Overload relay setting and calculation

52 Comments / Motor Control

How to Set Overload Relay Protection

An overload relay is a crucial device for motor control, designed to prevent motors from overheating or suffering winding damage due to excessive current. Properly setting the overload relay is essential for ensuring both motor protection and smooth operation.

Importance of Correct Overload Relay Settings

To avoid frequent trips and maintain efficient operation, the overload relay’s value must align with the motor’s full-load amperes (FLA).

  • Undersetting (Too Low): If the setting is lower than the FLA, the motor may trip frequently, causing interruptions in the process.
  • Oversetting (Too High): If the setting is higher than the FLA, the relay may fail to protect the motor during an overload, potentially leading to motor failure or winding damage.

How to Calculate Overload Relay Settings

The overload relay setting is determined based on specific electrical formulas to ensure accuracy and safety:

Practical Tips for Setting Overload Relays

  • Using the Motor Nameplate: Refer to the motor nameplate to determine the FLA. Typically, the overload relay setting should be 5–10% higher than the FLA.
  • Considering Service Factor: If the motor’s service factor is:
    • 1.15: Set the overload relay to 125% of the FLA.
    • 1.0: Set it to 115% of the FLA.

Example Calculation for Overload Relay Setting

Objective: Determine the correct overload relay setting for a 3-phase induction motor.

Motor Specifications

  1. Rated Power (P): 15 kW
  2. Voltage (V): 400 V (Line-to-Line)
  3. Frequency (f): 50 Hz
  4. Efficiency (η): 90%
  5. Power Factor (PF): 0.85

Step-by-Step Calculation

1. Calculate the Full Load Current (FLA)

2. Determine Overload Relay Setting

The overload relay setting is typically based on a percentage of the motor’s full load current. Common settings are:

  • For motors with Service Factor (SF) ≥ 1.15: Set to 125% of FLA.
  • For motors with SF < 1.15: Set to 115% of FLA.

The overload relay setting should be set to 35.45 A.

3. Check for Additional Adjustments

If the overload relay trips during motor start-up, some standards allow increasing the setting up to 140% of FLA for short duration :

Final Overload Relay Setting

  • Normal setting: 35.45 A
  • Adjusted (if necessary): Up to 39.70 A

These settings should be validated against the motor’s specific requirements and local safety standards.

Practical Notes

  • Always consult the motor’s nameplate for accurate data.
  • Confirm overload relay characteristics with the manufacturer’s manual.
  • Perform routine maintenance to ensure the relay operates reliably over time.

Reset Function: Manual vs. Automatic

Overload relays usually offer two reset options:

  1. Manual Reset: Requires manual intervention after a trip, allowing the operator to identify and resolve the root cause before resetting.
    • Recommended for troubleshooting and preventing repeated trips.
  2. Automatic Reset: Automatically resets the relay after a trip, which is convenient but may not address underlying issues.

Recommendation: Use the manual reset option to ensure a thorough analysis of tripping issues and to safeguard equipment integrity.

Conclusion

Properly setting the overload relay is vital for protecting motors from damage and ensuring efficient operation. By following the guidelines above and referring to the motor’s nameplate and manufacturer recommendations, you can achieve an optimal balance between safety and performance.

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52 thoughts on “Overload relay setting and calculation”

  1. gopalthulasidoss

    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.
    Regards
    Gopalthulasidoss

  2. chaithu

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

  5. Jamie

    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?

  6. lemau

    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.

  7. rajendratiwari

    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
    ETC…………

  9. G.Viswanathan

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

  11. KH

    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

  12. axel musonda

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

  13. AMritpal Singh

    Dear sir

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

  14. lemau

    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

    Thanks

  15. Rajesh Kumar

    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

  16. roy k

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

  17. axel musonda

    Hi
    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?

  18. Staci

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

  23. lemau

    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.

  24. Jose Eliazer

    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,

    Thanks
    Jose..

  28. Abhishek Chauhan

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

  29. Gerelyal Ben Maima

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

  31. amirastor

    Hi All,
    Sir,
    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

  32. Macky

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

  33. StephanB

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  37. chandresh thakar

    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

  38. Tony Tan

    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).
    Thanks

  40. Lava kumar.V

    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..

  41. vithoba

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

  44. uma mahesh

    hi sir ..
    how do you now how much current drawn for 1hp motor ?
    i want basic formula?
    please tell m,e sir…
    thanking you

  46. Sanoj

    Hai sir,
    You can set your overload relay at 105A and the Breaker size is 200A.

  47. Piyus Ranjan Sahu

    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?

  48. Lemau

    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?

  50. Anuj pandey

    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

  52. Peter

    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.

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