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)


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.


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.

author-pic

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.



12 Comments


  1. shamsuzzaman
    Aug 25, 2014

    i want to know how many torque of 15 kw motor


  2. 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.


  3. Haresh Rathod
    Mar 14, 2014

    what are the basis for above calculations?


  4. faizalmdz
    Nov 15, 2013

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


  5. 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


  6. 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


  7. sthirum
    Sep 14, 2013

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


  8. 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


  9. 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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