The insulation system for wires used in generators, electric motors, transformers, and other wire-wound electrical components is divided into different classes by temperature and temperature rise. The insulation system is sometimes referred to as insulation class. The different classes are defined by NEMA,[1] Underwriters Laboratories (UL),[2] and IEC standards.
| IEC 60085 class[3] | NEMA Class [4] | NEMA/UL Letter class |
Maximum hot spot temperature allowed |
Typical materials |
|---|---|---|---|---|
| Y | 90°C | Unimpregnated paper, silk, cotton, vulcanized natural rubber, thermoplastics that soften above 90 C [5] | ||
| A | 105 | A | 105°C | Organic materials such as cotton, silk, paper, some synthetic fibers [6] |
| E | 120°C | Polyurethane, epoxy resins, polyethylene terpthalate, and other materials that have shown usable lifetime at this temperature | ||
| B | 130 | B | 130°C | Inorganic materials such as mica, glass fibers, asbestos, with high-temperature binders, or others with usable lifetime at this temperature |
| F | 155 | F | 155°C | Class 130 materials with binders stable at the higher temperature, or other materials with usable lifetime at this temperature |
| H | 180 | H | 180°C | Silicone elastomers, and Class 130 inorganic materials with high-temperature binders, or other materials with usable lifetime at this temperature |
| 200 | N | 200°C | As for Class B,and including teflon | |
| 220 | 220 | R | 220°C | As for IEC class 200 |
| S | 240°C | |||
| 250 | 250°C | As for IEC class 200. Further IEC classes desiganted numerically at 25 °C increments. |
The maximum hot-spot operating temperature is reached by adding the rated ambient temperature of the machine (often 40°C), a temperature rise, and a 10 °C hot-spot allowance. Electrical machines are usually designed with an average temperature below the rated hot-spot temperature to allow for acceptable life. Insulation does not suddenly fail if the hot-spot temperature is reached, but useful operating life declines rapidly; a rule of thumb is a halving of life for every 10 °C.
Older editions of standards listed materials to be used for the various temperature classes. Modern editions of standards are proscriptive, only indicating that the insulation system must provide acceptable life at the specified temperature rise.
In large machines, different systems may be used according to the predicted temperature rise of the machine; for example, in large hydroelectric generators, stator windings may be Class B but the more difficult to cool rotor winding may be Class F.