Electrical Insulation · Standards Guide
Class F Insulation (155 °C) Explained
When a motor winding or an insulation sleeve is rated Class F, that single letter tells you how much heat it can take. Understanding thermal insulation classes helps you specify sleeves and coatings that will survive their operating temperature for a full service life.
In short (TL;DR)
Class F = 155 °C maximum continuous hot-spot temperature under IEC 60085. It sits between Class B (130 °C) and Class H (180 °C) and is the most common class for fiberglass insulation sleeves.
What are thermal insulation classes?
Electrical insulation degrades faster the hotter it runs. IEC 60085 classifies insulation by the maximum continuous temperature its hot-spot can tolerate while still achieving a reasonable service life. Each class is a temperature ceiling — stay below it and the insulation lasts; exceed it for long periods and life drops sharply.
Thermal class temperatures
| Class | Max temperature | Typical use |
|---|---|---|
| Class A | 105 °C | Older organic materials |
| Class E | 120 °C | Improved organic systems |
| Class B | 130 °C | Common in general motors |
| Class F | 155 °C | The industry workhorse |
| Class H | 180 °C | High-temperature service |
| Class N | 200 °C | Very high temperature |
Why Class F is the workhorse
At 155 °C, Class F gives designers meaningful thermal headroom over Class B without the cost of high-temperature Class H materials. That balance makes it the default for most motors, transformers and the fiberglass insulation sleeves that protect their wiring.
How coatings achieve Class F
For a fiberglass sleeve, the coating is what delivers the thermal and mechanical performance. Tentuo's TT-AC-909 water-based acrylic emulsion is engineered for Class F (155 °C): in testing it holds a stable finish with no cracking even after a continuous 180 °C × 48-hour bake, keeping its Class F insulation performance. It is also solvent-resistant and RoHS/REACH certified.
Frequently asked questions
What temperature is Class F insulation?
Class F insulation has a maximum continuous hot-spot temperature of 155 °C under IEC 60085. As long as the insulation system stays at or below that temperature in service, it can be expected to last a normal service life.
What is the difference between Class B, F and H insulation?
They are thermal classes defined by maximum operating temperature: Class B is 130 °C, Class F is 155 °C and Class H is 180 °C. A higher class tolerates more heat, which usually allows a more compact or harder-working design.
Why is Class F so widely used for fiberglass sleeves?
Class F (155 °C) offers a strong balance of heat resistance and cost, with enough thermal headroom for most motors, transformers and wiring. That makes it the default choice for fiberglass insulation sleeves, including Tentuo's TT-AC-909 acrylic-coated sleeves.
Does a coating determine a sleeve's insulation class?
The coating is a key part of the insulation system. Tentuo's TT-AC-909 water-based acrylic emulsion is engineered for Class F (155 °C) and keeps that performance even after a 180 °C × 48-hour bake test, with no cracking and a stable finish.
Need Class F fiberglass sleeve coatings?
See the full coating range for fiberglass insulation sleeves, or contact our engineers for samples and qualification support.