Motors Mounting

NEMA Dimensions

NEMA has standardized motor dimensions for a range of frame sizes. Standardized dimensions include bolt hole size, mounting base dimensions, shaft height, shaft diameter, and shaft length. Use of standardized dimensions allows existing motors to be replaced without reworking the mounting arrangement. In addition, new installations are easier to design because the dimensions are known. Standardized dimensions include letters to indicate the dimension’s relationship to the motor. For example, the letter C indicates the overall length of the motor and the letter E represents the distance from the center of the shaft to the center of the mounting holes in the feet (Picture 1). Dimensions are found by referring to a table in the motor data sheet and referencing the letter to find the desired dimension.

Picture 1: Motor dimensions


NEMA divides standard frame sizes into two categories, fractional horsepower and integral horsepower. The most common frame sizes for fractional horsepower motors are 42, 48, and 56. Integral horsepower motors are designated by frame sizes 143 and above. A T in the motor frame size designation for an integral horsepower motor indicates that the motor is built to current NEMA frame standards. Motors that have a U in their motor frame size designation, are built to NEMA standards that were in place between 1952 and 1964.

The frame size designation is a code to help identify key frame dimensions. The first two digits are used to determine the shaft height. The shaft height is the distance from the center of the shaft to the mounting surface. To calculate the shaft height, divide the first two digits of the frame size by 4. For example, a 143T frame size motor (Picture 2) has a shaft height of 3½ inches (14 ÷ 4).

Picture 2: 143T frame size motor


The third digit in the integral T frame size number is the NEMA code for the distance between the center lines of the motor feet mounting bolt holes. The distance is determined by matching this digit with a table in NEMA publication MG-1 (Picture 4). For example, the distance between the center lines of the mounting bolt holes in the feet of a 143T frame is 4.00 inches (Picture 3).

Picture 3: Distance between the bolt holes

Picture 4: Distance between the bolt holes table

IEC Dimensions

IEC also has standardized dimensions, but these dimensions differ from NEMA standards. An example of the IEC dimensions are shown in the following drawing on Picture 5.

Picture 5: IEC Dimensions

Mounting Positions

The typical floor mounting positions are illustrated in the following drawing on Picture 6, and are referred to as F-1 and F-2 mountings. The conduit box can be located on either side of the frame to match the mounting arrangement and position. The standard location of the conduit box is on the left-hand side of the motor when viewed from the shaft end. This is referred to as the F-1 mounting. The conduit opening can be placed on any of the four sides of the box by rotating the box in 90° steps.

Picture 6: F-1 and F-2 mountings


With modification, a foot-mounted motor can be mounted on a wall and ceiling. Typical wall and ceiling mounts are shown in the following illustration on Picture 7. Wall mounting positions have the prefix W and ceiling mounted positions have the prefix C.

Picture 7: Typical wall and ceiling mounts (W & C)

Mounting Faces

It is sometimes necessary to connect the motor directly to the equipment it drives. In the following example (Picture 8) a motor is connected directly to a gear box.

Picture 8: Motor connected to a gear box

C-face Motor

The face, or the end, of a C-face motor has threaded bolt holes. Bolts to mount the motor pass through mating holes in the equipment and into the face of the motor (Picture 9).

Picture 9: C-face motor

D-flange Motor

The bolts go through the holes in the flange of a D-flange motor and into threaded mating holes of the equipment (Picture 10).

Picture 10: D-flange motor