"U" FRAME: A previously used NEMA designation indicating frame size and dimension (prior to 1965 the standard frame sizes per horsepower rating). U frame motors are still commonly used in the automotive industry.

U.L. Underwriters Laboratories: An independent testing organization which examines and tests devices, systems and materials with particular reference to life, fire and casualty hazards. It develops standards for motor and control for hazardous locations through cooperation with manufacturers. U.L. has standards and tests for explosion-proof and dust ignition proof motors which must be met and passed before application of the U.L. label. http://www.ul.com/