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Properly Sizing Circuit Breakers and Fuses for Motors

“The motor keeps tripping my breaker and overload!”


Anyone who has worked around or with electric motors long enough has probably heard a very annoyed and frustrated technician utter that statement—or maybe they have even said it themselves.  Nuisance short circuit trips are a typical problem for many users and plants that operate electric motors. Nine out of ten times, however, the trip is caused by inappropriate pairing between the motor and either a circuit breaker or fuse, and not a defect in the motor.


First, the key to the statement above is the word “keeps,” which indicates that problem is repeating itself over and over again with the same motor and breaker configuration.  The technician resets the breaker, starts the motor, it runs for a while and—arrgh!—it tripped again.


Now, we know that the motor is not the problem in this scenario.  A trip caused by a defect in the motor, usually some kind of winding failure which has caused the motor to go to ground, would make the motor incapable of starting and tripping the circuit breaker or fuse more than once without substantial repair.  (Think:  One and it’s done, finito, toast.)


Since this issue is reoccurring, we can now look at the usual suspect for the nuisance short circuit trip—An undersized circuit breaker or fuse.  


The amount of current that electric motors use at start-up is vastly different than the current it uses after it is running.  Start-up current, depending on the size of the motor, can be 8-10 times greater than its normal operating full-load current, because an electric motor is essentially one long wire with very little resistance until its magnetic field has been built up in the windings.  A circuit breaker or fuse that does not account for the motor’s huge inrush of current at startup will trip repeatedly and be the source of a great deal of frustration.


Re-Evaluate Existing Circuit Breakers and Fuses for Premium Efficient Motors

In today’s age of lean manufacturing, equipment is being asked to do more— start more frequently, run more continuously, be more efficient.  In 2010, the electric motor industry responded to that demand with premium efficient motors and sweeping efficiency regulations that continue to evolve today.  What has this got to do with pairing circuit breakers and fuses?  


When premium efficient motors evolved into being, the motors were made more efficient by optimizing the windings which reduced their resistance and copper losses when running.  And yes, you guessed it, by reducing the winding resistance, the magnetizing inrush current increased dramatically.


This means that users and maintenance managers swapping out old motors for the newer premium efficient version need to evaluate existing circuit breakers and fuses during installation, as they may now be undersized and causing nuisance short circuit trips.


See the Chart Below for a General Guideline on Circuit Breaker and Fuse Sizes

If you are uncertain, don’t hesitate to contact a licensed professional electrical engineer to resolve motor protection problems.


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Posted on:  November 5, 2014

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