IEC vs. NEMA Contactor Ratings:

Part 1 Normal duty

Many customers ask for us to explain the difference between NEMA sizing and IEC sized contactors and starters, and how a dual rated IEC and NEMA contactor or starter compares with a NEMA only equivalent. In this two part tech tip we will explain how to size both NEMA and IEC size contactors, and how they differ in performance.

IEC ratings for contactors are sized based on application and motor ratings. For AC motors there are four utilization categories:

AC1- Resistive loads such as heaters with a power factor greater than .95

AC2- Starting, plugging, inching and jogging of slip ring motors.

AC3- Line starting and breaking during normal running of standard squirrel cage inductive motors. Application examples include lifts, conveyers, pumps, fans etc. This is considered ‘normal’ duty for AC motors.

AC4- Plugging, inching and jogging of squirrel cage and slip ring motors. Application examples include cranes, hoists, printing machines, etc.

IEC contactors are sized precisely based on one of the AC utilization ratings noted above. Normal duty or the AC3 rating is typically used for sizing the contactor. For example a 32 amp IEC contactor like our CRC 32 is rated for 32 amps full load current at 220 to 460VAC in AC3 applications. To size the contactor properly you will need to know the voltage, current and HP and use the appropriate AC rating.

NEMA contactors are similarly rated for general purpose line starting and breaking during running for standard squirrel cage motors. The major difference is that the NEMA ratings are broader and have a small amount of margin built into the ratings to cover the broader range. NEMA starters are rated by size: 00, 0, 1, 2, 3, 4, 5, 6 and 7. To size the contactor you need to know only HP and voltage.

At Cerus we dual rate our contactors for both NEMA and IEC ratings. Our NEMA rated contactors meet the NEMA sizing requirements and the IEC ratings for that particular size contactor. Using our CRC 32 as an example once more, we see it is also rated as a NEMA Size 1 contactor. To size a dual rated contactor like our CRC 32 you will need HP, voltage and current (current is needed to insure that the motor FLA does not exceed the contactor AC3 rating since this is a design limitation).

In summary, for dual rated or IEC rated contactors you will need to size a contactor or starter based on HP, voltage and current, while a NEMA contactor or starter is sized only by voltage and HP. For general duty applications (AC3), Cerus CRC contactors that are dual NEMA and IEC rated, have similar or superior performance to NEMA only contactors. Next time we will look at how the two compare in heavy duty AC4 applications with a large amount of plugging, inching or jogging.

NEMA vs. IEC contactors Part 2:

Heavy duty applications

Last time we looked at how NEMA and IEC contactors differ in sizing for general applications. This time we will look at the differences in applications which require plugging, inching and jogging, or AC4 utilization category as defined by IEC. As a reminder there are four main utilization categories for IEC contactors:

AC1- Resistive loads such as heaters with a power factor greater than .95

AC2- Starting, plugging, inching and jogging of slip ring motors.

AC3- Line starting and breaking during normal running of standard squirrel cage inductive motors. Application examples include lifts, conveyers, pumps, fans etc. This is considered ‘normal’ duty for AC motors.

AC4- Starting, plugging, inching and jogging of squirrel cage and slip ring motors. Application examples include cranes, hoists, printing machines, etc.

NEMA and IEC have normal duty ratings for contactors that follow AC3 utilizations as we talked last time. For AC4 use, both NEMA and IEC will require a derating.

NEMA sizing for plugging inching and jogging follows a standard sizing table. This table results in approximately a 20 to 70% derating compared with normal duty. This table does not specify, nor does it require a life cycle rating be published. So while a NEMA contactor may perform at this derating level, the impact on life cycle ratings is dependent on the manufacturer.

IEC deratings for AC4 depend on life-cycle ratings the end user wishes to maintain. With our CRC 32 as an example (which is also rated as a NEMA Size 1 contactor), the NEMA derating table will derate the contactor 50% at 460 volts. Using this NEMA derating for the contactor, the lifecycle rating for the contactor will also reduce to 10% of the AC3 rating.

It is difficult to compare NEMA contactors and IEC contactors for AC4 utilization, since NEMA life cycle ratings are usually unpublished information and is dependent on the manufacturer. In general though, the IEC rated contactor will need to be sized more carefully when used in AC4 applications than a NEMA only contactor. In both cases life cycles will be reduced and if a higher lifecycle is needed, then the contactor sizing will need to be derated further, or a higher size contactor will need to be selected to maintain the lifecycle rating and load rating desired.

This is probably the biggest area of misapplication when NEMA customers switch to IEC style starters. Lifecycle curves are available in our technical publications for contactors which can assist you in making the right selection for your application (insert weblink). A good rule of thumb with our CRC series contactors for AC4 applications is to use a contactor 2 to 3 times higher than the load rating at AC3 to have a similar life cycle rating. Or simply call us at Cerus to find the right size- NEMA or IEC!