Proper motor protection
with IEC versus NEMA
IEC and NEMA motor controls both work well in most scenarios: Select the best option for your application
By Brent Purdy
There is a wide range of motor controls available that meet one of the two dominant standards: National Electrical Manufacturers’ Association (NEMA) ICS 2 and International Electrotechnical Commission (IEC) 60947. NEMA and IEC standards evolved separately with distinct intentions, creating the differences apparent today. The NEMA philosophy was to design to a standard “frame” size to allow interchangeability among different manufacturers and to handle the worst-case applications. IEC, which originated in Europe, emphasized space and cost savings by applying only the required capacity to the load being served. Unlike NEMA, IEC does not categorize starters into specific frame sizes, but instead uses utilization categories (e.g., AC-3, AC-4) to rate devices.
These utilization categories are specific to an application and duty cycle. IEC relies on each manufacturer or an external laboratory to evaluate products for specific utilization categories. The IEC methodology means each manufacturer may have a number of different offerings covering one NEMA frame size. Widely used IEC utilization categories include:
- AC-1 for purely resistive or only slightly inductive loads, such as heater circuits
- AC-3 for routine starting and stopping of a squirrel cage motor and only occasional jogging
- AC-4 is similar to AC-3 but includes inching and plugging (jogging and reversing at speed), requiring significant instantaneous control of the motor current
NEMA devices under 100 A or 50 hp are generally twice the size and price of IEC devices. Consequently, IEC devices offer about double the selection of starter sizes between 2 and 900 hp, making proper component selection much more critical than it is with NEMA.
NEMA-type devices have a tougher, more robust design with more conservative ratings, but IEC, with its smaller size and lower cost, is the better option in many applications (figure 1). Some feel IEC devices cannot take abuse like a NEMA device, but proper selection of the starter and careful design of short-circuit and overload protection devices should provide satisfactory performance.
Figure 1. This NEMA starter and IEC starter are each sized for up to a 27-amp load, with the NEMA starter capable of a wider range of duty befitting its larger size and greater weight.
It is clear the NEMA standard wants to keep it simple, as there are 10 NEMA starter sizes between 2 hp and 900 hp, a range over which one IEC manufacturer has 20 sizes. The NEMA standard simplifies selection by offering devices suitable for a wide range of applications. The IEC standard has more starters, each suitable for a more specific and narrowly defined range of applications. This wide range of products makes it important to precisely design the system with consideration of duty cycle, motor horsepower, full load current, and other factors. The selection process is similar for both standards but more rigorous for IEC, and this difference often factors into the IEC versus NEMA decision.
Making the choice
NEMA and IEC starters are both rated for 1 million electrical operations, and the UL testing is identical for each. When selecting components, many designers stop at the two main differences between NEMA and IEC, size and cost. Although these are serious factors and the easiest to define, one should dig deeper into other factors affecting total life-cycle costs.
For a user with a well-defined application, IEC makes sense. The standard is a good fit for those users with the time and expertise to carefully design their motor control and protection circuits to precisely fit a well-known operating environment. That includes just about any original equipment manufacturer, because the extra upfront design cost will be quickly made up with less expensive IEC components.
For scenarios where motors are subject to frequent overload or where the motor load is less well defined, NEMA may be a better choice. For those doing a one-time design, particularly when operating parameters are less well known, it may be cheaper to just use NEMA and forego the precise design required for IEC. The table lists the leading factors favoring IEC and NEMA.
For example, if the maximum current is not well known, NEMA devices are often the best choice. A NEMA size 1 starter, with a maximum 10 hp at 480 V, has a continuous rating of 27 A, while an equivalent IEC device is rated for only 18 A, making it critical to know the exact characteristics of the load for IEC designs. In applications such as large undefined projects or systems that change duty and loading cycles frequently, the ease of specifying NEMA justifies the price and size restrictions. This is why the NEMA devices have a strong foothold in heavy industries such as petrochemical and steel.
NEMA devices also have more common and replaceable parts than IEC devices. So, when downtime is critical and a facility has a large installed base of units, this is an added benefit. IEC devices, on the other hand, are basically disposable below 100 A. This may be a hindrance or benefit depending on the user and the number and type of applications in a facility.
IEC starters require knowledge of the load and its duty cycle to be properly sized. For example, IEC starters list AC-1 and AC-3 utilization currents, which are the rated currents for totally different applications. Understanding the difference is essential to properly sizing a starter. A typical IEC 5-hp starter will be marked for 18 A (AC-1) and 7 A (AC-3). If an application involves frequent jogging or full voltage reversing, the starter may not be rated for that duty, so sizing the unit up one or two levels is advised.
NEMA starters, in general, are more robust. The springs, contacts, and other components can take more abuse, vibration, and shock. The starter coils are typically encapsulated to protect against contaminates, where IEC coils are only tape wrapped and coated. On the flip side, NEMA uses more coil power and needs more enclosure space and clearances.
NEMA starters are open design, so safety covers are often purchased separately or added in the field. IEC contactors and starters are typically IP20 finger-safe rated, which lends itself to a safer open panel design.
Finally, consider the modern control environment where motor control devices are integrated with the automation system. As compared to NEMA, IEC devices have many more options to network components together into a distributed control system using open fieldbus standards.
Overloads and single phasing
Although either IEC or NEMA components can be used in most every motor protection scheme, there are significant differences between the two standards regarding protection from thermal overload or single-phase conditions. Solid-state or electronic overload relays have become a popular solution for both IEC and NEMA starter applications, and these relays are significantly different from standard NEMA thermoelectric overload devices.
The standard NEMA overload relay uses bimetallic or eutectic alloy overloads. Electronic overloads deliver superior performance, but are usually offered as an option at additional costs. The standard overload relay uses removable heaters to actuate the protective relay, which must be manually reset at the device after a trip. But with electronic overloads, built-in automatic reset can be activated once an operator has confirmed a safe condition.
Another significant difference between IEC and NEMA overload relays is phase loss detection. This critical motor protective feature is standard on IEC starters, but must be added to a standard NEMA starter.
Although sizing an IEC starter can be difficult, standard NEMA overload sizing can also be tricky. Each heater element may represent a different amperage value based on the unit where it is installed. Each manufacturer has dozens of tables to size the heaters to the loads. A particular overload unit from a prominent manufacturer shows up in 16 tables and has a maximum full load current value ranging from 11.2 A to 14.0 A. The same unit also has other published values that are higher in rating for single-phase applications. Because these are thermoelectric devices, ambient temperature correction calculations may adjust ratings. Sizing errors are thus easy to make if one is not well versed in the procedure.
Protection solutions for IEC and NEMA
The motor control circuits for both IEC (figure 2) and NEMA (figure 3) consist of branch circuit protection, starter, and overload protection. UL lists IEC starters, with defined combinations of fuses and breakers, which can be used together for various motors and loads.
Figure 2. This IEC motor starter diagram shows typical symbols and terminal numbers used in the design of IEC devices.
This NEMA motor starter power and control diagram shows typical symbols and diagrams used in the design of NEMA devices.
A starter can be damaged after a short circuit even if the fault is safely cleared within the time allotted in the design parameters. The wider ranging capability of the NEMA device is an advantage in withstanding possible fault damage, but with proper short-circuit protection a fault that would damage an IEC starter is rare.
Poor selection of short-circuit/ground fault protection upstream of the IEC starter causes more welded contacts and “self-destructed” units than occurs with comparable NEMA components, but type 2 coordination can minimize this problem. Type 2 coordination is a protection category described in IEC 60947-4-1. Section 126.96.36.199 specifies that type 2 coordination requires that under short-circuit conditions, the starter shall cause no danger to persons or installations and shall be suitable for further use.
With type 2 coordination, there is very little difference in protection schemes for IEC and NEMA starters. Although the NEMA starters can have higher short-circuit current withstand ratings (SSCR) out of the box, using properly sized current-limiting breakers, or class CC or class J fuses, gives IEC similar SCCR ratings. It is not uncommon for IEC starters to achieve 65–100 kA with the proper fuse or breaker combination. The caveat is that the manufacturer has to test the unit as a combination, and the purchaser must use the exact test configuration components.
Although not universal, most NEMA starters are equipped with class 20 overloads and IEC with class 10. This means that IEC will handle 600 percent current for 10 seconds before tripping, while NEMA will handle the same amount for 20 seconds. At first glance, this seems to be an advantage to NEMA. However, this provides less protection from motor damage due to a locked rotor. For the applications that require long acceleration times, such as a heavily loaded conveyor or a crushing/grinding operation, the longer NEMA times are needed. In standard applications such as material handling, pumping, and packaging, the NEMA class 20 overload can permit damage to the motor during true overload conditions.
A common method many use to reduce failure concerns of IEC devices is to upsize the starter to the next range. This gives about 30 percent extra capacity, and works well according to many experienced engineers and electricians. For example, an IEC starter for a 10-hp, 480-V motor could be upsized to a 25-A device, making it comparable to a NEMA size 1 rating. This also saves more than 11 square inches of panel space.
Upsizing may be helpful in certain situations, but there is no need to upsize a 9-A IEC starter, equivalent to a NEMA size 00 rating, for a 2-hp, 480-V motor, as this is overkill and negates the size and cost advantages of IEC. The key to sizing IEC or NEMA starters is to evaluate the duty of the load, not just the amperage or horsepower.
IEC devices can handle standard across-the-line starting and stopping with no issues, even under high loads. However, environmental conditions that may damage contacts, such as excessive heat or airborne contaminates, are good reasons to upsize. Frequent reversing and jogging are AC-4 utilization categories that can damage a starter. Still, these effects are minimal, as NEMA states that AC-4 utilization may reduce an IEC starter’s electrical life by as little as 2 percent of its life span when compared to AC-3 utilization.
Excelling at an application
Automotive, petrochemical, steel, and pulp and paper industry end users often specify NEMA devices. The utilization category, such as plugging and inching large inertial loads, may require NEMA, but even when this is not the case, the standard is often preferred to ensure commonality across the entire facility.
A lifting hoist or large conveyors in a quarry or mine present high-inertial loads that may start, stop, reverse, and jog frequently. These types of installations are typically in harsh environments. In this scenario, motor control centers and NEMA starters are warranted.
IEC motor controls excel in applications such as the material handling and packaging industry, where massive conveyor systems and large numbers of small motors move material or packaged products. Loads are relatively light; installations are compact; and price sensitivity is high. Groups of IEC starters in industrial control panels are a compact solution. A NEMA motor control center controlling 50 or more ¾-hp to 1.5-hp motors is overkill in terms of cost and space requirements.
In many applications, tradition, specifications, or local opinion drives the decision to use NEMA or IEC components. Both styles of controls have areas where they naturally fit, but many traditional NEMA industries are embracing IEC control for cost or space saving. Many have predicted the death of NEMA for years, but it is still around and doing well. For the foreseeable future, there will be a place for NEMA, particularly in heavy industrial applications in the U.S.
For most situations either IEC or NEMA may be employed, but IEC devices require more forethought and planning. If done correctly, IEC controls can pay big dividends and offer a competitive advantage.