Class "B" vs. Class "F" Insulation - Requesting a motor with Class "F" insulation and receiving a motor marked with Class "B" insulation is not a problem because U.S. Electrical Motors builds all their motors with Class "F" insulation or better. When the nameplate indicates Class "B", this means the motor is designed to operate within Class "B" temperature rise limits. These limits are 80 degrees centigrade at 1.0 service factor, and 90 degrees centigrade at 1.15 service factor. For many years, it has been USEM's policy to nameplate open dripproof motors that meet this temperature rise criteria with the insulation Class "B", regardless of the actual insulation type. This gives the end user the information that the motor has a 90 degrees centigrade, or less, temperature rise at 1.15 service factor.

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