Incorrect insulation class also causes electric motor winding failure. Insulation classes are important because they determine a motor’s ability to withstand heat. There are four general classes of insulation, which are A, B, F, and H. Classes B and F are the most common. These classes can withstand temperatures up to 104ºF. Insulation is designed to perform up to the maximum rated winding temperature, which is the combination of allowed temperature and ambient temperature. Motors that are operated at a temperature higher than the recommended insulation class will suffer from reduced performance quality and a shorter service life.