How do NEMA- and IEC-style electromechanical motor control products differ?

In today's business environment, flexibility is an asset. If you're able to apply either IEC- or NEMA-style products, you have the flexibility to adapt to local traditions and product supply anywhere in the world. This allows you to take the best advantage of local availability and expertise, which can be an important competitive edge in a global economy. Before applying this flexibility, however, you need to know the differences between NEMA and IEC designs.

Differing philosophies

The NEMA philosophy emphasizes more robust designs for broader applicability. Ease of selection and breadth of application are two of the fundamental mainstays within its design philosophy.

Governing the design of NEMA motor control products is the NEMA Industrial Control Standard (ICS-2). This formal document contains all of the information about the design of electromechanical NEMA-style motor control.

The IEC philosophy, on the other hand, is application and performance. In selecting IEC devices, you need a more sophisticated level of knowledge about the application than is necessary when selecting a NEMA general-purpose device. For example, you need to know motor load, duty cycle, and full load current (FLC) when selecting an IEC contactor.

This requirement, which may be a drawback in some segments, is a significant advantage in Original Equipment Manufacturer (OEM) segments. For example, an equivalent hp-rated IEC device tends to be substantially smaller than its NEMA counterpart; this represents a significant advantage for most OEM designers. Also, IEC products tend to be less expensive.

NEMA features and benefits

Ease of selection is the fundamental design advantage of a NEMA-style starter. To effectively select a NEMA-style product, you only need to know the horsepower and voltage.

Serviceability is another key in the NEMA market. Most NEMA products are serviceable and provide front access to internal parts. Unless a total device replacement is necessary, contacts and coils of all sizes can be repaired usually without removing the device from the panel.

Reserve capacity is a consideration in the ease of selection of NEMA products. They are designed to be very robust and broadly applicable. They are "general-purpose," as opposed to being more specific and/or application intensive.

NEMA-style devices have interchangeable heater elements. In fact, a range of thermal units are available for the overload. This characteristic makes a NEMA product attractive on many construction jobs, where the ultimate motor schedule or the actual motor nameplate is not known until close to the start-up date.

The motor control center (MCC) or Enclosed Package Solution is a NEMA tradition. Most NEMA products are sold as starters, in either open-style or various enclosed versions.

IEC features and benefits

The large number of IEC contactor sizes allows you to more closely match the contactor to the load or application. Unlike NEMA, where the available sizes are standard across the industry, the number of different contactor size ratings varies by manufacturer.

Generally, an IEC starter is about half the size of a comparable NEMA starter. Above 100A (NEMA Size 3), however, the physical differences between NEMA and IEC are negligible.

IEC products are more modular in nature; they can snap onto a 35-mm, or in larger sizes, a 75-mm DIN rail. This eliminates the need to drill holes and mount each component individually. Thus, putting them into a panel is relatively easy.

Unlike NEMA starters, which typically are sold fully assembled, IEC starters typically are sold as components (a contactor, an auxiliary block, an overload relay block) so that you can make your own assembly. You simply select the accessories needed for the application and snap them together, without the use of tools.

Also unlike NEMA starters, IEC-style thermal overload relays typically have fixed thermal elements, with an adjustment range that may require replacing the complete overload relay when significant motor FLC changes due to application requirements.

A large number of IEC contactors are applied in group installations, and NEC Sec. 430-53 deals with this application facet. However, this isn't unique to IEC contactors, since you can install any starter this way.

IEC products, more often than not, are sold to OEMs, which typically install them in applications where multiple devices are needed to control different functions of a machine.

Type 2 coordination is an IEC standard that can be applied to a NEMA-style product as well. Type 2 protection is afforded by the use of a properly rated and class of fuse with a contactor or starter. This type of protection requires that, after short-circuit condition testing, the con-tactor or starter must be reusable without component or part replacement. Also, no significant change is allowed in the operating time-current characteristics of the overload relay. This is a coordinated protection standard for the short-circuit protective device, contactor, and overload relay. The rules are the same for both IEC- and NEMA-style devices. For Type 2 coordination, the typical higher withstand rating of a NEMA-style starter provides a greater selection of short-circuit protective devices. Type 2 coordination with IEC-style products will offer the same coordination and degree of protection; however, the selection of fuse classes may be more limited. For short-circuit currents up to 5000A, Type 2 protection for IEC-rated motor starters would require Class RK5, J, RK1, or CC fuses. For short-circuit currents up to 100kA, Class RK1, J, and CC fuses are required.

Compared with NEMA-style products, IEC-style products are more disposable than serviceable. In fact, IEC products below 100A typically are considered disposable; they can be removed from a DIN rail, thrown away, and replaced with a new product in a matter of minutes.

Self protected starters represent a new category of combination starters as defined in UL 508-E. Typically, such a starter has a contactor, an overload relay, and short-circuit protection. The device also undergoes a unique testing procedure, one that goes beyond that for conventional combination motor controllers. This procedure tests the starter at different fault levels, verifying that coordinated protection similar to Type 2 is provided.

Training

Training is one of the biggest differences between IEC- and NEMA-style products. Because of their application-specific designs, IEC products require a more precise application knowledge in the selection process. Many of the IEC variables aren't necessary when selecting a NEMA-style product.

Does this mean you need a lot more training before you can select an IEC product? Probably not; it just means that your training has to be very specific in nature. Selecting IEC products simply requires more time and knowledge upfront, to ensure the selection matches the application.

Once the proper selection is made, however, both IEC- and NEMA-style products will deliver similar performance.

SUGGESTED READING

EC&M Articles: "Short-circuit Protection for IEC Motor Contactors," May 1990. "Selecting NEMA and IEC Motor Controllers," July 1992. For copies, call (913) 967-1801.

RELATED ARTICLE: Table 1. NEMA-style design features and benefits

* Easy to select and apply: can be specified early in the project. Detailed motor load data is not necessary to size device.

* High withstand capability: suited to heavy-duty applications with little concern for derating.

* Wide range of field installable accessories.

* Easy to maintain: contacts are easily inspected; coils are quickly changed.

* Wide range of enclosure types: NEMA Type 1, 3R, 4, 4X, 7, 9, and 12.

Table 2. IEC-style design features and benefits

* Meet IEC standards: acceptable worldwide

* Compact size: saves panel space. (30% to 80% smaller than NEMA).

* 35 mm DIN rail mounting: saves installation time.

* 45 mm width and 45 x 60 mm mounting hole spacing on small contactors: interchangeability among manufacturers without panel modification.

* Fingersafe terminals: safety for operating and maintenance personnel.

* Ambient-compensated bimetallic overload relay: compensates for varying temperatures at the controller.

* Automatic reset, field-convertible: suitable for remote or inaccessible locations.

* Test-to-trip feature: simulates trip condition by mechanically operating trip mechanism, reducing start uptime.

* Broad range of factory-assembled starters: nonreversing and reversing; two-speed; combination; reduced voltage; duplex.

RELATED ARTICLE: STATE OF THE MARKET

The installed base of NEMA is largely North American in nature, with some sparse pockets around the globe. These pockets are found in areas with large U.S. specifying influences, such as U.S. oil companies doing business in Saudi Arabia and other countries in the middle east.

Over the past decade, however, many North American industries have switched from NEMA-style contactors and starters to IEC-style devices; however, that conversion has leveled off and has done so by market segment. Those in the material handling industries and OEMs in the commercial and light industries have just about completed the changeover to IEC. Meanwhile, those in process industries, such as petrochemicals, have largely stayed with NEMA-style devices.

In terms of units sold, IEC-style products outsell NEMA-style products by about three to one in the North American market. In dollar value, however, NEMA-style devices outsell IEC-style devices two to one. Price levels account for this difference, as the latter products offer a wider product range due to the closer matching of the contactor to the load. More sizes result in a greater variety of prices.

Predictions of doom for NEMA-style product business were made when IEC-style products were first introduced to the U.S. market. Those predictions can be equaled to the same made for electromechanical relays when programmable logic controllers (PLCs) were introduced; everyone said these relays would become obsolete. In both cases, this has not happened.