Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

# 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.

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.

### 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. Abdul-wahab Issahaku
Dec 16, 2022

I want to know the right contactors for?
15hp-motor——contactor size
20hp–!!————–!!
30hp–!!————–!!
7.5hp–!!————-!!

2. MOHAMMED SUMAIR ALI
Dec 15, 2022

Characteristics of Contactors and Starters (IEC 60947-4)
(Modification-1) Shall be AC-4 rated.
(Addition-1) Rated Insulation Voltage shall be a minimum of 660 VAC.
(Modification-2) Shall be rated for uninterrupted duty.
(Addition-2) Rated Operational Current shall be a minimum of 24 A.
(Addition-3) Contactors for motor service shall be selected based upon the
following:
a) On the assumption that all motors operate under 90% AC-3 duty and
10% AC-4 duty

Question:

As per the requirement of the specification ..the contactor Ampere rating for shall be minimum 24A for even for the motor will Less KW loads .

if Coordination is between MPCB,overload and motors there any impact of Contactor if it is oversized?

3. Vikki
Sep 09, 2021

Current setting of thermal overload relay.
थर्मलर्म ओवर लोड रिले की करंट सैटिंसै गटिं की जाती है।
Option 1 : According to noted current on motor name plate / मोटर नेम प्‍लेट पर अंकिअं त धारा के अनुसा नु र
Option 2 : On 15% less of supply voltage / सप्‍लाई वोल्‍टेज के 15% कम पर
Option 3 : On 10% more of supply voltage / सप्‍लाई वोल्‍टेज के 10% अधिक पर
Option 4 : On 10% less load current / भार धारा के 10% कम पर

4. 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

• Roberto
Feb 15, 2023

Thanks Vinay to inform about the error regarding to writedown the equation

5. 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.

6. Kush Lohana
Oct 05, 2020

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

7. 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,

Apr 25, 2020

for induction motor I will use AC7b?

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.

• Roberto
Feb 15, 2023

In fact the release shows that 8×6= 52 and not 42, but it also is wrong as well, so some correction or complementary variable and numbers must be added in this equation in order to explain this result.