If you have ever read a transformer nameplate, you have seen the letters: Class B, Class F, Class H. They look like a single number you either meet or miss. In practice they are the single most important lever in insulation selection — get the class wrong and you either overspend on margin you never use or quietly shorten the life of the unit. This guide explains what the classes actually mean, where the “hotspot” comes from, and how to map a temperature class to a real insulation material.
What a Thermal Class Actually Is
A thermal class is the temperature at which an insulating material is expected to retain its required properties for its rated service life — commonly defined as about 20,000 hours (roughly 3 years of continuous duty). The standard classes most relevant to transformers are:
| Class | Max temp (°C) | Typical materials |
|---|---|---|
| A | 105 | impregnated paper, cotton |
| B | 130 | DMD (polyester-based composites) |
| F | 155 | NMN, F-grade DMD, prepreg |
| H | 180 | NHN, silicone-bonded materials |
| N / C | 200 / 220+ | polyimide-based (NHN top tier) |
The number is not the surface temperature you feel — it is the hottest-spot temperature the material is allowed to reach.
The Hotspot: Why the Number Is Not the Room
A winding does not heat evenly. The temperature at the hottest point inside the coil is higher than the average by a margin the standards call the *hotspot increment*. The usual reference basis is:
Hottest-spot temp = Ambient (40°C) + Winding temperature rise + Hotspot increment (≈10°C)
So when a designer says “Class F,” they mean the hottest spot must stay at or below 155°C under a 40°C ambient. The average winding rise is therefore allowed to be about 105 K for a Class F design.
A common and useful trick: build with Class F materials but limit the temperature rise to Class B (an “F/B” design). You get the thermal headroom of F-grade materials while running cooler, which extends service life substantially. This is why many modern dry-type transformers are sold as “F/B.”
Mapping Temperature Class to Material
Once you know the allowable hottest-spot temperature, the material choice follows:
- Up to 130°C (Class B): DMD flexible composite is the cost-effective default for layer and phase insulation.
- Up to 155°C (Class F): NMN and F-grade DMD, plus DMD prepreg for coil interlayer and end insulation in dry-type units.
- Up to 180°C (Class H) and beyond: NHN (Nomex – polyimide – Nomex) for traction, wind, and high-temperature duty.
Browse the full range in our Flexible Composite Insulation category.
How to Specify: Three Steps
- Find the hottest spot, not the average. Use the rated ambient plus the declared temperature rise plus ~10°C hotspot. That is the number your insulation must survive continuously.
- Pick the class with margin. Choose a material whose class sits above the hottest-spot temperature. Running an F-grade material at a B-level rise is a cheap way to buy life.
- Confirm thickness and creepage separately. Thermal class is only one axis. Slot fill, creepage distance, and mechanical duty still have to be met — grade and thickness are separate decisions.
A Note on the Standards
The classes above follow IEC 60085 (and equivalent IEEE/ANSI practice), which defines the temperature index and the expected life at that temperature. When a supplier quotes a class, ask for the *tested* temperature index, not just a label — a material marked “Class F” should show a documented 155°C rating. This keeps procurement honest and stops a mislabeled composite from quietly limiting your transformer’s life.
Common Mistakes
- Confusing surface temp with class temp. Touching a warm tank tells you nothing about the coil hotspot. Specify from the declared rise, not the feel.
- Over-specifying. Putting NHN in a design that never exceeds 130°C pays for temperature margin that is never used.
- Under-specifying. Running Class B DMD where the hotspot reaches 160°C ages the composite early and trades a small saving for field failures.
- Ignoring the F/B trick. Many buyers don’t realize F-grade materials run at B-level rise can extend life at little extra cost.
Conclusion
Temperature class is the backbone of insulation selection: it turns a vague “keep it cool” into a number you can design and buy against. Read the hottest spot, pick the class with margin, and match the material — DMD for B, NMN/prepreg for F, NHN for H and above — and your transformer gets reliability without unused cost.
Need help mapping a temperature class to the right insulation for your transformer? Contact our team with your rated rise and duty — we will match the grade and thickness to your build.
Related reading: NMN vs DMD: How to Choose the Right Flexible Composite Insulation
Browse all flexible composites: Flexible Composite Insulation category