Understanding Metric or IEC Motors

IEC and NEMA motors are designed and built based on standards from different organizations, but are intended for the same purposes. NEMA, the National Electrical Manufacturers Association, is the industry authority on electrical equipment manufacturing in the USA. On the other hand, international standards are established by IEC, the International Electrotechnical Commission. One of the main differences between both organizations is that IEC uses the Metric System of Measurement, while NEMA standards are based on the Imperial system.

This article will provide an overview on how the technical specifications of IEC motors differ from those that follow NEMA standards. For a given application, it is generally possible to find suitable motors in both IEC and NEMA versions. However, it is important to understand the key differences between both sets of standards.

Motor characteristics can be separated into two broad categories: those that describe motor operation, and those that describe its physical construction.

Operating Characteristics: Differences Between IEC and NEMA Motors

Before purchasing a motor, you must determine if its operating characteristics are well-suited for the application at hand. The following are some key specifications to review:

  • Rated Power: Is the motor capable of moving the intended load?
  • Energy Efficiency: What energy expenses can be expected?
  • Insulation Class: What is the maximum temperature rise that the motor can tolerate?
  • Design Class: How does motor torque behave as speed increases? How high is the starting torque compared with the full-load value?

This section will provide an overview of how these specifications differ between IEC and NEMA motors.

Rated Power and Energy Efficiency

One of the main differences between IEC and NEMA motors is that shaft output is specified in horsepower (hp) for NEMA motors, but in kilowatts (kW) for IEC motors. However, this is not an issue when comparing them, since you just have to apply a conversion factor - 1 hp is equivalent to 0.7457 kW. Keep in mind that the rated power of IEC motors describes mechanical output at the shaft, not electric power input at the motor terminals, which is also measured in kW.

IEC and NEMA motors also differ in terms of their efficiency levels. For NEMA motors, there are three levels available: Standard, Energy Efficient and Premium. On the other hand, IEC motors are available in four efficiency tiers:

  • IE1 - Comparable to NEMA Standard
  • IE2 - Comparable to NEMA Energy Efficient
  • IE3 - Comparable to NEMA Premium
  • IE4 - Above NEMA Premium

Insulation Class

Some environments allow motors to dissipate heat more easily, while in other locations the motor may be required to tolerate a higher temperature rise. When selecting a motor, you must verify that its insulation class is suitable for the intended operating conditions. The insulation classes used by IEC motors are very similar to those of NEMA motors, with only two key differences:

  • NEMA motors have Class A, B, F and H insulation, where the rated temperature rise values are 60°C, 80°C, 105°C and 125°C, respectively.
  • IEC motors use the same temperature classes, but there is an additional class called E between A and B, with a rated temperature rise of 75°C.
  • In addition, temperature class F in IEC motors is rated for a temperature rise of 100°C, not 105°C

Design Class

A motor’s design class describes how its torque changes from 0 rpm to rated speed, and the design class must be selected based on load characteristics. NEMA motors are available in four design classes, A through D:

  • Classes A and B are intended for general-purpose applications, such as fans and pumps. Their average starting torque is 150% of the full-load value.
  • Class C is for high starting torque loads (250%), such as conveyor belts.
  • Class D is for applications with the highest starting torque (275%), such as mechanical punches.

On the other hand, IEC motors are available in two design classes. IEC Design N is comparable to NEMA Design B, and IEC Design H is comparable to NEMA Design C.

Differences Between Motor Frames and Enclosures

When selecting a motor for a specific application, it is also important to know its size and enclosure type. The frame size is used to specify support for the motor, while the enclosure type lets you determine if the motor will tolerate the dust and humidity conditions present at its intended location.

IEC and NEMA Motor Frame Sizes

Frame sizes for IEC and NEMA motors use different measurement units. For instance, all IEC motors display shaft centerline height in millimeters, and this applies for both two-digit and three-digit frames:

  • A IEC 80 frame indicates a shaft height of 80 mm.
  • A IEC 250M frame indicates 250 mm.

This is very different for NEMA motors, where two-digit frames are in sixteenths of an inch, while three-digit frames use the first two digits to describe shaft height in fourths of an inch:

  • A NEMA 48 frame has a shaft height of 3 inches (76.2 mm), making it similar to a IEC 80 frame in terms of size.
  • A NEMA 405T frame has a shaft height of 10 inches (254 mm), making it similar to a IEC 250M frame.

The third digit in a NEMA frame indicates motor length, but does not make reference to a specific measurement unit. Instead, it is necessary to check manufacturer tables for the corresponding length in inches.

Enclosure Specification Differences

With respect to enclosures, the main difference is that NEMA motors use descriptive specifications such as “Totally Enclosed, Fan Cooled”, while IEC motors use a two-digit IP code (International Protection Marking). The first digit in the IP code indicates protection against solid particles, while the second digit is for protection against liquids, and higher numbers indicate greater protection. For example, a motor with an IP65 enclosure has better protection that one with an IP 44 enclosure.


IEC and NEMA standards provide different ways to manufacture and label electrical motors, but neither of them can be considered better than the other. By understanding both IEC and NEMA labels, you can always select the right motor for each application, regardless of how technical specifications are presented.

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