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.
Related electrical guides & articles
I want to know the right contactors for?
15hp-motor——contactor size
20hp–!!————–!!
30hp–!!————–!!
7.5hp–!!————-!!
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?
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
There is a typographical error in the equation for Max Rated Torque. It should have been 9500 x KW / RPM; 5252 x HP / RPM
Thanks Vinay to inform about the error regarding to writedown the equation
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.
why hp not watt is being used for drawing Motor lock rotor current?
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,
your truly
for induction motor I will use AC7b?
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.
I don’t know how 8×6 becomes 42 in your calculation.
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.