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Home / Technical Articles / How to know if you set the correct current on a motor thermal overload relay

Start with full-load current…

The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay. Because of the variable voltages around the world, motors for pumps are made to be used at both 50 Hz and 60 Hz in a wide voltage range.

How do you know which current to set on a motor overload relay
How do you know which current to set on a motor overload relay (photo credit: Edvard CSANYI, EEP)

Therefore, a current range is indicated on the motor’s nameplate. The exact current capacity can be calculated when we know the voltage.

Motor thermal overload relay
Figure 1 – Motor thermal overload relay

Calculation example

When we know the precise voltage for the installation, the full-load current can be calculated at 254 Δ/440 Y V, 60 Hz. The data is indicated on the nameplate as shown on the illustration on below:

  • f = 60 Hz
  • U = 220-277 ∆/380 – 480 Y V
  • In = 5.70 – 5.00/3.30 – 2.90 A
The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay
Figure 1 – The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay

60 Hz data calculation:

  • Ua = actual voltage 254 ∆/440 Y V (actual voltage)
  • Umin = 220 ∆/380 Y V (Minimum values in the voltage range)
  • Umax = 277 ∆/480 Y V (Maximum values in the voltage range)
The voltage ratio is determined by the following equations:

UΔ = (UA – Umin) / (Umax – Umin)
which is in this case: UΔ = (254 – 220) / (227 – 220) = 0.6

UY = (UA – Umin) / (Umax – Umin)
which is in this case: UY = (440-380) / (480-380) = 0.6

So, UΔ = UY

Calculation of the actual full-load current (I)

Imin = 570/3.30 A
(Current values for Delta and Star at minimum voltages)

Imax = 500/2.90 A
(Current values for Delta and Star at maximum voltages)

Now, it is possible to calculate the full-load current by means of the first formula:

  • I for Delta values: 5.70 + (5.00 – 5.70) × 0.6 = 5.28 = 5.30 A
  • I for Star values: 3.30 + (2.90 – 3.30) × 0.6 = 3.06 = 3.10 A

The values for the full-load current correspond to the permissible full-load current of the motor at 254 ∆/440 Y V, 60 Hz.

Rule-of-thumb: The external motor overload relay is always set to the nominal current shown on the nameplate.

However if motors are designed with a service factor, which is then shown on the nameplate eg. 1.15, the set current for the overload relay can be raised by 15% compared to full-load current or to the service factor amps (SFA) which is normally indicated on the nameplate.

If the motor is connected in star = 440 V 60 Hz the overload relay then has to be set to 3.1 A.

Reference // The motor book by Grundfos

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Scott
    Aug 29, 2023

    Exactly the information I needed to understand how to PROPERLY set my thermal overload relay on a variety of 1-phase and 3-phase electric motors. The inclusion of accounting for motor Service Factors is especially important in the real world of electric motors. Thank you for authoring this article.

  2. Hagag jabir
    May 12, 2023

    Really,I need this excellent article

  3. Meta
    Nov 22, 2021

    Nice article

  4. Lydia Mbithi
    Oct 22, 2021

    Where has this 0.6 come from and what is it?

  5. Lascelles Dacres
    Oct 03, 2021

    Good day
    I am happy to know I can get assistance on your site.
    I am doing a family portable water project….with a 230v 10 hp 50 amp motor for the pump.
    Can you tell me the best amp setting for the contactor overload.

    L Dacres

  6. lammii befiqadu
    Sep 22, 2021

    thank for your cooperation and I m glad to see it. i will see for future partnership .

  7. Seidu Adams
    Mar 12, 2021

    How do I know the size of overload for a motor

  8. Razi abbas
    Jun 29, 2020

    Do you have diagram for relay

  9. aklilu
    May 25, 2020

    it is a good document.

  10. Clement Amegnigan
    Dec 02, 2019

    I would like to have all the news about electrical engineering

  11. Maxwell mbiyu
    Nov 23, 2019

    It was very nice about. Automation knowledge congratulations

  12. Owerri Imo State Nigeria
    Nov 22, 2019

    15 by 12 meters conference Hall Electrical piping and wiring.

    • luckson
      Dec 09, 2019

      95 Eden North Sunnyside pretoria

  13. Subrata Kumar Bera
    Nov 22, 2019

    The above explanation is good . Normally , O/L replay is connected in one of the phase of delta connection in the starter panel. In case of high HP motor , we provided CT in 3 phase connection for current & voltage checking. In that case can we provide O/L Relay by using CTs in the main income of starter panel.

  14. Apolonio Nisperos
    Nov 21, 2019

    we can also check the actual current and set the overload relay.

  15. Apolonio Nisperos
    Nov 21, 2019

    always refer to the motor nameplate

  16. Sanket Patil
    Sep 13, 2019

    Setting a pick up current for an overload relay is specified in any IEC standard

  17. ronaldo cortez
    Jul 24, 2019

    this only for 3phase how about single phase connection

  18. Shafi
    Mar 29, 2019

    Good explanation . Need specific value of current at which relay should trip the motor with 440V Y and 3.1A setting.
    Also plz explain if overload setting is made at nominal current of motor, mentioned on nameplate . Then what is the use of 1.15 or 1.25 times rated current setting. Thanks

    Dec 08, 2018


    Oct 29, 2018


  21. Umar
    Sep 20, 2018

    I need more diagrams about motor connections

  22. Glen Sullivan
    Sep 20, 2018

    Thank you for a good explanation. I have a couple of people I work with that believe if you have a fast acting overtemp relay, you dont need overloads. If it gets hot, it’s overloaded. Not very fast enough for lost phases or locked rotors. I am not sure if it’s even legal as the standard calls for overload fitted to motors over 370w. Has anyone here got a correct answer?

    • Samir Ismail
      Dec 02, 2018

      Dear Sullivan
      Normally the Fast Acting O.T.R. have an embedded sensor(s) in the motor armature coils, suppose for some reasons these sensors are no more working, then your motor is not protected at all.
      It’s preferable to use the usual Over Load protection mean as the “Main Protection One” and the F.A.O.T.R. as a “Backup”, in this case you realize a complete protection of your motor against the overload events.

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