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Motor Frame Size

Modified on 2012/06/28 16:32 by Jeff Joslin Categorized as Electrical



Over the last century or so electric motors for industrial use have evolved significantly. They have increased in efficiency and reduced in size. In addition, the motor ratings have been standardized by the National Electric Manufacturers Association (NEMA).

One of the primary standardizations has been in the area of motor frame sizes. Motors rated with the same horsepower, speed, and enclosure will normally have the same frame size from different motor manufacturers. Obviously, this has great benefits for equipment owners faced with the prospect of changing motors.

THREE GENERATIONS (pre 1952,1952-1964, 1964-present)

The standardization effort over the last forty years has resulted in one original grouping of frame sizes called original. In 1952, new frame assignments were made. These were called U frames. The current T frames were introduced in 1964 and continue to be the standard frame designation for electric motors. There are still many U frame motors in service that will have to be replaced in the future. There are fewer original frame size motors (pre-1952) but they may be found on some OWWM. The replacement of these motors continues to be a challenge and it is hoped that this information will assist in finding suitable replacement motors for your OWWM.


The attached PDF file shows the standard frame size assignments for the three different generations of motors. There are separate tables for open drip proof (table 1) and totally enclosed fan cooled (table 2) motors. For each horsepower rating and speed, there are three different frame sizes. The first is the original frame size, the middle one is the U (1952-1964) frame size, and the third one is the T (1964-present) frame.

Base mounting hole spacing (also referred to as the E and F dimensions) and shaft height (referred to as the D dimension) will remain constant for all frames having the same three digits regardless of vintage. Most of the dimensions are standard dimensions that are common to all motor manufacturers.


Over the years, motors' horsepower ratings have continued to increase. This is in part due to improvements made in insulating materials. As a result of this improved insulation, motors can be run much hotter than they could in the past. This allows more horsepower to be developed from a given motor size. The original NEMA frame sizes ran at very low temperatures. The U frame motors were designed for use with Class A insulation, which has a rating of 105 degrees Celsius. T frame motor designs are based on utilization of Class B insulation with a temperature rating of 130 degrees Celsius. In order to address the increase in horsepower available from a motor, the shaft and bearing sizes had to be increased. You will find that the original 254 frame motor (5 HP at 1800 RPM) had a 1-1/8 inch shaft. The 254U frame (7-1/2 HP at 1800 RPM) has a 1-3/8 inch shaft, and the current 254T frame (15 HP at 1800 RPM) has a 1-5/8 inch shaft. Bearing diameters were also increased to accommodate the larger shaft sizes and heavier loads associated with the higher horsepowers. Of course this means changing motors may necessitate sourcing of new pulleys, etc.


The term "fractional horsepower" is used to cover those frame sizes having two digit designations as opposed to the three digit designations that are found in Tables 1 and 2. The frame sizes that are normally associated with industrial fractional horsepower motors are 42, 48, and 56. For these motors, the frame size designates a particular shaft height, shaft diameter, and face or base mounting hole pattern. Frame designations are not based upon horsepower and speed, so it is possible that a given horsepower and speed combination may be available in one, two or even three three different frame sizes. The two digit frame number is based on the shaft height in sixteenths of an inch. Therefore, a size 48 frame motor will have a shaft height of 48 ÷ 16 = 3 inches. A 56 sized frame motor would have a shaft height of 3-1/2 inches. Size 56 frame motors are the largest of the fractional horsepower rated motors are may actually be found in horsepower ratings up to 3 HP and in some rare cases, 5 HP. This makes the term "fractional" somewhat misleading.


The term Integral Horsepower Motors generally refers to those motors having three digit frame sizes such as 143T or larger. In these motors, the centerline shaft height (D dimension) above the bottom of the base is the first two digits of the frame size divided by four. For example, a 254T frame would have a shaft height of 25 ÷ 4 = 6.25 inches. Although the last digit does not directly relate to an inch dimension, larger numbers do indicate that the rear bolt holes are moved further away from the shaft end bolt holes (the F dimension becomes larger).


In addition to the standard numbering system for frames, there are some standardized variations that are denoted by the following suffixes to the NEMA number.

C Designates a C flange/face mounted motor, e.g., "NEMA 56C frame". 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 designate 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 S in a motor frame 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 A T indicates that the motor is of the 1964 and later T frame vintage.

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

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


Manufacturers use prefixes to specify variations within a frame size, such as the overall motor length. There is no standardization of prefixes and you must check with the manufacturer to learn the meaning of a prefix.


Frame sizes (PDF)

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