When replacing an electric motor, most buyers start with the kW rating. That is important, but it is only one part of the specification. The motor also has to physically fit the machine, line up correctly with the coupling or gearbox, and match the existing mounting arrangement. This is where electric motor frame size becomes critical.
A motor with the correct kW rating can still be the wrong replacement if its frame size, shaft height, shaft diameter or mounting dimensions are different from the original motor. The result can be a motor that does not bolt onto the existing base, sits too high or too low for the coupling, or requires modifications before it can be installed.
This electric motor frame size chart covers common four-pole, 400 V three-phase motors from 0.37 kW to 15 kW and explains what the IEC frame number means, why the same motor power can sometimes have different frame sizes, and what you should check before ordering a replacement motor.
How IEC Motor Frame Sizes Work
The IEC frame size is primarily a way of identifying the mechanical dimensions of an electric motor. One of the most important dimensions for a foot-mounted motor is the shaft centre height, measured from the mounting surface to the centre of the motor shaft.
For example, an IEC frame 80 motor has a nominal shaft centre height of 80 mm, while a frame 100 motor has a nominal shaft centre height of 100 mm. This dimension is important because it determines where the motor shaft sits relative to the machine, coupling, pulley or gearbox it is driving.
Common IEC motor frame numbers include 56, 63, 71, 80, 90, 100, 112, 132, 160, 180, 200, 225, 250, 280 and 315. You may also see letters such as S, M and L after the frame number. These letters identify different dimensional variants within the same frame-height family. For example, a 132S and 132M have the same nominal shaft centre height but can have different mounting and body dimensions.
This is why frame size should not be treated as simply another number on the nameplate. It is directly connected to whether the motor can physically fit the installation. If you are replacing a motor that already drives a gearbox, coupling or conveyor, the frame size needs to be checked alongside the other mechanical dimensions before you order.
Electric Motor Frame Size Chart: 0.37 kW to 15 kW
The following table represents a typical four-pole, 400 V three-phase industrial motor configuration commonly used for general industrial applications.
| Motor Power | Motor Current Approx. | Typical Frame Size |
|---|---|---|
| 0.37 kW | 1.0 A | 71 |
| 0.55 kW | 1.5 A | 80 |
| 0.75 kW | 2.0 A | 80 |
| 1.1 kW | 2.5 A | 90 |
| 1.5 kW | 3.3 A | 90 |
| 2.2 kW | 4.8 A | 100 |
| 3.0 kW | 6.3 A | 100 |
| 4.0 kW | 8.2 A | 112 |
| 5.5 kW | 11.0 A | 132 |
| 7.5 kW | 14.5 A | 132 |
| 11.0 kW | 20.5 A | 160 |
| 15.0 kW | 28.5 A | 160 |
These values are useful as a selection reference, rather than a universal rule for every manufacturer. Actual motor current and frame size can change depending on efficiency class, pole count, motor design and manufacturer.
For that reason, if you are buying a replacement motor, the safest approach is to compare the actual nameplate and dimensional drawing of the existing motor with the proposed replacement. The table can help you narrow down the likely frame, but the manufacturer’s specification should be the final reference.
What Does Motor Frame Size Actually Tell You?
The frame number gives you an indication of the motor’s physical size and mounting geometry. For a standard foot-mounted IEC motor, shaft centre height is one of the most important dimensions, but it is not the only one that matters.
A replacement motor can have the same shaft centre height while still having differences in shaft length, shaft diameter, foot-hole spacing, body length or terminal-box arrangement. This is particularly important when the motor is being installed into an existing machine where the available mounting space is already fixed.
For example, suppose an existing machine has a 7.5 kW motor. You find another 7.5 kW motor from a different manufacturer and assume it will be a direct replacement. Electrically, the two motors may appear similar, but if the mounting feet, shaft dimensions or coupling position are different, the new motor may require an adapter or mechanical modification.
That is why motor frame size should be checked together with the actual dimensional drawing, especially for replacement applications.
Why the Same kW Motor Can Have Different Frame Sizes
It is tempting to think that motor power determines the frame size, but the relationship is not that simple. The kW rating tells you the motor’s power output, while frame size describes important physical dimensions. Several other characteristics can influence the final motor design.
Pole Count Can Change the Motor Dimensions
Motor speed is closely related to pole count. On a 50 Hz supply, a typical two-pole motor operates around 2800 rpm, a four-pole motor around 1400 rpm, and a six-pole motor around 900 rpm under normal operating conditions.
For the same power rating, a slower motor needs to produce more torque than a faster motor. That can influence the motor’s active materials and physical construction. Therefore, you should not assume that a 7.5 kW two-pole motor will have exactly the same physical dimensions as a 7.5 kW four-pole or six-pole motor.
This becomes particularly important when the motor is connected to a gearbox. If the required machine speed is much lower than the motor speed, the gearbox ratio has to be selected accordingly. Our gearbox ratio and conveyor drive guide explains how motor RPM, output speed and reduction ratio are related.
Efficiency Class Can Also Affect Construction
Efficiency class is another factor that can influence motor construction. An IE3 motor is designed to achieve lower electrical losses than a lower-efficiency motor, and the internal design can therefore differ even when the nominal kW rating remains the same.
This does not mean that every IE3 motor will have a larger frame than every IE2 motor. It simply means that kW alone should not be used to predict physical dimensions.
If you are deciding whether the additional purchase cost of an IE3 motor is justified by lower electricity consumption, our IE2 vs IE3 motor efficiency comparison explains the running-cost side of that decision for Pakistani industrial applications.
Manufacturer Design Matters Too
Different manufacturers can use different construction approaches while still working within standardised IEC dimensions. Important mounting dimensions are standardised, but other dimensions can vary.
This is particularly relevant when replacing a motor from one manufacturer with a motor from another. Before ordering, compare the shaft dimensions, foot dimensions, centre height and mounting arrangement rather than relying only on the frame number.
Frame Size and Motor Mounting Are Not the Same Thing
Another common source of confusion is treating frame size and mounting type as the same specification.
They are related, but they describe different things.
Frame size tells you about the motor’s physical dimensional family, while mounting configuration tells you how the motor is attached to the machine.
A motor may be:
- Foot mounted
- Flange mounted
- Foot and flange mounted
For example, B3 is commonly associated with foot mounting, while B5 and B14 are flange mounting arrangements.
If you are ordering a motor that will be mounted directly to a machine or gearbox, you therefore need to check both the frame size and mounting configuration. Our B5 vs B14 flange mounting guide explains the difference between these configurations and why the mating components need to be compatible.
What If the Motor Is Connected to a Gearbox?
This is where frame-size selection becomes even more important.
Consider a typical industrial drive:
Electric Motor → Gearbox → Conveyor
The motor provides the input power and speed, while the gearbox reduces the speed and increases the available output torque.
For the complete drive to work correctly, several specifications have to match. The motor needs to be suitable for the gearbox input, while the gearbox needs to be correctly sized for the load.
You therefore need to look at:
Motor:
kW + RPM + frame size + shaft + mounting
Gearbox:
Ratio + body size + torque + input configuration
Application:
Load + output speed + duty cycle + starting conditions
For example, an NMRV worm gearbox may be suitable for a conveyor because it provides speed reduction and high output torque in a compact arrangement. You can see the available configurations on the NMRV worm gearbox range page.
For heavier applications, a different type of gear reducer may be more appropriate depending on the required torque, efficiency and duty cycle.
Checking Motor Frame Size Against a Gearbox
When a motor is connected directly to a gearbox, the frame and mounting arrangement become part of the mechanical interface.
A gearbox can have the correct ratio and torque rating but still be unsuitable if the motor cannot be mounted correctly to its input.
Before ordering a motor for an existing gearbox, check:
- Motor frame size
- Motor shaft diameter
- Motor shaft length
- Flange configuration
- Flange dimensions
- Gearbox input dimensions
- Motor RPM
- Motor power
This is particularly important for complete gearmotor assemblies because the motor and gearbox are effectively one mechanical system.
If you are unsure about the gearbox body size as well, our NMRV 63, 75 and 90 comparison explains why the gearbox body size needs to be selected according to torque rather than simply matching it to motor power.
Ordering a Replacement Motor
When you are replacing an existing motor, the selection process is different from choosing a motor for a completely new machine.
With a new machine, you can design the mounting arrangement around the motor you choose. With an existing machine, the mounting base, shaft position, coupling and available space may already be fixed.
That means the existing motor becomes your dimensional reference.
Start by recording the information on the motor nameplate:
- Motor power
- Voltage
- Current
- Frequency
- RPM
- Number of poles
- Efficiency class
- Frame size
- Mounting arrangement
Then measure the important mechanical dimensions if the motor is being replaced:
- Shaft diameter
- Shaft length
- Shaft centre height
- Foot-hole spacing
- Overall motor length
- Mounting-hole dimensions
- Flange dimensions where applicable
This information gives the supplier enough detail to identify a replacement that is electrically suitable and mechanically compatible.
Why Shaft Height Matters
Shaft height is one of the most important reasons frame size matters in a replacement motor.
Imagine that a motor is driving a gearbox through a coupling. The motor shaft and gearbox input shaft need to be correctly aligned. If the replacement motor’s shaft centre is higher or lower than the original, the coupling can operate under unwanted misalignment.
That can increase vibration and place additional stress on the coupling, motor bearings and gearbox input bearing.
The same principle applies to belt drives, chain drives and other transmission systems where the motor shaft has to line up with another rotating component.
This is why a motor with the same kW rating is not automatically a suitable replacement.
The motor has to fit the mechanical system as well as satisfy the electrical requirement.
What Do the S, M and L Letters Mean?
Motor frame designations can include letters after the frame number.
You may see examples such as:
- 90S
- 90L
- 132S
- 132M
- 132L
The number identifies the frame-height family, while the letter distinguishes different dimensional variants.
This is useful because two motors can share the same shaft centre height while having different mounting footprints or body lengths.
For replacement applications, this distinction can be important. If the old motor is a particular S, M or L variant, replacing it with another variant may require checking the actual foot spacing and dimensional drawing before installation.
Motor Frame Size for New Installations
Frame size is not only important for replacement motors. It should also be considered when designing a new machine.
If you are building a conveyor, mixer, packaging machine or other industrial system, the motor mounting arrangement should be included in the mechanical design from the beginning.
For a conveyor, for example, you may first calculate the required load torque and output speed, then select the appropriate gearbox and motor combination. Once the motor is selected, its frame and mounting dimensions can be incorporated into the machine base.
Our electric motor selection guide for conveyor systems covers the broader selection process, including load, torque, speed, power, gearbox selection and environmental conditions.
This approach is better than selecting a motor purely by kW and trying to make it fit the machine afterward.
Frame Size Is Only One Part of Motor Selection
It is important not to make the opposite mistake and focus so heavily on frame size that the other specifications are ignored.
A motor can have exactly the right frame size and still be unsuitable for the application.
You also need to consider:
Power:
Does the motor have enough kW for the load?
Speed:
Is the RPM suitable for the machine or gearbox?
Voltage and phase:
Does it match the available electrical supply?
Efficiency:
Is IE2, IE3 or another efficiency class appropriate?
Duty:
Can it operate for the required number of hours?
Environment:
Does the enclosure and construction suit dust, heat or moisture?
Mounting:
Does the B3, B5, B14 or other arrangement match the machine?
Gearbox:
If a reducer is being used, is the motor mechanically compatible with its input?
For gearbox-driven applications, the motor and reducer should therefore be treated as one complete drive system rather than as two unrelated components.
Frame Size and Gear Motor Selection
A complete gear motor combines the electric motor and gearbox into one drive unit. This can simplify installation, but it does not eliminate the need to check motor frame and mounting specifications.
For example, if you need a conveyor drive, you may select a motor based on the required power and then select the gearbox according to the required output speed, torque and service factor.
The gearbox may be an NMRV worm gearbox for a compact, right-angle drive, or another gear reducer if the application requires a different construction.
If you are unsure whether a separate motor and gearbox or a complete gear motor is the better option, the decision should be based on the required speed, torque, mounting arrangement, duty cycle and available installation space.
How to Identify the Frame Size of an Existing Motor
The easiest place to start is the motor nameplate.
Many industrial motors have the frame designation printed alongside the kW, voltage, current and RPM information. If the frame size is not clearly visible, the motor’s dimensional drawing or manufacturer documentation can normally provide it.
If documentation is unavailable, you can measure the motor.
The most useful measurements are:
- Distance from mounting base to shaft centre
- Shaft diameter
- Shaft length
- Distance between mounting holes
- Mounting-hole dimensions
- Overall motor dimensions
- Flange diameter and bolt pattern, if flange mounted
A clear photograph of the nameplate plus a photograph of the motor mounting arrangement can also make identification much easier when requesting a replacement.
Electric Motor Frame Size Selection Checklist
Before placing an order, confirm the following information.
Electrical specifications
- Motor power in kW
- Voltage
- Frequency
- Number of phases
- Current
- RPM
- Number of poles
- Efficiency class
Mechanical specifications
- IEC frame size
- Shaft centre height
- Shaft diameter
- Shaft length
- Foot-hole spacing
- Mounting
Common Motor Frame Size Selection Mistakes
One of the most common mistakes is assuming that the same kW means the same physical motor. As explained above, pole count, efficiency class, manufacturer and construction can all affect the dimensions.
Another mistake is checking only the frame number while ignoring the shaft. Two motors from different manufacturers may share a nominal frame designation but still require the shaft and coupling dimensions to be checked before a replacement is installed.
A third mistake is forgetting the mounting arrangement. A foot-mounted motor and a flange-mounted motor can have the same electrical rating while being completely different installation options.
Finally, buyers sometimes select a motor first and only afterward discover that the gearbox input, coupling or machine base does not match. Checking the motor, gearbox and machine as a complete system avoids this problem.