Overload relay setting and calculation

Overload relay is the one of important device for motor control.It can prevent our motor from overheat or winding burning due overload of ampere.

We need to setting the value of overload relay properly depend on our application and motor full load ampere.If we setting low from FLA,it can cause motor trip c0ntinues and process not running smoothly.

But if we setting to high from FLA,the overload relay cannot protect the motor if overload happen.It can cause the motor failure or winding burns.So we must calculated and get a right setting for the overload relay.

How to setting Overload Relay protection?

Basically to setting the overload value,we refer to this formula :

1) IB ≤ In ≤ IZ

IB = Anticipated operating current of the circuit
IZ = Current carrying capacity of conductor or cable or Motor
In = Rated current of protective device

2) I2 ≤ 1,45  x IZ

IZ = Current carrying capacity of conductor or cable or Motor

For adjustable protective devices, In corresponds to the value set.
I2 = The current which causes tripping of the protective device under the conditions specified in the equipment regulations (high test current).

Common practice

Normally for overload relay setting depend on FLA ( Full Load Ampere ) of motor.We can see at the NAMEPLATE of motor.Normally setting for overload is 5% until 10 % more than FLA.

But it is depend on operation and functional of motor.For more detail setting,please refer manual guide of motor from manufacture.

We also can setting overload relay value depend on value of service factor motor.Example if service factor 1.15,we can setting 125% from FLA and if service factor is 1.0,we can setting 115% from FLA of motor.

 Manual and automatic reset.

Normally,overload relay have this 2 reset function selection.We can choose manual or automatic reset after it trigger overload from motor.

I recommended for manual reset function because we can know when it trip and we can troubleshooting the root cause for tripping issue.So after we found the root cause,we can reset back the overload relay and can continue the operation.


Lemau is Owner of Electricneutron.com .Involved in Electrical & Automation Engineering since 2001. Passionate to shared all experience & knowledge about Electrical.

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  1. hi sir,
    I have one question about the relay settins in real time application. that is
    While changing the relay setting in live air circuit breaker Have we switch off the breaker or no need?
    I will give you one answer please check whether thwt is correct or not?
    While increasing the relay setting of the air circuit breaker no need to switch off the circuit breaker.If we want to decrease the relay setting means we have to switch of the circuit breaker is it correct if this is wrongmeans plese give me correct idea and what is the actual reason and correct answer.

  2. chaithu says:

    plz send me all type of cntroling diagrams and explanation
    thnks for giving this information to me

  3. cephaspet says:

    u guys are good thanks somuch

  4. dear sir
    what is formula of curve of overload protection class 10?
    in circuit breaker siemens 3wn6173

  5. Jamie says:

    Hey all,

    I was wondering why my overload relay in an air compressor with a star delta starter is downsized compared to the motor fla?

    • Hi Jamie..
      Your are compressor still running or always stop due to overload relay trip?

      If air-compressor still running :-
      1) check your overload relay condition.Perform test trip to ensure it is function properly,if not change it.
      2) Check your Full Load Ampere using Clamp on meter for actual value and compare with your setting.

      If your compressor always trip :-
      1) Use your actual FLA value and make calculation and set a new setting for your overload relay.

  6. Dear Sir
    i would like to inform you i need electrical easy formula.or all electrical coculletion because my math matik is very week
    1) druop volteg
    2) callculetion of size of cable for equpment ( motol or etc)
    3 from kva to mva 4) motor polarizetion 5) transformer steblity test

  7. G.Viswanathan says:

    How to check the functionality of Over Load relay without connect the load (May be by control circuit testing)

  8. Hai, I would like to ask,

    I found a single line diagram, overload setting @ current/voltage module is 63A…630A.
    Breaker is 250AF/250AT
    Load is 15.75kW
    My calculation: 15.75*1000/sqrt(3)/440V/0.8=25.8A

    My question is
    1) is the overload setting too high? should 25.8A within the overload setting? Can I say minimum value of overload setting cannot lower than 25.8A?
    2) is the MCCB rating too high?current only 25.8A, but the breaker is 250AF?normally, we refer to which value, AF or AT (Because I have seen some stated 160AF/100AT)?why there is two AF and AT for MCCB?

    Thank you

  9. axel musonda says:

    Dear sir
    Am very pleased finding were we can share knowledge about electrical engineering .plis let me receive emails on the email above

  10. AMritpal Singh says:

    Dear sir

    i have a compressor having rating current 0.8A @220volt 5oHz. How can i calculate the overload protection for this compressor .

    • Hi,
      You can used roughly calculation as per below example :-
      Normally I set my overload protection around 115% to 120% from Full Load current.
      For your application :
      (0.8 x 120)/ 100 = 0.96 Ampere –> 1 Ampere


  11. Rajesh Kumar says:

    Dear Sir,

    Please solve my problem ,I want to run a three phase squirrel cage induction motor of 10 HP,415 volts rating by DOL Starter,Star Delta Starter and VFD(30Hz).
    Please tell me the overload relay range and its setting,contactor rating,TPN MCB rating for backup fuse protection and cable size in each case.Sir,please give me the full calculation for this.I shall be highly thankfull for this.
    With Regards,
    Rajesh Kumar

    • Hello sir,
      your motor kw : 7
      full load amps : 12.98
      you can set the relay range ; 13 A ( 8.9 A to 13.5 A )
      Suitable MCB : C class 32 A
      4 sq copper cable .1 sq copper cable can carry upto 5 A so its suitable to ur application..

  12. i have a three phase motor full load current [ name plate] 15 A @ 415v how can set over load relay please telme callculation

    • axel musonda says:

      I want to know the exact setting of an electronic overload relay used on rigs ie type GL 90 main Motor rated current is 73amps service factor 1.15 voltage of 550 and 55kw running a 140cc hydraulic pump?

    • normal practice –> 15A x 110% = 16.5 AMP for overload relay setting

  13. Very good info. Lucky me I recently found your blog by chance
    (stumbleupon). I have book-marked it for later!

  14. Sahjahan Ali Ali says:

    How setting for overload relay of motor formula.
    Please send me.

  15. sitaram patel says:

    wow its v usefull link

  16. What does extra low voltage mean?

    • Extra low voltage is reduced voltage using transformer to 50 VDC or 120 VDC.Below from this voltage range we call extra low voltage.
      The main purpose of Extra low voltage is to eliminate electrical shock.Equipment that located in swimming pool,public area or kindergarten normally used Extra low voltage.
      This voltage also can be generate and synchronized with solar system and it green method to have electricity power supply.We also can reduce electricity consumption by using LED with Extra low voltage power supply.

      • Jose Eliazer says:

        Dear Sir,
        i Recently got know about your blog , really pretty good,
        i would like to know about Extra low voltage 50 or 120VDC in transformer, In this case Please provide details about How DC voltages used on transformer , is there coupling circuits , if you have circuit diagrams please send me,


  17. Thanks you

  18. Msu Barry Gbenekanu says:

    I really like this website.
    Keep it up.

  19. Abhishek Chauhan says:

    please send me formula for calculate the setting of any type of relay

  20. Gerelyal Ben Maima says:

    Feeling confident and well that this is the most interesting site that will really help.

  21. Fortune says:

    What is the overload setting for HV Motor of FLA= 303 @ 6.6V

  22. amirastor says:

    Hi All,
    Help me please, our cable between softstater and overload relay keep burning out. exactly at outgoing to softstater. it is to control 3phase motor, 36A, 18.5kW at nameplate.
    Softstater – Siemens 3RW3036 (22kW)
    Overload.R. – Eaton ZB65-40

  23. Macky says:

    I want a well label diagram of a relay and it applications in electircal panels

  24. StephanB says:

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  25. Dear sir
    What is the meaning of the thermal overload method-curve&curve 46

  26. what is the suitable relay for 15 hp star delta motor

  27. Ramesh Chandra Pahan says:

    I Know Power Factor Improvement Setting & Billi Meter Setting

  28. chandresh thakar says:

    sir my oueveshan is ht motor 3.3kv 2ookw my fault in in ht feeder in trip IN AMP. and I0 . INCRESE AND FEEDER TRIP

  29. Tony Tan says:

    Dear Sir,
    May I know how to calculate the tripping relay(mA) for RCCB.
    In what condition I shall used 30mA or/and 100mA(tripping relay).

  30. vithoba says:

    55kw AC induction motor how to select overload relay and cont actor and breaker plz tell me

  31. dayakar says:

    what is mean by “mA” value on MCB’s ??? how can we calculate this value

  32. I have a 45kw motor with a FLC of 86 A. It used to take 72A earlier but due to some changes done to process parameters is taking 93A for 10 seconds with a frequency of 2 times a minute. That means each phase of 93A there is a gap of 20 seconds and then the motor runs on full load. The MPCB has been kept at 80A but is not tripping yet. Is my motor safe, or do I need to change it to previous state?

    • Hi,
      you can know your motor ampere limit at namplate.Please find service factor (SF) and times with motor FLA.That the ampere your motor can stand.
      My opinion,if 93A is only for 20 sec,it still ok.I quest your starter is star delta right?what is your MCB ampere value?

  33. I have 22kW 30HP motor so how can I cal.MCB rating ,ct rating, contactor size OL relay current setting .explain in detail by using formulae to cal. All parameters

  34. how to select the OLR for 7.5kw feeder in DOL and START DELTA circut.

  35. what can be the problem of this motor 19.5A, 415v.i ave installed 50A contactor,a phase failure and i ave set the overload at 22A.the motor has burnt twice what can be the problem.

  36. Moti Singh says:

    Dear Sir,
    Please tell me what check point of over load relay & MPCB before failure’s.

  37. Ranganathan says:

    I found this website is very useful and each and everyone has their own answer that give us oppertunity to think further.Thru this website we all from diffrent country getting closer together.Thanks Guy.

  38. Overload setting formulae plz

  39. which relay renge should be used with 30 hp sub motor with DOL starter

  40. sir, how we find triping time of circuit breaker by using rely

  41. V. Sankar says:

    I have a compressor rated at 230V 7.5HP FLA on the motor is 36 amps 60hz.
    How to calculate to set the overload relay?

  42. jolly igiebor says:

    How do I know the amps of contactor and overload relay to use for, 2kw, 3kw, 5kw

    • Hi,Please find your equipment full load ampere or you can estimate with calculation.After that you can sizing your contactor and overload relay depend on FLA value..

  43. Aqeel says:

    Any body who ca help me……….?
    how to draw thermal characteristics curve for specific induction motor.

  44. Hi, after reading this awesome paragraph i am too
    happy to share my experience here with friends.

  45. Gideon says:

    What causes the over heating of over load?

  46. md aziz kamel says:

    pls advice
    i had rewinding 3 phase motor 45kw,FLA 80A.After installation and test run about 12 hours the motor trip again .aftet check was found the motor winding already burnt.fyi overload setting is 95A.is it any possibility of this problem..

  47. D.Waghmare says:

    What is two metal use in bimetallic stip in thermal over load relay

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