Electrical insulation classes A, E, B, F and H explained
What insulation thermal classes mean, their temperature limits, and common materials used in each class. · Updated
What a thermal class means
Insulation classes describe the maximum temperature an insulation system is designed to withstand in continuous service. They are defined in IEC 60085 and are used for motors, transformers and other windings. The class applies to the complete insulation system tested together, not just one material.
Classes and temperatures (IEC 60085)
- Class Y: 90 °C
- Class A: 105 °C
- Class E: 120 °C
- Class B: 130 °C
- Class F: 155 °C
- Class H: 180 °C
- Higher classes (200, 220, 250 °C) are used for special high-temperature designs.
Materials commonly used
- Class A: impregnated paper, pressboard, cotton and similar organic materials.
- Class E and B: polyester films and enamelled wires, mica and glass-based materials with suitable binders.
- Class F and H: glass fibre and mica with high-temperature resins, aramid paper such as Nomex, polyimide film such as Kapton, silicone-based materials.
- Always confirm the rating of the exact grade and the system it will be used in.
Frequently asked questions
What temperature is class F insulation?
Class F is rated for 155 °C under IEC 60085.
What temperature is class H insulation?
Class H is rated for 180 °C under IEC 60085.
Can I upgrade a motor's insulation class by changing one material?
Not reliably. The class applies to the whole insulation system, including wire enamel, slot liners, sleeving, varnish and leads. All components must suit the target class.