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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. 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