In short. A transformer’s percentage impedance sets the maximum current it can pass into a short circuit on its secondary. The lower the impedance, the higher the fault current. Impedance tolerance, X/R ratio, vector group and earthing all affect the numbers used in a study.
Reading the nameplate
- Rating (kVA) and voltage ratio.
- Percentage impedance (%Z): the voltage that must be applied to the primary to drive rated current through a shorted secondary, as a percentage of rated voltage.
- Vector group (such as Dyn11), which determines phase shift and zero-sequence behaviour.
- Tap range and position, which shift the impedance slightly.
Estimating secondary fault current
Assuming a strong source, the secondary fault current is approximately the rated secondary current divided by the per-unit impedance.
Ifault ≈ Irated ÷ (%Z ÷ 100)
For example, a transformer with 5 percent impedance limits the fault to about twenty times rated current. Adding the source impedance lowers it a little.
| %Z | Approx. multiple of rated current |
|---|---|
| 4 | 25 |
| 5 | 20 |
| 6 | 16.7 |
| 8 | 12.5 |
Tolerance and its effect
Impedance has a manufacturing tolerance. For the maximum fault calculation use the lower end, for the minimum use the upper end, as the study standard prescribes. Ignoring tolerance can underestimate the maximum fault level.
X/R ratio
Larger transformers have higher X/R, which raises the DC offset and peak current. Use the transformer’s data or typical values from the standard.
Vector group and zero sequence
Delta-star transformers with earthed star isolate zero-sequence currents from the primary side. This matters for earth-fault levels and protection design. Delta windings block zero-sequence flow, which affects how earth faults on the secondary appear on the primary. See transformer protection.
Parallel transformers
Two transformers in parallel give a higher fault level than one. Check breaker ratings for the paralleled case and unequal impedances, which cause unequal load sharing.
Generator-fed transformers
When a generator feeds the transformer, the fault current is limited by the generator’s reactance and the transformer, and is lower than from the utility. Run both cases. See generator contribution.
Illustrative example
A 1,600 kVA, 6 percent transformer at 415 V has rated secondary current of about 2,226 A, so the approximate secondary fault current with a strong source is about 37 kA. A fast check like this tells a reviewer whether a software result of, say, 60 kA is suspect.
Transformer impedance as the limit on fault current
Percent impedance Z% is the voltage that must be applied to the primary to circulate rated current with the secondary shorted, expressed as a percentage of rated voltage. Low impedance gives better voltage regulation and higher fault current.
Secondary fault current ≈ rated current ÷ (Z% ÷ 100), for an infinite source
Typical impedance by rating (indicative, confirm nameplate)
| kVA | Typical Z% | Fault multiple of rated current |
|---|---|---|
| 100 to 630 | 4 to 5 | 20 to 25 |
| 800 to 2,500 | 5 to 6.25 | 16 to 20 |
| Above 2,500 | 6 to 8 | 12 to 16 |
Other transformer data that matters
- Vector group: a Dyn11 transformer isolates zero-sequence current, which affects earth-fault paths and harmonic circulation.
- Tap position: changes impedance slightly and voltage ratio.
- Tolerance: standards allow impedance tolerance, so use the lower bound for maximum fault current.
Parallel transformers
Paralleling two units doubles fault contribution. Check that switchgear is rated for the parallel condition even if normal operation is split.
Impact on generator systems
A step-up or step-down transformer between the generator and the load changes the fault level seen by each side. It also affects the load a generator can start, because of the series impedance. See generator contribution.
Common mistakes
- Ignoring tolerance.
- Using rated kVA of a different tap.
- Forgetting paralleled transformers.
What to send us
Send transformer nameplate photographs. Send your application details. See also transformer sizing.
Questions engineers ask
Why does a higher %Z reduce the fault current?
More of the voltage drops across the transformer itself, limiting the current.
Is the quick formula exact?
No. It assumes a strong source and ignores cables. Use it as a check.
How does the vector group matter?
It affects phase shift and earth-fault behaviour, important for protection.
Should I use nameplate or test values?
Test reports are better if available. Otherwise use nameplate with tolerance.
Values are illustrative. Use actual data for studies.