Choosing between IEC and NEMA motor controls

Key Concepts NEMA and IEC motor controls differ in size, ruggedness, and complexity. The solid-state overload relay is common to both NEMA and IEC motor controls.

By Jack Smith, Senior Editor, Magazine March 1, 2001

Key Concepts

NEMA and IEC motor controls differ in size, ruggedness, and complexity.

The solid-state overload relay is common to both NEMA and IEC motor controls.

Keep the application in mind when choosing between NEMA and IEC.

The more things change, the more they stay the same, it has been said. This statement could be applied to the ongoing debate about National Electrical Manufacturers Association (NEMA) vs International Electrotechnical Commission (IEC) motor controls and protection.

During the past decade, overload motor protection mechanisms have changed substantially and become similar between NEMA and IEC motor controls. However, contactors have changed very little.

NEMA and IEC – the differences

Size, ruggedness, durability, complexity, perceived quality, and domestic vs global markets are the primary differences between NEMA and IEC motor controls (Fig. 1).

Size is the most apparent outward difference between NEMA and IEC motor controls. The NEMA contactor is physically larger than the IEC contactor for a given current rating. The larger size implies that ruggedness and durability are inherent in the NEMA motor control.

Traditionally, NEMA motor controls are perceived to be simpler than their IEC counterparts. Despite the perceptions about size and assumptions about motor control complexity, neither the NEMA nor the IEC is intrinsically better or safer (Fig. 2).

Historically, NEMA motor controls have been sold primarily in domestic markets, while IEC motor controls have been sold primarily in the European/global markets. In Europe, much emphasis is placed on small size and efficient use of materials. This statement is true in general – not only with motor controls. While Europe’s emphasis on small size, effective use of materials, and the high priority given to international trade have led to the type of product that is designed to IEC standards, those criteria have not been given the same priority in the United States.

Domestically, the emphasis has been placed on reliability and maintainability. Also, the practice has been to provide performance in almost all applications without the need to go through a lengthy and complicated selection procedure. The U.S. has a high level of automation, requiring that equipment be maintained quickly, rather than having to buy a new product. The emphasis has been on easy installation with a high level of confidence in the integrity of the installation and the equipment.

NEMA and IEC – the similarity

Despite the differences between NEMA and IEC motor controls, the two types have a major similarity. This similarity – the solid-state overload relay – has evolved during the past decade. Solid-state overload relays have replaced both bimetal and eutectic trip mechanisms in many applications. However, it should be noted that both mechanical overload devices are still used – bimetal primarily.

Solid-state overload relays are found in the majority of both NEMA and IEC motor controls (Fig. 3). There is little difference between solid-state overload relays used for either type. In some applications, the same solid-state overload relay can be used in NEMA and in IEC units, leaving the contactor and enclosure the main differences between the two.

Advantages of using a solid-state overload relay with either NEMA or IEC controls are:

  • Increased adjustability

  • Better motor protection

  • Minimized nuisance tripping

  • Jam protection.

  • NEMA sizes

    NEMA contactor frame sizes vary from 00 to 9. The table lists NEMA contactor frame sizes for three-phase, single-speed, full-voltage, magnetic controllers for nonplugging and nonjogging duty. Each size can be applied to a range of motor loads. For example, with a 460-V motor, a NEMA Size 1 starter can be used with a motor load up to 10 hp.

    NEMA contactor sizes do not apply to IEC devices. Typically, IEC devices are application-rated products. Instead of using NEMA sizes, IEC contactors are sized by ampere ratings. However, the most significant differences between NEMA and IEC controllers are not in how they are named or rated, but in the design decisions made when applying them. NEMA products are generally designed to handle a range of applications. IEC products are much more application sensitive, by design.

    Tripping classes

    The tripping class indicates the time within which an overload must trip if an overcurrent condition occurs. Specifically, an overload device must trip in a time period m its class (m 10 sec for Class 10) when subjected to an electrical current of at least 600% of the overload device’s rating. It should be noted that 600% of the device’s rating is the minimum of a range (traditionally this range is between 6 and 10 times the overload device’s rating) and is, therefore, the trip point that many strive to use. At a minimum of 600% of the overload device’s rating, it must trip in the following times:

    Class 10A 2-10 sec
    Class 10 4-10 sec
    Class 20 6-20 sec
    Class 30 9-30 sec

    Classes 10 and 20 overload protection is available for most motor applications, whether using most traditional eutectic overload relays, or new electronic overload designs. The greater functionality of some solid-state overload relays allows Class 10, 15, 20, or 30 overload protection.

    The quick trip characteristic of a Class 10 overload relay is necessary for motors with short locked-rotor time capability. A Class 20 overload relay is used when its slower trip characteristic is necessary to provide additional accelerating time. Similarly for Class 15 and 30, a major consideration is the trip time characteristic. Trip time characteristics of overload relays do not differ between NEMA and IEC motor controls.

    Nuisance tripping

    Tripping is designed into electrical systems to provide equipment protection. An unwanted trip is a nuisance regardless of whether it is a NEMA or IEC motor control. Nuisance tripping is simply any trip that wasn’t designed to happen, and can occur because of ill-considered design, improperly installed or maintained equipment, and incompatible or misapplied components.

    When troubleshooting or analyzing the electrical circuit and equipment application, the goal is to identify the cause of the trip. The next step is to make adjustments, redesign, and/or replace components as indicated by the analysis.

    One or more of the following conditions can cause a motor to become thermally overloaded, thereby causing a legitimate trip:

    • Increased torque that is too high in continuous operation

    • Startup or braking times too long

    • Switching frequency too high

    • Locked-rotor condition

    • Connecting or switching on a fault

    • Poor supply voltage or frequency (high or low)

    • Phase loss

    • Insufficient cooling.

    • If the motor overload trips before any of these has occurred, the trip is considered a nuisance.

      Nuisance tripping can occur with any type of protection device – thermal overloads, solid-state overload relays, or circuit breakers. Solid-state overload relays may provide a wider range of full load amp (FLA) adjustment to help alleviate the nuisance trip quicker without having to replace the device.

      The most common nuisance trip is when, because of the load, the motor takes too long to reach full speed or running current. The overload trips without allowing the motor to reach operating speed.

      The newest solid-state overload relays provide microprocessor-based electronic protection for motors. They have many programmable functions for providing equipment protection and include diagnostics to warn of potential problems. Because diagnosis is often the real challenge when faced with nuisance tripping, diagnostic functions can make dealing with nuisance tripping easier.

      Typically, nuisance tripping does not vary from NEMA to IEC. Circuit design, installation and maintenance issues, and misapplications can occur as easily in one type of application as it could the other. Care must be taken when designing and/or applying either NEMA or IEC components to minimize the frequency or severity of nuisance tripping occurrences.

      Choosing a motor control

      The differences between NEMA and IEC motor controls offer you choices. The similarity of their solid-state relays offers the best of both types. When selecting a motor control, consider both the application and the resources you can devote to it.

      Plant Engineering magazine extends its appreciation to Rockwell Automation and Siemens Energy & Automation for their assistance in the preparation of this article.

      Solid-state overload relay basics

      Jack Smith

      An overload relay responds to an electrical load and operates at a preset value of overload. It is usually a current relay intended to protect a load, such as a motor or its controller. It does not necessarily protect itself.

      The solid-state overload relay works by sensing the current through the motor and main contactor. Current transformers (CTs) sense the current flowing through the motor leads and main contactor. Signals from the CTs are applied to comparison circuits, which drive the disconnect circuits.

      Some overload relay circuits use a known, precision, low-ohm resistance capable of dissipating the heat generated by the current flowing through it and the motor. The voltage drop across this known precision resistance (which correlates directly with its current) is applied to a circuit that compares the sensed current to a reference voltage. This reference voltage is usually adjustable. When the sensed voltage drop, which accurately represents the motor current, reaches the predetermined reference voltage, the comparitor circuit triggers a disconnect circuit, which, in turn, causes the contactor to open. This action removes the current from the motor.

      The above explanations are greatly simplified to convey one theory of operation and do not address every motor protection scheme. In practice, solid-state overload relays are more complex, and have an abundance of parameters that must be considered when adjustments are made.

      NEMA and IEC design characteristics

      Jack Smith

      Selecting the appropriate motor control for a given application is critical to achieving the maximum productivity from the installation. Each installation should be evaluated independently because the requirements of the specific application dictate which type of device is appropriate.

      This table is a brief summary of the design characteristics of NEMA and IEC devices.


      Designed to meet the standards and requirements of the North American market Designed to meet the standards and requirements of international markets
      Larger size when compared to an IEC motor control with similar horsepower ratings Smaller size when compared to a NEMA motor control with similar horsepower ratings
      Simple system of sizes for easy selection of a product that provides a high level of performance in the majority of applications More application intensive; requires additional technical expertise to ensure proper selection and application
      Rugged, general purpose design Highly functional design, rated close to ultimate capabilities
      Available in starter assemblies using bimetal or eutectic alloy overload relays with field replaceable heater elements to cover a wide range of motor horsepower ratings without changing overload relays Overload relays with integral heater elements that cannot be replaced in the field
      Solid-state overload relays available Solid-state overload relays available
      Wide variety of overload relays available Normally sold as a component for field assembly
      Easily replaceable coils and contacts Contacts replaceable only on larger sizes
      Most designs have molded or encapsulated coils Most designs have tape wound coils

      NEMA contactor frame sizes

      Size of controllerContinuous current rating,1 ampsHPHPHPHP

      00 9 1.5 1.5 1.5 2
      0 18 3 3 5 5
      1 27 7.5 7.5 10 10
      2 45 10 15 25 25
      3 90 25 30 50 50
      4 135 40 50 75 100
      5 270 75 100 150 200
      6 540 175 200 300 400
      7 810 300 600
      8 1215 450 900
      9 2250 800 1600

      1 See NEMA ICS 2-321.20

      Issues to consider when selecting a motor control device

      Jack Smith

      When trying to decide between using NEMA or IEC motor control devices, or a combination of the two, the following questions should be taken into consideration. Determining the answers to these questions can help you make an educated choice.

      • How much of a time investment can be made when selecting the proper motor control?

      • How much of a technical personnel investment can be made when selecting the proper motor control?

      • What is the real cost of the extra panel space that NEMA motor control devices typically require?

      • Is the NEMA and/or IEC device appropriate for the load/life performance needed in the application?

      • Does the plant operation make it necessary for you to be able to perform general maintenance-such as changing contacts-on these devices?

      • How easily can you obtain service help and/or replacement devices?

      • Does the contactor provide the flexibility that may be required for field installed options or factory supplied specials?

      • What are the costs associated with field failures of a misapplied device (repair costs, lost productions costs, etc.)?