Class F and Class H are commonly used insulation classes for dry-type transformers. They are often discussed together with transformer temperature rise, but these two specifications are not the same.
The insulation class defines the thermal capability of the insulation system, while temperature rise indicates how much the transformer winding temperature increases above the specified ambient temperature during operation.
Understanding the difference helps engineers select a suitable transformer for different operating conditions.
What Are Class F and Class H?
The main difference is the temperature capability of the insulation system:
Class F: 155°C insulation system temperature rating
Class H: 180°C insulation system temperature rating
These values describe the thermal endurance of the insulation system. They should not be interpreted as the normal operating temperature of the transformer.
A Class H transformer, for example, does not necessarily operate at 180°C. Actual winding temperature depends on temperature rise, ambient temperature, loading, cooling, and transformer design.
Temperature Rise vs. Insulation Class
Temperature rise and insulation class are separate technical parameters.
Temperature rise refers to the increase in winding temperature above the surrounding ambient temperature under specified operating conditions.
For example, if the ambient temperature is 40°C and the specified winding temperature rise is 100 K, the corresponding winding temperature would be approximately 140°C under that condition.
The insulation class indicates the thermal capability of the insulation system. Therefore, a transformer can use Class H insulation while being designed with a relatively low temperature rise.
When comparing transformers, both insulation class and temperature rise should be considered.
How Ambient Temperature Affects Transformer Temperature
Ambient temperature is an important factor in transformer thermal design.
A simplified relationship is:
Winding Temperature ≈ Ambient Temperature + Temperature Rise
Therefore, a higher ambient temperature generally results in a higher operating temperature under the same load and cooling conditions.
For projects in hot climates, such as the Middle East and Africa, the specified ambient temperature should be considered when selecting the transformer.
Ambient temperature is an important factor in transformer thermal design.
A simplified relationship is:
Winding Temperature ≈ Ambient Temperature + Temperature Rise
Therefore, a higher ambient temperature generally results in a higher operating temperature under the same load and cooling conditions.
For projects in hot climates, such as the Middle East and Africa, the specified ambient temperature should be considered when selecting the transformer.
Which Should You Choose: Class F or Class H?
The choice should be based on the project requirements rather than insulation class alone.
Class F may be suitable for many standard commercial and industrial dry-type transformer applications.
Class H may be considered when the project requires a higher insulation temperature capability or has more demanding thermal conditions.
Other factors should also be evaluated, including:
Temperature rise
Ambient temperature
Transformer loading
Cooling method
Installation conditions
Altitude
Enclosure and ventilation requirements
Conclusion
Class F and Class H describe different insulation temperature capabilities: 155°C for Class F and 180°C for Class H.
They should not be confused with transformer temperature rise. A higher insulation class does not automatically mean a lower operating temperature or better overall transformer performance.
For dry-type transformer selection, insulation class, temperature rise, ambient temperature, cooling method, and actual operating conditions should be evaluated together.
