Transformer Efficiency Classes Explained
Reviewed by the Fengguang engineering team · Last updated 8 Nov 2026
Efficiency classes are not a simple ranking of quality. They describe loss limits at defined ratings and conditions. Learn how to compare them fairly.
What an efficiency class is
An efficiency class sets maximum loss values at defined reference conditions, usually at rated voltage, rated frequency and a specified reference temperature. It allows buyers to compare designs on the same basis. It is not a guarantee that every unit in the class has the same losses, and it does not replace the project datasheet.
Why the highest class is not always right
A higher class usually means a larger core, more conductor material or a different winding design. That can increase first cost, mass and dimensions. For a lightly loaded or intermittent site, the energy saving may not repay the price difference. The right class comes from a life-cycle calculation and the buyer’s specification.
Check the test basis
Confirm the reference temperature, the loss values, the tolerance and the test standard. Ask for the guaranteed values that will be checked in the routine test, not only the class label.
| Decision factor | Question to ask |
|---|---|
| Load profile | How many hours at what load? |
| Tariff | What is the cost of a kWh over the life? |
| First cost | How much does the higher class add? |
| Space / mass | Does the larger unit fit? |
| Specification | Which class does the buyer require? |
FAQ
Is a higher efficiency class always better?
It is better on losses, but not always on total cost. Check the load profile, tariff, first cost and physical constraints.
Do efficiency classes apply to dry-type transformers?
Yes, the relevant standard and class depend on the transformer type and market. Confirm the exact standard and loss table for the project.
Can I compare classes across manufacturers?
Only if the losses are quoted at the same reference conditions and standard. Ask for a like-for-like loss table.
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Need this applied to a project?
Send the application, voltage class and site conditions through the inquiry form. The team will check the transformer family and the document scope.
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