Which classes are the most common NEMA motor insulation classes?

The most common NEMA (National Electrical Manufacturers Association) motor insulation classes are B and F. Class B insulation can operate at temperatures up to 130°C and is widely used due to its good balance of thermal resistance and cost-effectiveness. It is often suitable for general-purpose motors. Class F insulation, with a higher temperature limit of 155°C, is used in applications where motors encounter higher thermal stresses.

Motors with Class F insulation can provide a longer lifespan and reliability under strenuous operating conditions compared to lower-rated insulation classes. The prevalence of B and F insulation classes reflects their suitability for a wide array of applications and their ability to handle various operating temperatures and environments.

The other options refer to combinations of insulation classes that are not as commonly utilized in industry. For instance, Class A has a lower temperature rating and is less frequently used in modern motor applications, while Class C and Class H serve specialized functions that do not dominate the standard motor insulation landscape.