Winding Resistance Temperature Correction Calculator
Winding resistance is measured at whatever temperature the transformer happens to be — after a run, in the sun, or after a soak test. To compare that reading with the factory record or a previous test, it must be corrected to a common reference temperature. This calculator applies the IEC 60076-1 / IEEE C57.152 correction and, when three phase readings are entered, the phase-to-phase unbalance.
Key takeaways
- Correction is a ratio, not a subtraction: R2 = R1 × (K + T2) / (K + T1), with K = 235 for copper and 225 for aluminium.
- The reference is usually 75 °C for oil-immersed transformers. Use the temperature written in the specification or the factory record, not a default chosen by habit.
- Phase unbalance is judged after correction. Comparing raw readings taken at different temperatures invents a fault that is not there.
- A pass on this calculation does not clear the transformer. Always compare with the factory record and the previous test as well.
1. What this calculator does — and what it does not
It converts a measured winding resistance from the measurement temperature to a chosen reference temperature, and it computes the phase-to-phase unbalance once all three phases have been corrected to the same reference. It works for copper and aluminium windings and accepts either a single reading or a three-phase set.
It is a comparison aid, not a diagnostic. It does not tell you whether a transformer is fit for service, does not replace the factory acceptance record, and does not account for temperature gradients inside a large winding. Where a standard, a utility specification or the manufacturer’s instruction book sets a different reference temperature or limit, that document governs.
2. Winding resistance temperature correction calculator
Formula: IEC 60076-1 / IEEE C57.152. Copper K = 235 (IEEE C57.152 uses 234.5 — the difference is below 0.2% at normal winding temperatures). Aluminium K = 225. Confirm the reference temperature and the acceptance limit in the governing specification.
3. How to read the result
The correction factor moves the reading in the right direction
If the winding was measured below the reference temperature, the factor is greater than 1 and the corrected resistance is higher. If it was measured above the reference, the factor is less than 1 and the corrected resistance is lower. A factor far from 1 is normal: a copper winding measured at 25 °C rises by about 19% when corrected to 75 °C.
Unbalance is a phase comparison, not an absolute limit
The tool computes (Rmax − Rmin) / Raverage × 100% after correction. A common screening limit is 2%, but the number in your specification — or the factory record — is the one that counts. A single phase reading that is high or low can point to a tap-changer contact, a joint, a poorly made connection during the test, or a genuine winding problem. Check the test connections before condemning the transformer.
All three phases must be at the same temperature — or corrected individually
The three-phase form uses one measurement temperature. If the phases were measured at materially different temperatures, correct each one separately in single-reading mode and compare the corrected values. Do not average raw readings taken at different temperatures.
4. Formula and constants
| Quantity | Expression | Notes |
|---|---|---|
| Temperature correction | R2 = R1 × (K + T2) / (K + T1) | R in mΩ or Ω, as long as both readings use the same unit. |
| Copper constant | K = 235 | IEC 60076-1. IEEE C57.152 uses 234.5; the practical difference is negligible. |
| Aluminium constant | K = 225 | Confirm the winding material from the nameplate before using it. |
| Phase unbalance | (Rmax − Rmin) / Raverage × 100% | Compute after correcting all three phases to the same reference. |
| Typical reference | 75 °C | Some specifications use 85 °C; the factory record states the one that applies. |
5. Correction factors at common measurement temperatures
Factor to convert a reading to 75 °C. Use the calculator above for any other temperature or reference.
| Measured at | Copper (K = 235) | Aluminium (K = 225) |
|---|---|---|
| 20 °C | 1.216 | 1.224 |
| 25 °C | 1.192 | 1.200 |
| 30 °C | 1.170 | 1.176 |
| 40 °C | 1.127 | 1.132 |
| 50 °C | 1.088 | 1.091 |
| 60 °C | 1.051 | 1.053 |
6. Frequently asked questions
What temperature should I correct to?
Use the reference temperature in the specification or the factory record. For oil-immersed power transformers that is commonly 75 °C; some specifications and some manufacturers use 85 °C. Mixing references between tests is a common source of false trends.
Why 235 for copper and 225 for aluminium?
They are the inferred temperature coefficients of resistance used in the correction formula. IEC 60076-1 uses 235 for copper and 225 for aluminium; IEEE C57.152 uses 234.5 for copper. At normal winding temperatures the difference between 235 and 234.5 is well below the measurement uncertainty.
Should I correct before or after comparing phases?
Correct first. Phase comparison only means something when all three readings refer to the same temperature. If the phases were measured at different temperatures, correct each one separately and then compare.
What phase unbalance is acceptable?
A 2% phase-to-phase difference is a common screening limit, but the governing specification or the manufacturer’s instruction book may set a different one. A value inside the limit is not a clean bill of health: compare with the factory record and the previous test, and investigate any step change.
Does this calculator work for aluminium windings?
Yes — select aluminium and the tool uses K = 225. Confirm the winding material from the nameplate; using the copper constant on an aluminium winding biases every corrected value.
7. Where to go next
For the test procedure and what the readings mean, see transformer winding resistance testing. For where this test sits in a commissioning programme, see the transformer commissioning test sequence. The TWO-1100S winding ohmmeter measures the resistance this calculator corrects; if you need a configuration for a specific transformer, request a quote.
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