?
This is the maximum from the transformer with an infinite upstream source. Real values are lower once source impedance and cable impedance are included.
Why does it matter?
Breakers and busbars must be rated for at least the prospective fault current at their location.
Sources and further reading
Related calculators
More in Telecom Power & Passive Infrastructure · All calculators
This calculator gives estimates for planning only. Electrical installations must be designed, checked and carried out by qualified personnel following local codes and the equipment manufacturer's data. Never rely on this result alone for safety-critical decisions. See our disclaimer.