NEMA Insulation Classes

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

• T(^oF) = [T(^oC)](9/5) + 32

¹⁾ Allowable temperature rises are based upon a reference ambient temperature of 40^oC. Operation temperature is reference temperature + allowable temperature rise + allowance for "hot spot" winding.

Example Temperature Tolerance Class F:

40 ^oC + 105 ^oC + 10 ^oC

    = 155 ^oC

In general a motor should not operate with temperatures above the maximum. Each 10 ^oC 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^oC 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.

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm