Transformer Voltage Regulation Calculator
Secondary voltage drop under load from impedance, resistance, load and power factor. With the formula, a worked example and typical values.
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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.
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About this calculator
This page calculates regulation and secondary voltage on load from impedance, resistance, load, power factor and 1 more input. 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
A leading power factor can give negative regulation (voltage rise); this calculator covers lagging loads only.
Cross-check: 8 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
- Voltage regulation from the resistance, reactance, load and power factor entered.
What remains unverified
- The impedance and loss figures: take them from the test certificate or data sheet.
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 60076-1, 2011 Ed 3.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Power-transformer rating and performance context. The calculator uses user-entered resistance/impedance data and an approximate regulation relationship; confirm the test basis and sign convention. |
| Cross-check test cases | 8 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 |
|---|---|
| Impedance (%) | 5 |
| Resistance (%) | 1 |
| Load (fraction of rating) | 1 |
| Power factor | 0.8 |
| No-load secondary voltage | 415 V |
- X = √(5² − 1²) = 4.899 %
- Regulation = 3.7945 %
- Vload = 415 × (1 − 0.03794) = 399.25 V
Assumptions
- Lagging power factor
- Primary voltage held at rating
- Load balanced
A leading power factor can give negative regulation (voltage rise); this calculator covers lagging loads only.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Impedance (%) | (number) | greater than 0, up to 100 | |
| Resistance (%) | (number) | 0 or more, up to 100 | Copper loss as % of rating; reactance is derived from Z and R |
| Load (fraction of rating) | (number) | greater than 0, up to 1.5 | |
| Power factor | (number) | greater than 0, up to 1 | Rough starting point only. Resistive heating and incandescent loads are 1.0; a fully loaded induction motor is typically 0.8 to 0.88 and falls sharply when lightly loaded; discharge lighting depends on its ballast or driver. For real work use the nameplate or a measured value. |
| No-load secondary voltage | V | greater than 0 V |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Regulation (%) | (number) | Shown to 2 decimal places | |
| Secondary voltage on load | V | Shown to 1 decimal place |
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 no-load secondary voltage
| No-load secondary voltage (V) | Regulation | Secondary voltage on load (V) |
|---|---|---|
| 41.5 | 3.79 | 39.9 |
| 104 | 3.79 | 100.1 |
| 208 | 3.79 | 200.1 |
| 415 | 3.79 | 399.3 |
| 830 | 3.79 | 798.5 |
| 1,660 | 3.79 | 1,597 |
| 4,150 | 3.79 | 3,992.5 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60076 series (power transformers)
Impedance, losses and cooling class are manufacturer values; enter the figures from the transformer data sheet. 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 impedance (%), resistance (%), load (fraction of rating), power factor and no-load secondary voltage. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Lagging power factor. Primary voltage held at rating. Load balanced.
Which calculators should I use next?
Common next steps are Transformer Efficiency and Losses 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. 8 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.
- Transformer Formulas: Current and Fault Level: Transformer formulas for turns ratio, full-load current, impedance and fault current
- Transformer Tap Changer Formulas: Transformer tap changer formulas: secondary voltage at a primary tap, the tap for a target voltage
- Voltage Change and Flicker Formulas: Voltage change formulas: step change from a load, the dip as the ratio of load to short-circuit level
More Transformer Engineering calculatorsAll engineering calculatorsTransformer Voltage Regulation in Elecmetric