Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter

Calculate Size of Contactor, Fuse, C.B, O/L  of DOL Starter

  • Calculate Size of each Part of DOL 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 , Overload Relay of Starter is Put before Motor.

Basic Calculation of Motor Torque & Current:

  • Motor Rated Torque (Full Load Torque) =5252xHP/RPM
  • Motor Rated Torque (Full Load Torque) =5252×5/750=35 lb-ft.
  • Motor Rated Torque (Full Load Torque) =9500xKW/RPM
  • Motor Rated Torque (Full Load Torque) =9500x(5×0.746)/750 =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 over Load Relay:

  • Thermal over Load Relay (Phase):
  • Min Thermal Over Load Relay setting =70%xFull Load Current(Phase)
  • Min Thermal Over Load Relay setting =70%x4= 3 Amp
  • Max Thermal Over Load Relay setting =120%xFull Load Current(Phase)
  • Max Thermal Over Load Relay setting =120%x4= 4 Amp
  • Thermal over Load Relay (Phase):
  • Thermal over Load Relay setting =100%xFull Load Current (Line).
  • Thermal over Load 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.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 13 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. 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.

49 Responses to Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter

  1. asimali856 says:

    thanks……sir

    Thanks & regards

    Asam Ali

    Al Mariah United Group Abu Dhai – U.A.E Cell No.: +971 567077962

    ________________________________

  2. Tamer says:

    why did you eliminate the efficiency of the motor & the p.f value from your calculations ?

    I think the rated current =(5*746)/(1.732*415*0.8*0.8)=8 Amp.

    Thanks

  3. Rudru Naikar says:

    send me diploma cet question papers

  4. dushyant kumar says:

    good materil

  5. Mohan says:

    very useful notes u provided, keep doing this

  6. Arun S says:

    Dear Sir,
    What is mean by Instantaneous Trip and Inverse Trip for Size of Circuit Breaker?

    Regards
    Arun S

  7. Krishna says:

    What does 3.31,3.7,4.6,0.37,2.95 values indicate sir in DOL Starter sizing calculations

  8. jaikumar says:

    Interesting&knowledgeable hope to see more quality& informative articles

  9. viral makwana says:

    Motor name plate do not indicate locked rotor current code, how to know that?

  10. BASANAGOWDA PATIL says:

    Why normally relay setting is 1.3 times of the motor HP.Is there any exact formula to calculate the same ?

    How to calculate relay setting for motors with different RPM.Ex.10 HP/1000 RPM,10HP/1500 RPM, 10 HP/3000 RPM.

  11. mohd shamse alam says:

    Sir may you tell me to setting of relay 3 phase 10 hp motor 380 volts cos.8 And how to calculate please tell formula’s

  12. Harish says:

    superbbbbbbbbbbbbbbb

  13. abbas says:

    please tell me about overload, mcb, mccb,fuses selection about dol ,star delta starter in

  14. RAHUL says:

    Dear sir

    Thanks for the valuable information.For 5 HP motor the letter code is A or F. This is my doubt. please make it clear if you are free sir.

    warm regards

  15. Hector says:

    Hi, first of all, thanks for sharing this information. Second, I have a question: Doesn’t the contactor should be sized in comparison with the conductor size?, which must be 125% of the FLA of the load (If PF is higher or equal than 0.8). I make this cuestion because if the conductors are sized 125% of the FLA, and the contactor sized on 100%, there might be a voltage drop, or a hot spot in the contactor. What do you think, sir?

  16. Tuhin Mia says:

    Thank You Sir.
    Tuhin
    From
    Bangladesh
    Mobile:+8801712427773

  17. pankaj patel says:

    3+N

  18. Steve Arreola says:

    Thank you Sir…Greetings from West Texas

  19. Jinesh akabari says:

    What is calculation of 5hp ,415v induction motor protection panel ?

  20. Henry says:

    Hi there I have an under floor heating matt with a full load of 10000kw/ 42 amps.

    Wanted to no what size contactor I would need
    Thanks Henry

  21. sudesh says:

    how it can be possible that a 5 hp motor is rated for 47 A. as u have taken 47A. for calculating motor starting torque (i.e. at the Basic Calculation of Motor Torque & Current topic). Plz tell me sir..

  22. Mr. SOVANN says:

    Hello Sir, I am Mr.Sovann,I am living in Cambodia. I would like to ask u some question. My country use 380v for three phase, and 220v for single phase, 50Hz, and Pf 0,85.
    Question: i have a motor thee phase, its power is 3kw. but i don’t know how to choose a Motor Circuit Breaker, a Contactor, an overload relay,and its cross-section area of wire to be suit with it. Could u show me how to choose all of that devices? or could i have some of formulas about them?
    Thanks in advance
    Good luck!

  23. dinesh das says:

    Well.I hope,I will learn more….

  24. Jagat says:

    wat is d difference between slip ring induction motor and squirrel cage.??

  25. waleed khalid says:

    what is the different will occure if we use method of starting like star delta or soft starter

  26. prabhu says:

    thanks

  27. Rohit says:

    sir pls tell me the overload relay size for 45 kw 3 ph motor

  28. Raj says:

    Actually, it is very useful.

  29. Jignesh says:

    Hi Sir,

    Good to know that you are tremedous on Electrical calculations tools in excel itself.

    Would like to have help on Motor FLA Calculation Sheet,Contactor sizes ( NEMA ).

    Sharing your knowledge in such a platform is appreciable matter.

    Thanks,
    Jignesh vaghela
    Jignesh22
    1085@gmail.com

  30. Sakthivel says:

    how to calculate power contactor coil current.

  31. manuel joseph says:

    it is very good for improving knowledge

    thanks

  32. chirag chaniyara says:

    plz give 11 KW slip ring induction motor installation,commissioning and installed equipment list?

  33. sivasubramaniyam says:

    sir, the fuse rating formula come ac and dc load or same?

  34. Durga Khatri says:

    please give me the rating of MCCB contractor and types of starter for the squirrel cage induction motor of 112Kw ??? that need to operate blending blower in cement industry.

  35. D.SUGANYA says:

    how to find fuse rating of 3 phase induction motor

  36. Ken says:

    Your information about fuse size for motors was helpful as I have been searching for a simple table to give me some sense of the accuracy of my calculations. Thank you. But I must inform you that I looked further and I now know that there is an error in your NEC 430.52 table. You have the multiplier factors under the wrong headings. In other words, they are reversed. For a Single Phase Motor (for example), your table shows a Non-Time Delay Fuse value of 175% of FLC and 300% for a Dual Element (Time Delay Fuse). In fact the NEC 430.52 table shows that For a Single Phase Motor a Non-Time Delay Fuse should be 300% of FLC and 175% for a Dual Element (Time Delay Fuse). Your math example below your table is also incorrect because it follows the reversed data from the table. I only point this out because I have much respect for you and have read good advice from you many times. I checked a few other sources and your table is the only one with the reversal. I would not want your readers to be misled as I know you would not either. Thank you for sharing your wisdom online. Be well,

  37. artium says:

    How to size conatctors for wound rotor motor 400V 132KW?

  38. ijaz ali says:

    thank u soo much it, s was too good!

  39. Chinnappan says:

    Thank you Sir.
    I have feed it in to Excel sheet with formulae for future reference.
    Similarly I want to know for resistive (heater) load. Kindly help me.

  40. karthick says:

    Sir,
    How do i calculate the Contactor Ratting by only with the known values oh KW,V,& A

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