Electrical Motor Insulation Classification:

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, Underwriters Laboratories (UL), and IEC standards.

Electrical Motor insulation and components
Electrical Motor insulation and components

Electrical motor insulation systems are rated by standard NEMA (National Electrical Manufacturers Association) classifications according to maximum allowable operating temperature:

IEC 60085
Thermal class
Old IEC 60085
Thermal class
Letter class
Maximum hot spot (Operation)
temperature allowed
Relative thermal
endurance index (°C)
Typical materials
90 Y 90°C >90 – 105 Unimpregnated paper, silk, cotton, vulcanized natural rubber, thermoplastics that soften above 90° C
105 A 105 A 105°C >105 – 120 Organic materials such as cotton, silk, paper, some synthetic fibers
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 and Alconex GOLD)
250 250°C >250 As for IEC class 200. Further IEC classes designated numerically at 25 °C increments.

T(°F) = [T(°C)](9/5) + 32


Allowable temperature rises are based upon a reference ambient temperature of 40°C. Operation temperature is reference temperature + allowable temperature rise + allowance for “hot spot” winding.

Eg. Temperature Tolerance Class F:

40°C + 105°C + 10°C = 155°C

In general a motor should not operate with temperatures above the maximum. Each 10°C rise above the rating may reduce the motor lifetime by one half. It is important to be aware that insulation classes are directly related to motor life.

Example – a motor operating at 180°C will have an estimated life of:
only 300 hours with Class A insulation
1800 hours with Class B insulation
8500 hours with Class F insulation
tens of thousands of hours with Class H insulation

Temperature Tolerance Class B is the most common insulation class used on most 60 cycle US motors. Temperature Tolerance Class F is the most common for international and 50 cycle motors.