Horizontal or Vertical Shaft Down. UL Recognized for US and Canada (E47479). NEMA 48 frame three-phase general purpose AC motors have a standard 1/2" diameter, 1-1/2" length shaft whose center is 3 above the base mounting surface. Variations to the frame's standard dimensions and design are indicated by a letter designation following the frame number. These frame standards and letter designations are defined by the National Electrical Manufacturers Association (NEMA).