Fault Level Calculator
Fault MVA from fault current and voltage. With the formula, a worked example and typical values.
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
This page calculates fault level from line voltage and fault current. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
For the reverse, I = S / (√3 × V). Per-unit base values are in the Per-Unit calculator.
Cross-check: 6 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: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- Conversion between fault current and fault level at the voltage you enter.
What remains unverified
- Where the fault level comes from: the source data and the maximum or minimum case are yours to confirm.
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 level in three-phase AC systems is the governing context. The calculator converts a supplied three-phase fault current to MVA; it does not calculate the fault current itself. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
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 |
|---|---|
| Line voltage (L-L) | 415 V |
| Fault current | 25 kA |
- S = √3 × 415 V × 25 kA = 17.97 MVA
Assumptions
- Symmetrical three-phase fault
- Voltage is the pre-fault line-to-line value
For the reverse, I = S / (√3 × V). Per-unit base values are in the Per-Unit calculator.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Line voltage (L-L) | V | greater than 0 V | |
| Fault current | kA | greater than 0 kA |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Fault level | MVA | 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.
How the result changes with line voltage (l-l)
| Line voltage (L-L) (V) | Fault level (MVA) |
|---|---|
| 41.5 | 1.8 |
| 104 | 4.5 |
| 208 | 9.01 |
| 415 | 17.97 |
| 830 | 35.94 |
| 1,660 | 71.88 |
| 4,150 | 179.7 |
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 line voltage (L-L) and fault current. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Symmetrical three-phase fault. Voltage is the pre-fault line-to-line value.
Which calculators should I use next?
Common next steps are Per-Unit Base Values and Transformer Fault Current. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 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
- Per-Unit Formulas: Base Values and Impedance: Per-unit system formulas: base current and base impedance from MVA and kV
- Transformer Through-Fault Current Formulas: Transformer through-fault formulas: the rated currents
More Electrical Protection calculatorsAll engineering calculatorsFault Level (kA to MVA) in Elecmetric