Transformer Fault Current Calculator
Calculate the prospective three-phase short-circuit current at a transformer secondary from kVA, voltage and impedance percent, with formula and examples.
Try your own numbers
Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
About this calculator
Use this for a first estimate of the short-circuit current available at the secondary terminals of a transformer, for example when checking the breaking capacity of the breaker or switchgear that will be connected there.
Limits of this calculator
This is an upper bound. It treats the supply as an infinite source and ignores cable impedance, so the real fault current at a downstream board is lower. It is not a substitute for a short-circuit study, and the equipment rating must still be read from the manufacturer data.
Cross-check: 10 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Infinite source, terminal fault, cable impedance ignored (upper bound).
What this calculator checks
- Terminal three-phase fault current from kVA, voltage and transformer impedance, with an infinite source.
What remains unverified
- Source (utility) impedance and cable impedance, so the value is an upper bound.
- Minimum fault level, needed for protection to operate.
- Motor contribution and the IEC 60909 voltage factor.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60909-0, 2026 Ed 3.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Short-circuit current calculation for three-phase AC systems. The calculator is a transformer-impedance estimate and does not assemble a complete network model or apply every IEC correction factor. |
| Cross-check test cases | 10 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Transformer rating | 1,000 kVA |
| Secondary voltage (L-L) | 415 V |
| Impedance | 5 |
- Full-load current = S / (√3 × V) = 1,391.2 A
- Isc = 1,391.2 / 0.05 = 27,824 A
Assumptions
- Infinite source (utility impedance ignored)
- Terminal fault, cable impedance ignored
- Gives an upper bound
Including source and cable impedance lowers the value. Use as a conservative first check.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Transformer rating | kVA | greater than 0 kVA | |
| Secondary voltage (L-L) | V | greater than 0 V | |
| Impedance | (number) | greater than 0, up to 100 | Percent, from the nameplate |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Fault current at terminals | kA | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Transformer rating (kVA) | Fault current at terminals (kA) |
|---|---|
| 250 | 6.96 |
| 500 | 13.91 |
| 1,000 | 27.82 |
| 1,600 | 44.52 |
| 2,000 | 55.65 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60909-0 (short-circuit currents in three-phase AC systems)
- IEC 60947-2 (low-voltage circuit-breakers)
- IEC 60898-1 (circuit-breakers for household and similar use)
This is a first estimate. A protection study and the manufacturer's ratings and time-current curves decide the final selection. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter transformer rating, secondary voltage (L-L) and impedance. Change the unit of a value with its selector and the result stays the same.
Is this the same as a short-circuit current calculator?
It answers the most common version of that question: the three-phase fault level at a transformer secondary. Faults further along a cable run, or with a weak supply, need the source and cable impedance included, which this calculator does not do.
Why is the result called an upper bound?
Adding the utility source impedance and the cable impedance always lowers the fault current. Ignoring them gives the highest value, which is the safe side for a first check of breaker breaking capacity.
What impedance do I enter?
Use the impedance voltage in percent from the transformer nameplate or data sheet. Typical distribution transformers are around 4 to 6 percent, but use the actual figure.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 10 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Short-Circuit Formulas: Fault Level by MVA: Short-circuit formulas using the MVA method: transformer fault level, adding the source, three-phase fault current
- Transformer Through-Fault Current Formulas: Transformer through-fault formulas: the rated currents
- Per-Unit Formulas: Base Values and Impedance: Per-unit system formulas: base current and base impedance from MVA and kV
- Differential Protection Slope Formulas: Transformer differential protection formulas: CT secondary currents, the differential and restraint currents
More Electrical Protection calculatorsAll engineering calculatorsTransformer Fault Current in Elecmetric