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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

CodeMin.Max.
A13.14
B3.153.54
C3.553.99
D44.49
E4.54.99
F52.59
G2.66.29
H6.37.09
I7.17.99
K88.99
L99.99
M1011.19
N11.212.49
P12.513.99
R1415.99
S1617.99
T1819.99
U2022.39
V22.4
  • 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 MotorTime Delay FuseNon-Time Delay Fuse
Single Phase300%175%
3 Phase300%175%
Synchronous300%175%
Wound Rotor150%150%
Direct Current150%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 MotorInstantaneous TripInverse Time
Single Phase800%250%
3 Phase800%250%
Synchronous800%250%
Wound Rotor800%150%
Direct Current200%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

ApplicationContactorMaking Cap
Non-Inductive or Slightly Inductive ,Resistive LoadAC11.5
Slip Ring MotorAC24
Squirrel Cage MotorAC310
Rapid Start / StopAC412
Switching of Electrical Discharge LampAC5a3
Switching of Electrical Incandescent LampAC5b1.5
Switching of TransformerAC6a12
Switching of Capacitor BankAC6b12
Slightly Inductive Load in Household or same type loadAC7a1.5
Motor Load in Household ApplicationAC7b8
Hermetic refrigerant Compressor Motor with Manual O/L ResetAC8a6
Hermetic refrigerant Compressor Motor with Auto O/L ResetAC8b6
Control of Restive & Solid State Load with opto coupler IsolationAC126
Control of Restive Load and Solid State with T/C IsolationAC1310
Control of Small Electro Magnetic Load ( <72VA)AC146
Control of Small Electro Magnetic Load ( >72VA)AC1510

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 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

44 Comments


  1. DAYANAND
    Jun 07, 2019

    Which contactor is best suitable for 110kw/150hp slip ring induction motor…stator current196amp/rotor current275amp


  2. Md Taiyab Alam
    Sep 23, 2018

    8×6=52 How it is possible


  3. Faisal
    Aug 15, 2018

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


  4. 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


      • SUNDARRAJAN
        Jul 13, 2020

        5252*5/750=35 T=5252*HP/RPM


  5. Jayananda Mohanty
    Jul 18, 2018

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


    • satyajit nayak
      May 10, 2019

      20×1.5=30


  6. 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?


  7. Prabhakara
    Apr 17, 2018

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


  8. 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.


  9. 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.


  10. 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.


  11. Randy Sink
    Dec 10, 2016

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


  12. SAIBAL BOSE
    Nov 11, 2016

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


  13. 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?


  14. 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???????


  15. kazim
    Sep 14, 2015

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

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