Transformer Percent Impedance Test Calculator
Provides a transformer percent-impedance estimate from test voltage on the rated winding after confirming that test current matches the rated-current basis.
- Percent impedance
- %Z
- Primary-to-secondary voltage ratio
- x
- Test-current deviation
- %
- Current-match tolerance used
- %
- Test-to-rated current ratio
- x
Calculation details
- Calculation basis
- Boundary
Recent results
Formulas
- \(a = \frac{V_{\mathrm{primary,rated}}[\mathrm{V}]}{V_{\mathrm{test\ winding,rated}}[\mathrm{V}]}\)
- \(\Delta I_{\%} = \frac{\left|I_{\mathrm{test}}[\mathrm{A}]-I_{\mathrm{rated}}[\mathrm{A}]\right|}{I_{\mathrm{rated}}[\mathrm{A}]}\times100\)
- \(I_{\mathrm{ratio}} = \frac{I_{\mathrm{test}}[\mathrm{A}]}{I_{\mathrm{rated}}[\mathrm{A}]}\)
- \(Z_{\%} = \frac{V_{\mathrm{test}}[\mathrm{V}]}{V_{\mathrm{test\ winding,rated}}[\mathrm{V}]}\times100\)
A transformer percent impedance test calculator determines percent impedance (%Z) from the voltage measured on the rated test winding while the transformer is carrying its rated-current basis. The result expresses the measured impedance-test voltage as a percentage of that winding’s rated voltage.
This calculation is used to verify and document a transformer impedance-test record when the test is performed on the rated winding at the correct current basis. The displayed result is Percent impedance, along with the transformer voltage ratio and a check that the measured test current agrees with the stated rated-current basis.
The calculation does not convert transformer impedance to ohms. It also does not produce transformer-limited fault current, available fault current, protective-device information, equipment short-circuit ratings, conductor ampacity, voltage-drop results, or a fault-study result.
Rated-Winding Test Conditions
The calculation applies only when Impedance-test voltage is measured on the winding identified by Rated test-winding voltage and the measured Impedance-test current matches the Rated current basis within the selected Current-match tolerance.
The required entries are:
| Input | Electrical purpose |
|---|---|
| Rated primary voltage (V) | The primary voltage used for the transformer ratio record |
| Rated test-winding voltage (V) | The rated voltage of the winding on which impedance-test voltage was measured |
| Rated current basis (A) | The rated-current basis applicable to the impedance test |
| Impedance-test voltage (V) | The measured voltage applied on the rated test winding during the test |
| Impedance-test current (A) | The measured test current; it must match the rated-current basis |
| Current-match tolerance (%) | The permitted difference between measured test current and rated current |
Primary-to-secondary voltage ratio provides a ratio-record check between Rated primary voltage and Rated test-winding voltage. It does not change the percent-impedance arithmetic.
Percent-Impedance Formula
When the measured test current matches the rated-current basis, percent impedance is:
\(\displaystyle \text{Percent impedance} = \left( \frac{\text{Impedance-test voltage}} {\text{Rated test-winding voltage}} \right) \times 100\)
The calculator also evaluates the test-current relationship:
\(\displaystyle \text{Test-to-rated current ratio} = \frac{\text{Impedance-test current}} {\text{Rated current basis}}\)
\(\displaystyle \text{Test-current deviation} = \left| \frac{\text{Impedance-test current} - \text{Rated current basis}} {\text{Rated current basis}} \right| \times 100\)
The calculated Test-current deviation must not exceed Current-match tolerance for the stated rated-current basis to be accepted by this arithmetic check.
Calculation Example
For a transformer record with these values:
| Input | Value |
|---|---|
| Rated primary voltage | 12,470 V |
| Rated test-winding voltage | 480 V |
| Rated current basis | 90.211 A |
| Impedance-test voltage | 27.6 V |
| Impedance-test current | 90.211 A |
| Current-match tolerance | 1% |
The percent impedance is:
\(\displaystyle \text{Percent impedance} = \left( \frac{27.6\text{ V}} {480\text{ V}} \right) \times 100 = 5.75\%Z\)
The resulting calculation values are:
| Result | Value | Interpretation |
|---|---|---|
| Percent impedance | 5.75%Z | Test voltage as a percentage of the 480 V rated test winding |
| Primary-to-secondary voltage ratio | 25.9792 x | (12,470 / 480) |
| Test-current deviation | 0% | Impedance-test current exactly matches Rated current basis |
| Current-match tolerance used | 1% | Allowed deviation entered for the arithmetic check |
| Test-to-rated current ratio | 1 x | (90.211 / 90.211) |
A result of 5.75%Z means that 27.6 V was measured on the 480 V rated test winding while the test was performed at the 90.211 A rated-current basis.
Test Record Verification
A valid result depends on correctly identifying the tested winding. Rated test-winding voltage must be the voltage rating of the same winding where Impedance-test voltage was measured. Using the primary voltage in the denominator when the test voltage was measured on the secondary winding produces a percentage that does not represent the stated test condition.
The current fields serve a separate verification function. Rated current basis establishes the current level required for the impedance-test arithmetic, while Impedance-test current records the actual measured value. If the measured current differs from the rated basis by more than Current-match tolerance, the voltage measurement is not being evaluated at the stated rated-current condition.
Field and Engineering Limits
This is reverse-direction impedance-test arithmetic only. It assumes that the test voltage was measured on the rated winding and that the impedance-test current corresponds to the rated-current basis.
A percent-impedance result should not be treated as an impedance in ohms or entered directly as a field fault-current answer. Any fault-current, equipment-duty, protective-device, conductor, feeder, branch-circuit, raceway, voltage-drop, or protection decision requires separate engineering inputs and the applicable equipment data, installation conditions, and AHJ requirements.
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FAQs
Which voltage belongs in the percent-impedance formula?
Use the rated voltage of the same winding where the impedance-test voltage was measured. The test voltage and rated voltage must use the same winding basis.
Why must test current match rated current?
The direct voltage-ratio calculation assumes the measured impedance-test voltage was taken at the rated-current basis. This page rejects a larger mismatch than your entered tolerance.
Does this calculate transformer fault current?
No. It only reverse-calculates percent impedance from a matched test basis and does not output impedance ohms or fault current.