Insulation system

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The electrical 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 electrical insulation system is sometimes referred to as insulation class or thermal classification. The different classes are defined by NEMA,[1] Underwriters Laboratories (UL),[2] and IEC standards.


IEC 60085
Thermal class[3]
Old IEC 60085
Thermal class[3]
NEMA Class [4] NEMA/UL
Letter class
Maximum hot spot
temperature allowed
Relative thermal
endurance index (°C)[3]
Typical materials
90 Y 90 °C >90 - 105 Unimpregnated paper, silk, cotton, vulcanized natural rubber, thermoplastics that soften above 90 °C [5]
105 A 105 A 105 °C >105 - 120 Organic materials such as cotton, silk, paper, some synthetic fibers[6]
120 E 120 °C >120 - 130 Polyurethane, epoxy resins, polyethylene terephthalate, and other materials that have shown usable lifetime at this temperature
130 B 130 B 130 °C >130 - 155 Inorganic materials such as mica, glass fibers, asbestos, with high-temperature binders, or others with usable lifetime at this temperature
155 F 155 F 155 °C >155 - 180 Class 130 materials with binders stable at the higher temperature, or other materials with usable lifetime at this temperature
180 H 180 H 180 °C >180 - 200 Silicone elastomers, and Class 130 inorganic materials with high-temperature binders, or other materials with usable lifetime at this temperature
200 N 200 °C >200 - 220 As for Class B, and including teflon
220 220 R 220 °C >220 - 250 As for IEC class 200
S 240 °C Polyimide enamel (Pyre-ML) or Polyimide films (Kapton, MAGNEKON IMIDIKON(TM) and Alconex GOLD)
250 250 °C >250 As for IEC class 200. Further IEC classes designated 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 temperature increase.

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.

Categories of Insulation[edit]

There are five categories of insulation used by the three different classes of equipment; functional, basic, supplementary, double and reinforced. [7]

See also[edit]


  1. ^ "NEMA Insulation Classes".
  2. ^ E. Alfredo Campo (ed.), Selection of polymeric materials: how to select design properties from different standards William Andrew, 2007 ISBN 0-8155-1551-0 page 170
  3. ^ a b c International Electrotechnical Commission Standard 60085 Electrical Insulation- Thermal Evaluation and Designation, 3rd edition, 2004 ,page 11 table 1
  4. ^ NEMA standard MG-1 Motors and Generators
  5. ^ M. A. Laughton, D. F. Warne (ed), Electrical engineer's reference book, 16th edition Newnes, 2003 ISBN 0-7506-4637-3, page 7-3
  6. ^ Donald G. Fink and Wayne H. Beaty (ed), Standard Handbook for Electrical Engineers, Eleventh Edition, Mc Graw Hill, 1978, ISBN 0-07-020974-X, page 7-12
  7. ^ "Understanding IEC Appliance Insulation Classes: I, II and III". Fidus Power. 6 July 2018.

Further reading[edit]

  • Greg Stone (ed.), Electrical insulation for rotating machines: design, evaluation, aging, testing, and repair, Wiley-IEEE, 2004 ISBN 0-471-44506-1