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NEMA Frame/Shaft Sizes

Frame numbers are not intended to indicate electrical characteristics such as horsepower. However, as a frame number becomes higher, so in general does the physical size of the motor and the horsepower. There are many motors of the same horsepower built in different frames. NEMA (National Electrical Manufacturers Association) frame size refers to mounting only and has no direct bearing on the motor body diameter. This may vary from manufacturer to manufacturer.

In any standard frame number designation there are either two or three numbers. Typical examples of frame numbers are 48, 56, 145, and 215. The frame number relates to the “D” dimension (distance from center of shaft to center bottom of mount). For a two digit frame divide the frame number by 16 to arrive at this dimension. For example, in the two-digit 56 frame, the “D” dimension is 3½” (56 divided by 16 = 3½”). For the “D” dimension of a three-digit frame number, consider only the first two digits and use the divisor 4. In frame number 145, for example, the first two digits divided by the constant 4 is equal to the “D” dimension, 14 divided by 4 = 3½”. Similarly, the “D” dimension of a 213 frame motor is 5¼”, 21 divided by 4 = 5¼”.

By NEMA definition, two—digit frame numbers are fractional frames even though 1 HP or larger motors may be built in them. Three-digit frame numbers are by definition integral frames. Refer to NEMA Standard Dimension Chart and variations below.


In addition to the standard numbering system for frames, there are some variations:

C Designates a C flange/face mounted motor. This is the most popular type of face mounted motor and has a specific bolt pattern on the shaft end to allow mounting. The critical items on C face motors are the bolt circle (AJ dimension), rabbet diameter (AK dimension) and the shaft size (U dimension). C flange motors always have threaded mounting holes in the face of the motor.

D The D flange/face mounted motor. This motor has a special type of mounting flange installed on the shaft end. In the case of the D flange, the flange diameter is larger than the body of the motor and it has clearance holes suitable for mounting bolts to pass through from the back of the motor into threaded holes in the mating part. D flange motors are not as popular as C flange motors.

H Used on some 56 frame motors, H indicates that the base is suitable for mounting in either 56, 143T, or 145T mounting dimensions.

J This designation is used with 56 frame motors and indicates that the motor is made for jet pump service with a threaded stainless steel shaft and standard 56C face.

JM JM designates a special pump shaft originally designed for a mechanical seal. This motor also has a C face.

JP Similar to the JM style of motor having a special shaft, the JP motor was originally designed for a packing type of seal. The motor also has a C face.

S  designates that the motor has a short shaft. Short shaft motors have shaft dimensions that are smaller than the shafts associated with the normal frame size. Short shaft motors are designed to be directly coupled to a load through a flexible coupling (Love-Joy, etc.). They are not designed for use in belt drive applications.

T  indicates that the motor is of the 1964 and later T frame vintage.

U  indicates that the motor is of the 1952-1964 U frame vintage.

Y  indicates that the motor has a special mounting configuration. No details may be assumed regarding this designation. These motors are generally made to manufacturers specifications. These are the hardest motors to find suitable replacements.

Z  indicates the existence of a special shaft. The shaft may be longer, larger, or have special features such as threads, holes, etc.


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