Insulation class was the original method used to distinguish insulating materials operating at different temperature levels. Letters were used for different designations.
Letter classifications have been replaced by insulation system temperatures in degrees Celsius. The system temperature is the maximum temperature at the hottest spot in the winding (coil).
Graphical representations of six insulation systems recognized by Underwriters’ Laboratories, Inc. (UL) are shown in Figure A (Below). These systems are used by Acme for a large part of the product line.