How to Determine Your NEMA Motor Frame Size

When it comes to replacing a motor, frame size is an important factor to get right. Manufacturers have collaborated to standardize frame sizes to allow motors to be more easily replaced, meaning you can swap a Baldor motor for a WEG motor and vise versa, assuming all the other specifications align.

Use the below links to jump to different sections of this article:

• Motor frame size chart
• What is frame size
  • Fractional Horsepower
  • Integral Horsepower
• What do the letters in frame size mean?
• How to Measure NEMA frame size
• How to find your frame size

Standard frame sizes for motors with the same HP, speed, and enclosure to be replaced with motors with the same ratings but from different manufacturers.

Motor frame size standardization happened in three stages. All frame sizes up until 1952 are referred to as original frame sizes. A large standardization took place in 1952, these frame sizes are referred to as U-Frames. And then a final standardization took place in 1964, these frame sizes are referred to as T-Frames. Many original frame sizes and U-frames are still in action today and will eventually disappear as those motors are replaced.

Motor Frame Size Chart

There are two groups that have individually contributed to the standardization of electric motors, their ratings, and their sizes. The National Electrical Manufacturers Association, also known as NEMA, completed the standardization of motors in North America. The International Electrotechnical Commission, also known as IEC, completed the standardization of motors for the rest of the world, namely Europe.

NEMA assigns the frame sizes based on enclosure, horsepower, and speed. Whereas IEC doesn’t include output power or speed in their frame size designations. NEMA uses the imperial system in inches, while IEC uses the metric system in millimeters. As you can see in the chart below, the frame sizes are quite close, but often out by 3 or 4 mm, which is a large enough difference to make the motors not interchangeable.

Download the NEMA and IEC frame size comparison here.

Download the full NEMA and IEC frame chart here.

What Is Electric Motor Frame Size?

The frame size of an electric motor defines the geometric values of its physical size:

1. The outer dimensions of the motor flange & bolt circle
2. Diameter and length of the motor shaft and the size of the key if it has one
3. Length & width of the mounting holes in the feet

One key dimension that isn’t indicated by frame size is the overall length of the motor’s housing. This is a factor to consider when replacing a motor with the same frame size if your motor’s being installed in a tight application.

Fractional Horsepower Motors

You may have noticed that fractional horsepower motors have fewer numbers in their frame size as compared to integral horsepower motors (1 HP and above). Fractional horsepower frame sizes have only two digits instead of three, with common frame sizes such as 42, 48, and 56. These frame sizes only indicate shaft height, shaft diameter, and the mounting hole pattern.

There is also a couple of letter designations that could come after these numbers:

• C – Frame sizes with a “C” indicate that the motor is a C-flange mounted motor. There will be threaded holes on the face of the motor to mount the C-flange.
• H -Frame sizes with an “H” indicate that the base can be mounted in 56, 143T, or 145T mounting dimensions.
• J – Frames sizes with a “J” can be found on 56 frame motors and indicate that the motor has a threaded stainless-steel shaft and standard 56C face.

When selecting a replacement motor, it’s important to note that frame size does not determine the horsepower or speed of the motor. You can find the same HP and speed in three different frame sizes. So, when selecting your replacement, you must know what frame size, HP, speed, and enclosure you need.

Integral Horsepower Motors

Integral horsepower motors have three-digit frame sizes starting at 143T and getting larger from there. Motors that have the same three-digit frame size have the same base mounting hole spacing and the same outer dimensions of the motor flange, diameter and length of the motor shaft (and the size of the key if it has one), mounting hole diameter and bolt size, and bolt circle diameter.

What Do the Letters in Frame Size Mean?

There are many variations in frame sizes, and many of these include the addition of different letters. Below are the meanings of each letter:

• C – Frame sizes with a “C” indicate that the motor is a C-flange mounted motor. There will be threaded holes on the face of the motor to mount the C-flange.
• D – Frame sizes with a “D” indicate that the motor is a D-flange mounted motor. The flange will have a larger outer diameter than the motor to allow bolts to pass through the back of the motor and attach to the flange.
• JM – Frame sizes with “JM” indicate a special pump shaft with a mechanical seal and a C-face.
• JP – Frame sizes with “JP” indicate a special shaft with a packing seal and a C-face and are also used in pump applications.
• S – Frame sizes with an “S” indicate that the shaft is shorter than the regular size related to the specific frame size. These shafts are for flexible coupling rather than belt driven applications.
• T – Frame sizes with a “T” indicate that the motor is from the frame size standardization from 1964 or later.
• U – Frame sizes with a “U” indicate that the motor is from the frame size standardization from 1952 to 1964.
• Y – Frame sizes with a “Y” indicate a special mounting configuration. The letter doesn’t determine the exact configuration but does indicate that it is not standard.
• Z – Frame sizes with a “Z” indicate a special shaft. The letter doesn’t determine exactly what makes the shaft “special” but does indicate that it is not standard.

How to Measure NEMA Motor Frame Size

Fractional Horsepower Motors

When it comes to the two-digit frame sizes of fractional horsepower motors, the two-digit number indicates the shaft height in sixteenths of an inch. For example, a frame size of 42 means that the motor has a shaft height of 2.62 inches or a frame size of 48 means that the motor has a shaft height of 3 inches.

Integral Horsepower Motors

When it comes to the three-digit frame sizes of integral horsepower motors, the first two digits of the motor frame size indicate the shaft height in quarters of an inch. This is measured through the centerline from the bottom of the base. For example, a 143T frame has a shaft height of 3.5 inches.

The third number of the motor frame size indicates the distance between the rear bolt holes and the shaft end bolt holes. This number doesn’t represent a specific value in inches, but the higher the number, the larger the distance between the bolt holes.

How to Find My Motor Frame Size

There are two places where you can find your motor’s frame size. The first location is easy to locate right on your motor on the nameplate. See if you can find the frame size on the motor nameplate in the image below. The second location is in your motor manual provided by the motor manufacturer.

Check out this video series on motor nameplates for more details on your motor nameplate.

Summary

When it comes to electric motor replacements, sometimes you can’t find the exact same motor. The original motor could be discontinued or out of stock with a delivery date that is too far away. Thankfully, with the standardization of motors over the last few decades, you can easily find a replacement motor from a different manufacturer.

Ensuring that your replacement motor has the correct frame size helps ensure that the new motor fits right into place. If you’re having issues finding the correct replacement for your application, get in touch with our motor experts. They’d be happy to help you with your electric motor projects.