Correct Sizing of Motor Overloads

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mull982

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I have a 40hp 480V motor with a FLA rating of 47A. The only overload that I have avaliable is an electronic adjustable type with the lowest setting being 64A. I want to run this motor ASAP however I am concerned that the lowest setting of 64A on the overload will be too high to protect the motor adequately. I know in the code it allows sizing of the overloads to be 125% FLA and even sized up to 140% FLA for some cases. The 64A setting will give me a value of 136% of full load for this motor.

Will this setting work for this motor or will I possibly have problems down the road. The motor is a NEMA B type motor.
 
Is the motor marked with a service factor or temperature rise? If not, overload protection is limited to 130%. You device exceeds that as you know. Will it cause a problem? Who knows? Will your motor ever experience an overload? Some motors operate for a lifetime without ever experiencing an overload or tripping an overload device. The installation you are contemplating apparently does not meet the guidelines of the NEC. Nobody can tell you that is OK. It is in violation of the NEC. If there is a problem, you will not have the NEC to back you up because you knowingly violated it.
Your call....
 
Motor OL Protection

Motor OL Protection

mull982 said:
I have a 40hp 480V motor with a FLA rating of 47A. The only overload that I have avaliable is an electronic adjustable type with the lowest setting being 64A. I want to run this motor ASAP however I am concerned that the lowest setting of 64A on the overload will be too high to protect the motor adequately. I know in the code it allows sizing of the overloads to be 125% FLA and even sized up to 140% FLA for some cases. The 64A setting will give me a value of 136% of full load for this motor.

Will this setting work for this motor or will I possibly have problems down the road. The motor is a NEMA B type motor.
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As already stated you may be limited to 130% for your OL protection. If you do not want to obtain a different OL unit, you may want to investagate fuses as an option for OL protection. You should be able to meet the 130% limitation with the proper fuse protection (Dual-Element properly sized). Bussman should have info on-line that may help you.
 
I agree with the others, but if you are in a jam and it is possibly costing you money by not running the motor, I would take my chance and run it and have the proper overload over nighted.
 
It turns out the motor has a service factor of 115%. Therefore with a FLA of 47.7A and the 115% service factor I can use the 65A overload setting and be under the 140% rating stated in the NEC. Using the 65A overload setting I will be at 136% and should be o.k.

Thanks for the help.
 
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