Which contactor is better - IEC, NEMA or Definite Purpose?

Which contactor is better - IEC, NEMA or Definite Purpose?young professional deciding between nema iec and definite purpose contactor

Summary: The choice of contactor type depends most on who is making the decision, and what they value the most.  NEMA = ROBUST, Definite Purpose = "budget friendly", and IEC = versatile.


There are reasons for three different types of contactors, and there are reasons one makes more sense than the other... depending on who you ask.

What makes each type unique?

  • NEMA - This type is the "tank" of the contactor world.  It is big, has large contacts that makes it capable of handling even severe applications.  NEMA devices are also easy to maintain - repair it and put it back into service.  I used to work in tech support for a market leader of NEMA motor control, and it was not unusual to hear of NEMA units operating for 40 years.
    robust tank

  • Definite Purpose - As the name implies, this type has an intended use, and performs in that use well.  It has an attractive price point, but don't expect too much more of it though. Definite Purpose (a.k.a "DP") is perceived as being very budget friendly.
  • IEC - The IEC contactor provides the best flexibility.  In addition to the attractively small size, IEC contactors often have more accessories and circuit design options as well.  IEC is very designer-friendly you might say.

What is the same?

  • How they work - A contactor is a contactor essentially.  NEMA, IEC and DP contactors each have a coil that closes a set of contacts when power is applied to the coil.
  • UL ratings - While they are classified differently, all three types can be certified to a UL standard. 
  • Selection - How these contactors are selected is also very similar.  If it is a motor, then the UL horsepower rating is used to select the size.  Definite purpose may have Full Load Amp (FLA) and Locked Rotor Amps (LRA) ratings that is used to select the size.

So what dictates when each type is used?

Essentially, it boils down to two main factors: duty expectation, and the cost view.  What does that mean?  We'll explain below...

What is the duty expectation?

This essentially is how the contactor will be used.  How often will it turn on, and how long will it operate before turning off? What load will it be operating? How long should it operate before needing replacement or servicing?

Answers to these questions will shed light on which type is best for your application:

  • NEMA contactors are most often used on strenuous applications, such as frequent starting and stopping, or jogging (a rapid short interval used to adjust position).  NEMA contactors will last much longer than other contactor types.  They also are designed to be maintained where parts can be quickly replaced.
  • Definite Purpose are most often used in less strenuous applications. A good example is an HVAC unit or a refrigeration unit.  The thermostat kicks the unit on, it runs for several minutes, then turns off for a while.  Not too challenging, and thus not too much is needed.
  • IEC contactors are the middle ground... good operational life, but with enough flexibility to meet machine design needs.  IEC can be used in more strenuous applications, but they are often oversized according to application-specific load-life curves.

How long is your cost view?

What do I mean by this?  A cost view depends on who you ask.  If you are talking to the user of the equipment/machine, they likely consider cost in terms over the full life of the unit... from initial purchase to servicing to the end of its usable operation. If you are talking to a contractor, they don't want a call back due to a contactor problem.  Their cost view gets much higher if they have to return to a job site.  If you are talking to a machine manufacturer, they may be thinking in cost terms of the initial contactor purchase.  They likely aren't interested in paying for any more benefit beyond the intended use and operation period in which the machine is under warranty.

Where are these types most often used?

  • NEMA contactors and starters are most often used where operational life is more valued.  This is where the end user has more influence on the type used, such as an Motor Control Center (MCC) specified by the owner of the equipment, or a municipality that desires extended reliability on their water/wastewater equipment.
  • Contractors like using NEMA, not only because they are very reliable, but also because they can often buy them in control packages ready to mount to the wall and wire.
  • Commercial type Original Equipment Manufacturers (OEM's) of equipment like HVAC units, commercial cooking equipment, refrigeration units an compressor units often prefer Definite Purpose.  Why? Because these are "light duty" operations in what can be a very competitive space where extra accessories and design options just aren't needed.
    commercial refrigeration unit
  • Machinery OEMs often prefer IEC contactors.  It is not unusual to have many equipment loads controlled in a single machine control panel, where panel space is at a premium.  Thus the small size of the IEC contactor, and the design versatility is valued.
  • Outside North America, you will likely find IEC used.  NEMA and Definite Purpose are not used much in other regions of the world.

More information and resources

  • More information on contactors.
  • Short-Circuit Current Ratings are important to all three contactor types. Learn more on SCCR.
  • IEC contactors have more to offer in terms of sustainability.  Learn how.

Do you agree?

Did you like this content?  Or do you have a different perspective?  Let us know in the comments.



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