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Home / Technical Articles / Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)

Calculate size of each part of DOL motor starter for the system voltage 415V, 5HP three phase house hold application induction motor, code A, motor efficiency 80%, motor RPM 750, power factor 0.8 and overload relay of starter is put before motor.

Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)
Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)

Basic Calculation of Motor Torque and Current

  • Motor Rated Torque (Full Load Torque) = 5252xHPxRPM
  • Motor Rated Torque (Full Load Torque) = 5252x5x750 = 35 lb-ft.
  • Motor Rated Torque (Full Load Torque) = 9500xKWxRPM
  • Motor Rated Torque (Full Load Torque) = 9500x(5×0.746)x750 = 47 Nm
  • If Motor Capacity is less than 30 KW than Motor Starting Torque is 3xMotor Full Load Current or 2X Motor Full Load Current.
  • Motor Starting Torque = 3xMotor Full Load Current.
  • Motor Starting Torque = 3×47 = 142Nm.
  • Motor Lock Rotor Current = 1000xHPx figure from below Chart/1.732×415

Locked Rotor Current

Code Min. Max.
A 1 3.14
B 3.15 3.54
C 3.55 3.99
D 4 4.49
E 4.5 4.99
F 5 2.59
G 2.6 6.29
H 6.3 7.09
I 7.1 7.99
K 8 8.99
L 9 9.99
M 10 11.19
N 11.2 12.49
P 12.5 13.99
R 14 15.99
S 16 17.99
T 18 19.99
U 20 22.39
V 22.4
  • As per above chart Minimum Locked Rotor Current  = 1000x5x1/1.732×415 = 7 Amp
  • Maximum Locked Rotor Current = 1000x5x3.14/1.732×415 = 22 Amp.
  • Motor Full Load Current (Line) = KWx1000/1.732×415
  • Motor Full Load Current (Line) = (5×0.746)x1000/1.732×415 = 6 Amp.
  • Motor Full Load Current (Phase) = Motor Full Load Current (Line)/1.732
  • Motor Full Load Current (Phase) = 6/1.732 =4Amp
  • Motor Starting Current = 6 to 7xFull Load Current.
  • Motor Starting Current (Line) = 7×6 = 45 Amp

1. Size of Fuse

Fuse as per NEC 430-52

Type of Motor Time Delay Fuse Non-Time Delay Fuse
Single Phase 300% 175%
3 Phase 300% 175%
Synchronous 300% 175%
Wound Rotor 150% 150%
Direct Current 150% 150%
  • Maximum Size of Time Delay Fuse = 300% x Full Load Line Current.
  • Maximum Size of Time Delay Fuse = 300%x6 = 19 Amp.
  • Maximum Size of Non Time Delay Fuse = 1.75% x Full Load Line Current.
  • Maximum Size of Non Time Delay Fuse = 1.75%6 = 11 Amp.

2. Size of Circuit Breaker

Circuit Breaker as per NEC 430-52

Type of Motor Instantaneous Trip Inverse Time
Single Phase 800% 250%
3 Phase 800% 250%
Synchronous 800% 250%
Wound Rotor 800% 150%
Direct Current 200% 150%
  • Maximum Size of Instantaneous Trip Circuit Breaker = 800% x Full Load Line Current.
  • Maximum Size of Instantaneous Trip Circuit Breaker = 800%x6 = 52 Amp.
  • Maximum Size of Inverse Trip Circuit Breaker = 250% x Full Load Line Current.
  • Maximum Size of Inverse Trip Circuit Breaker = 250%x6 = 16 Amp.

3. Thermal Overload Relay

Thermal Overload Relay (Phase):

  • Min. Thermal Overload  Relay setting = 70%x Full Load Current(Phase)
  • Min. Thermal Overload Relay setting = 70%x4 = 3 Amp
  • Max. Thermal Overload  Relay setting = 120%x Full Load Current(Phase)
  • Max. Thermal Overload Relay setting = 120%x4 = 4 Amp

Thermal Overload Relay (Phase):

  • Thermal Overload Relay setting = 100% x Full Load Current (Line).
  • Thermal Overload Relay setting = 100%x6 = 6 Amp

4. Size and Type of Contactor

Application Contactor Making Cap
Non-Inductive or Slightly Inductive ,Resistive Load AC1 1.5
Slip Ring Motor AC2 4
Squirrel Cage Motor AC3 10
Rapid Start / Stop AC4 12
Switching of Electrical Discharge Lamp AC5a 3
Switching of Electrical Incandescent Lamp AC5b 1.5
Switching of Transformer AC6a 12
Switching of Capacitor Bank AC6b 12
Slightly Inductive Load in Household or same type load AC7a 1.5
Motor Load in Household Application AC7b 8
Hermetic refrigerant Compressor Motor with Manual O/L Reset AC8a 6
Hermetic refrigerant Compressor Motor with Auto O/L Reset AC8b 6
Control of Restive & Solid State Load with opto coupler Isolation AC12 6
Control of Restive Load and Solid State with T/C Isolation AC13 10
Control of Small Electro Magnetic Load ( <72VA) AC14 6
Control of Small Electro Magnetic Load ( >72VA) AC15 10

As per above chart:

  • Type of Contactor = AC7b
  • Size of Main Contactor = 100%X Full Load Current (Line).
  • Size of Main Contactor = 100%x6 = 6 Amp.
  • Making/Breaking Capacity of Contactor = Value above Chart x Full Load Current (Line).
  • Making/Breaking Capacity of Contactor = 8×6 = 52 Amp.

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Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

34 Comments


  1. Md Taiyab Alam
    Sep 23, 2018

    8×6=52 How it is possible


  2. Faisal
    Aug 15, 2018

    Can you please explain . How Motor rated torque value is 35 lb-ft????


  3. Chandan
    Aug 07, 2018

    How it’s 5252*5*750=35 lb-ft it’s coming 19,695,000 please clarify..


    • nikunj
      Aug 30, 2018

      motor rated or full load torque= 5252XHp / RPM


  4. Jayananda Mohanty
    Jul 18, 2018

    A motor full load current ratting 20 amp .how olr setting formula


  5. basel farkoh
    May 18, 2018

    May you please explain how it is possible to have the Motor Starting Current (Line) = 45 Amp bigger than the
    Maximum Locked Rotor Current = 22 Amp?


  6. Prabhakara
    Apr 17, 2018

    hi
    plz send me 0.5 hp, 1 hp and 10 hp motor slide rules


  7. cecilio
    Apr 05, 2018

    i think there is something wrong on the value of F max., it should be 5.59 not 2.59 under locked rotor current.


  8. zuber shaikh
    Nov 06, 2017

    Hi,
    I have an EC-DC motor of 0.4 KW. I want to command the motor from a PLC output 24 AC and i want to control the speed of the motor with 0-10 vdc. Could you please guide me for selection of control relay for starting the motor. The motor data is 230 VAC 50Hz single phase.
    Also can i use control relay for commanding the motor On and Off.


  9. David Castillo
    Jul 16, 2017

    The page before was better because it allowed copying or downloading what I published in PDF now and only lets print it bad that bad serivicio. I was better before.

    • Edvard
      Edvard
      Jul 16, 2017

      David, you can still create the same PDF using https://www.printfriendly.com/

      Just drop an URL od article and everything is the very same. Try it.

      Or you can install PDF printer on your Windows/Linux/OSX and print by choosing it. It’s simple.


  10. Randy Sink
    Dec 10, 2016

    Where can I find the required magnetic contactor size for a motor given the motor current or HP?


  11. SAIBAL BOSE
    Nov 11, 2016

    Hi,
    I need tips on how to do sizing of UPS & Automatic Power Factor Correction


  12. Darshan
    Dec 03, 2015

    We are going to manu.. 2HP single phase monoset pump for agriculture purpose.240V & max curretnt 11.5A. difficulty is i am not getting that how much degree of thermal fuse we can use?


  13. zubair akhtar
    Oct 08, 2015

    Mr.shafqat
    I have a question for you.
    I have a two speed motor having twelve(12) input terminals.when we provide supply on twelve terminals what happend???????


  14. kazim
    Sep 14, 2015

    Pls I want to know why you chose motor starting torque to be 3x full load current. Thanks


  15. Hesham Elhamaki
    Jul 15, 2015

    Dear Gents
    Please support me to select fuse size for 3 phase motor with LRA 11.4 A V=400, inpout power 1.21Kw, current = 2.3A and how to calculat it?, fause PH (3N)


  16. ghulam jaffar
    Jun 21, 2015

    how we select aa magnetic contactor wth respect to motor


  17. Sanjul Seth
    Jan 31, 2015

    Dear Sir/Mam ,

     I have completed bachelor’s degree in Electrical & Electronics engineering branch in 2014 . I have 6months experience  in designing electrical panels ( APFC, MCC & PCC panels)& switchgears and software skills in electrical autocad.
    Can you please suggest doing PLC, SCADA automation course will hike up my career or not
    to get a higher package. If yes please do suggest Institutes for the same
    Regards
    Sanjul


  18. Mohamed
    Jan 08, 2015

    The starting current is 45A , maximum size of none time delay fuse is 11A ,
    Why it will not blow?


  19. Mohamed
    Jan 08, 2015

    Why Code F minimum locked rotor current =5A , maximum locked rotor current =2.59A


  20. Mohamed
    Jan 08, 2015

    Motor starting torque=3 times of motor full load torque or current
    3 times or 2 times it is very big difference


  21. shamsuzzaman
    Aug 25, 2014

    i want to know how many torque of 15 kw motor


  22. Gbenro
    Jun 08, 2014

    How will you calculate the contactor, circuit breaker, thermal overload and size of power cable and control cable for DOL starter when u are give a 3KW induction motor. Thank u.


  23. Haresh Rathod
    Mar 14, 2014

    what are the basis for above calculations?


  24. faizalmdz
    Nov 15, 2013

    What is the basis of above calculation….
    Provide International Standard for Calculation….
    Thank you


  25. adipriya
    Sep 17, 2013

    1.Can we say what is the no load current for a motor if it’s full load current,voltage,power ,power factor are known


  26. shafqat.ali
    Sep 17, 2013

    5252x5x750 = 35 lb-ft. ????
    9500x(5×0.746)x750 = 47 Nm ???????


    • susanne
      Sep 17, 2013

      It should be:
      5252×5/750 = 35 lb-ft.
      9500x(5×0.746)/750 = 47 Nm


      • Shafqat Ali
        Sep 18, 2013

        Yes u r right


  27. sthirum
    Sep 14, 2013

    From where you got multiplying factor, Is there any std/ ref in IS, IEC.etc.


  28. sam atlantis
    Sep 13, 2013

    Motor Full Load Current (Line) = KWx1000/1.732×415

    Why the power factor is not included in the calculation although it has been give in the details?


    • Alok
      Feb 27, 2014

      p.f is taken in the calculation, but it is missing in the formula


  29. sandeepayyagari
    Sep 12, 2013

    great sir,can u describe about selection of transformer according to connected load, necessary erection steps for transformer erection

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