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

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.

## 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.

• Min. Thermal Overload Relay setting = 70%x4 = 3 Amp
• Max. Thermal Overload Relay setting = 120%x4 = 4 Amp

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

1. 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)

2. ghulam jaffar
Jun 21, 2015

how we select aa magnetic contactor wth respect to motor

3. 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

4. Mohamed
Jan 08, 2015

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

5. Mohamed
Jan 08, 2015

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

6. 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

7. shamsuzzaman
Aug 25, 2014

i want to know how many torque of 15 kw motor

8. 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.

9. Haresh Rathod
Mar 14, 2014

what are the basis for above calculations?

10. faizalmdz
Nov 15, 2013

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

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

12. 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

13. sthirum
Sep 14, 2013

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

14. 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

15. sandeepayyagari
Sep 12, 2013

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