# Thermal O/L Relay setting

Home Electrical Engineering Forum General Discussion Thermal O/L Relay setting

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• #10472
Keymaster

what is the approperiate setting for O/L shall it be 115% of the FLA or of the actual current of the motor or the same as FLA of the motor

some one says 115% of FLA and others says 100% FLA and other says 125%FLA

i heared many things so i'm confusing

#12046
electricalexpert65
Participant

Ali said:

what is the approperiate setting for O/L shall it be 115% of the FLA or of the actual current of the motor or the same as FLA of the motor

some one says 115% of FLA and others says 100% FLA and other says 125%FLA

i heared many things so i'm confusing

Not more than the rated full load current of the motor.

#12069
Keymaster

i think that the range of o/l could be 115%. but o/l setting could not more then it,s full load current of that motor.

#12071
erickench
Participant

NEC 430.32 gives the rules for sizing motor overloads. You must either determine the service factor which should be on the nameplate or determine the marked temperature rise also on the nameplate.

Motors with a marked service factor 1.15 or greater                                           125%

Motors with a marked temperature rise 40'C or less                                            125%

All other motors                                                                                                  115%

#12105
Keymaster

hi erikench,

thanks for information. i want to know whether class of insulation will affect the  overload setting .

#12119
Spir Georges GHALI
Participant

electricalexpert65 said:

Ali said:

what is the approperiate setting for O/L shall it be 115% of the FLA or of the actual current of the motor or the same as FLA of the motor

some one says 115% of FLA and others says 100% FLA and other says 125%FLA

i heared many things so i'm confusing

Not more than the rated full load current of the motor.

Dear Ali ;

First, we should the numonal current of the motor ” In ” from the name plate, them we adjust the O/L relay eactly as the ” In ” value, because :

For each thermal relay we have a Tripping Curve, by which we can find that there's no any tripping order if the motor current is equal to In, but if it's more than In the thernal relay needs some time ” accordignly the the tripping curve ” to stop the motor. that means if you adjust the thermal relay at 115% of In, your motor will run on overload for a time bigger than the time if adjusted on 100% of In, so, if the motor can accept it you can do it, but normally we don't do it. In some application, we need to run the motor with an overload ( we should know it ) for some time without tripping ( normally smal time ), in this case, we adjust at 100% of In and from the tripping curve we can know the tripping time at this overload, so we can decide if it will be OK or we should re-adjust, but the more important is the protection of the motor.

By the way, I want to draw your attention that there's many Tripping curves accordingly of the class of thermal relay ” Class 5, 10, 20 ” and for the Electronic Thermal Relays there's more, so, accordingly to the kind of application ” Standard  or Severe ” we do the choice of the kind of tripping curve.

Regards.

#12626
Keymaster

100 % agree with ghali . but  should have to pay attention if the motor is located in haz. the motor shall not overheat and surface temperature going beyond the temperature class  in case the overload relay is not well set.

#12711
Spir Georges GHALI
Participant

crosswind50000 said:

100 % agree with ghali . but  should have to pay attention if the motor is located in haz. the motor shall not overheat and surface temperature going beyond the temperature class  in case the overload relay is not well set.

Dear ;

Please explain more clear what is the relation between the surface's temperature and the Overload relay ?  Noting that the working of Overload relay is depending on the circulated current value and not of the heat temperature value.

#12726
electrician87
Participant

@electricalexpert65 said:
Ali said:

what is the approperiate setting for O/L shall it be 115% of the FLA or of the actual current of the motor or the same as FLA of the motor

some one says 115% of FLA and others says 100% FLA and other says 125%FLA

i heared many things so i’m confusing

Not more than the rated full load current of the motor.