How to use this calculator
- Choose single- or three-phase and enter the rated kVA and both voltages (line-to-line for three-phase).
- Enter the no-load loss P0 and the load loss Pk at rated current from the test report or datasheet.
- Enter the actual load as kVA, as kW with power factor, or as a measured current on the secondary side.
- Read the loading, losses and efficiency. The tool also gives the load at which efficiency peaks.
How it works: the formula
Load (copper) loss is I²R loss in the windings. Current is proportional to load, so this loss scales with the square of loading. No-load (core) loss depends on voltage and frequency and stays almost constant whenever the transformer is energised. Efficiency is highest where the two losses are equal.
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| S_rated | Nameplate rating | kVA |
| S_load | Actual load | kVA |
| x | Loading as a fraction of rating | — |
| P_0 | No-load (core, iron) loss | W |
| P_k | Load (copper) loss at rated current | W |
| η | Efficiency | % |
Worked example
A 1000 kVA, 11 kV / 415 V transformer with P0 = 1700 W and Pk = 10 500 W, carrying 750 kVA at PF 0.9:
Assumptions
- Losses entered are at reference temperature and rated voltage.
- Currents ignore magnetising current and voltage regulation under load.
- Cooling-fan or pump losses are not included in the efficiency.
Limitations
- Overload capability depends on ambient temperature, the load cycle and the cooling class. IEC 60076-7 (oil-immersed) and IEC 60076-12 (dry-type) are the loading guides, together with the manufacturer's limits.
- Harmonic currents increase eddy and stray losses beyond the x² scaling. Transformers feeding non-linear loads may need a K-factor rating or derating.
- Efficiency at a single operating point says nothing about energy losses over a varying load cycle. Weight the result by hours at each load for that.
References
Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.
- IEC 60076-1, Power transformers — Part 1: General (definitions of no-load loss, load loss and routine tests).
- IEC 60076-7, Loading guide for mineral-oil-immersed power transformers; IEC 60076-12 for dry-type transformers.
- IEEE Std C57.12.00, General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
- IEEE Std C57.91, Guide for Loading Mineral-Oil-Immersed Transformers.
Frequently asked questions
Where do I find P0 and Pk?
On the factory test report, or in the maker's datasheet as "no-load losses" and "load losses" (sometimes "short-circuit losses" at 75 °C or 120 °C reference). Regional ecodesign or efficiency rules may also set maximum values for your size.
Why is efficiency highest below full load?
Core loss is fixed and copper loss grows with load squared. Efficiency peaks where they are equal, which for many distribution transformers is somewhere around 30–50 % load.
Can I run a transformer above 100 %?
Sometimes, for limited periods. It depends on ambient temperature, the preceding load and insulation ageing. That judgement belongs to the loading guides and the manufacturer, not to a simple calculation.
Which current should I use for cable sizing?
Use the full-load current of the relevant winding for conductors rated to the transformer, together with the protection rules in your code. The actual current tells you how much headroom is left.