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.

## 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

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.

Randy Sink

Where can I find the required magnetic contactor size for a motor given the motor current or HP?

SAIBAL BOSE

Hi,

I need tips on how to do sizing of UPS & Automatic Power Factor Correction

Darshan

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?

zubair akhtar

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???????

kazim

Pls I want to know why you chose motor starting torque to be 3x full load current. Thanks

Hesham Elhamaki

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)

ghulam jaffar

how we select aa magnetic contactor wth respect to motor

Sanjul Seth

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

Mohamed

The starting current is 45A , maximum size of none time delay fuse is 11A ,

Why it will not blow?

Mohamed

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

Mohamed

Motor starting torque=3 times of motor full load torque or current

3 times or 2 times it is very big difference

shamsuzzaman

i want to know how many torque of 15 kw motor

Gbenro

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.

Haresh Rathod

what are the basis for above calculations?

faizalmdz

What is the basis of above calculation….

Provide International Standard for Calculation….

Thank you

adipriya

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

shafqat.ali

5252x5x750 = 35 lb-ft. ????

9500x(5×0.746)x750 = 47 Nm ???????

susanne

It should be:

5252×5/750 = 35 lb-ft.

9500x(5×0.746)/750 = 47 Nm

Shafqat Ali

Yes u r right

sthirum

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

sam atlantis

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

p.f is taken in the calculation, but it is missing in the formula

sandeepayyagari

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