Overload Relay Sizing for Motor - Permissible OverAmperage?

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hou628

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Hello,

I am working with a 5HP motor that has an FLA of 6.7A and a SF of 1. The overload relay for this motor is currently sized at 6.7A * 1.15 to give 7.7A. How much higher of a rating am I allowed to proceed past 7.7A to stay within code? The motor trips on occasion and a suggestion has been made to increase the relay setting to 12 -15 [A] even though this is much higher than the 1.15 multiplier. Please provide feedback. Thanks!
 
You can go up to 125% if the motor has a marked service factor of 1.15 or greater, or a marked temperature rise of 40°C or less.

Note base value is actual nameplate current rating... not the 430 tables.
 
Great...thanks. I have an SF of 1, so I'll keep it at 7.7A. Just wanted to ensure that higher was not acceptable for an SF of 1
 
hou628 said:
Hello,

I am working with a 5HP motor that has an FLA of 6.7A and a SF of 1. The overload relay for this motor is currently sized at 6.7A * 1.15 to give 7.7A. How much higher of a rating am I allowed to proceed past 7.7A to stay within code? The motor trips on occasion and a suggestion has been made to increase the relay setting to 12 -15 [A] even though this is much higher than the 1.15 multiplier. Please provide feedback. Thanks!
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More details of what you have would be helpful. Lets start with what is the typical current draw when running?

Most adjustable overload devices have the 1.15 multiplier built in and you set the dial to motor nameplate, most thermal element selection tables also incorporate the 1.15 multiplier into the table and you make selection based on motor nameplate.

So sounds to me like you have too much load for the motor, unless you give some detail that gives some other clues to what may be wrong here.

One other issue could be using class 10 overload device when starting conditions maybe need a class 20 overload - this would usually be a situation of where it takes some time to accelerate the load when starting and the overload doesn't hold long enough to achieve full speed, or even a frequently stop - start cycle or frequent reversing can be an issue - though those kind of issues may require derating the motor just to help prevent it from overheating.
 
Thanks for the reply. The running amperage is around 5.2A. During startup, it does not exceed 5 A as the load is minimal on startup.
 
hou628 said:
Thanks for the reply. The running amperage is around 5.2A. During startup, it does not exceed 5 A as the load is minimal on startup.
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What type of overload unit is being used?

Like I said many of them already have 1.15 to 1.25 factor already built into the dial settings or selection charts, so if you add 1.15 factor to a unit that already is at 1.15 - net result is protection level of about 132%. If that is what you have and are tripping it - your motor is overloaded- raising the overload setting even more is just a way of getting to the finish line faster when it comes to burning out that motor.
 
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