Search

Premium Membership

Study specialized technicalg articles, papers & video courses in low/high voltage areas. Save 50% on all courses at the EEP Academy with the Enterprise Plan.

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.

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
author-pic

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.

43 Comments


  1. VINAY
    May 08, 2021

    There is a typographical error in the equation for Max Rated Torque. It should have been 9500 x KW / RPM; 5252 x HP / RPM


  2. santhosh
    Mar 22, 2021

    Hi, we have a 425hp FCMA starter for motor. We have used Run contactor 1050A where as customer had prescribed for 800A. And start contactor 800 we have used whereas customer needed 500A. Does this create any problem with the product performance. Please suggest.


  3. Kush Lohana
    Oct 05, 2020

    why hp not watt is being used for drawing Motor lock rotor current?


  4. waleid
    Jul 09, 2020

    Dear
    first thank you for this website with my wish for more successful, if i have a 92 kw/380v/0.8/184 amp submersible pump and i want to reduce the inrush current how much could reduce the starting current if i use soft starter,
    your truly


  5. Fadi
    Apr 25, 2020

    for induction motor I will use AC7b?


  6. Pradeep
    Sep 04, 2019

    How will you calculate the contactor, Motor Protection circuit breaker, Molded CasecCircuit Breaker, thermal overload and size of power cable and control cable for DOL starter and Star Delta Starter Upto 5 HP & Above 5 HP induction motor. Thank u.


    • Agbuke andrew
      Jan 18, 2021

      I don’t know how 8×6 becomes 42 in your calculation.

Leave a Comment

Tell us what you're thinking... we care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

twenty one  −  14  =  

EEP Academy Video Courses

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning.