# Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter

## Calculate Size of Contactor, Fuse, C.B, O/L  of DOL Starter

• Calculate Size of each Part of DOL 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 , Overload Relay of Starter is Put before Motor.

### Basic Calculation of Motor Torque & Current:

• Motor Rated Torque (Full Load Torque) =5252xHP/RPM
• Motor Rated Torque (Full Load Torque) =5252×5/750=35 lb-ft.
• Motor Rated Torque (Full Load Torque) =9500xKW/RPM
• Motor Rated Torque (Full Load Torque) =9500x(5×0.746)/750 =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)==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 over Load Relay:

• Thermal over Load Relay (Phase):
• Min Thermal Over Load Relay setting =70%x4= 3 Amp
• Max Thermal Over Load Relay setting =120%x4= 4 Amp
• Thermal over Load Relay (Phase):