Sizing of contactor and overload relay for 3 Phase DOL starter

Data required for selection of DOL starter

  • KW rating of motor
  • Operating voltage
  • Application
  • KVA code if available

All the above data can be obtained from the name plate details on the motor.


Parameters to be considered during selection

  • Full load current of motor
  • Operational voltage and coil supply voltage
  • Type of load and duty cycle
  • Making and breaking capacity

Calculation of full load current (FLC)

Input power to the motor P =√3 V.I.Cosφ KW

P is rated power of motor in KW
V and I are voltage and current respectively

From this, we can calculate the full load current of motor

Ifl =(P × 1000)/(√3 V.Cosφ) A

Operational voltage and coil supply voltage

  • Power contacts of the contactor must be rated not less than the operating voltage of motor(as per name plate details).
  • Coil voltage/control voltage of contactor must be determined based on control circuit design.

Type of load and duty cycle

  • IEC 60947-5-1 defines the requirements for electromechanical devices for control circuits.
  • It indicate the type of electrical load and duty cycle of the loads to ease selection of contactors and relays.

Utilisation categories for contactors according to IEC/EN60947-4-1

(Mobile users, kindly drag the table left for complete view)

Type of currentUtilization categoryTypical application
ACAC-1Non-inductive or low-inductive load, resistance furnaces
AC-2Slipring motors: starting, switching off
AC-3Suirrelcage motors: starting, switching off while running
AC-4Squirrel-cage motors: starting, regenerative braking, reversing, jogging

Making and breaking capacity

Making/breaking capacity of a contactor is the maximum current that it can make or break safely. It must be equal to or greater than the locked rotor current.

Locked rotor current = (HP × 1000  x (KVA/HP))/(√3V.Cosφ) ampere

(KVA/HP) is indicated on the nameplate details of the motor based NEMA code.


Let us calculate the contactor and overload rating of DOL starter for the following motor:

7.5KW(10HP), Three phase squirrel cage induction motor, 415 volts, 50Hz, 1450RPM, efficiency: 90%, pf: 0.85, KVA code: H

Full load current Ifl =(P × 1000)/(√3 V.Cosφ) A
= (7.5 × 1000)/(√3 x415x0.85) = 12.28A

Locked rotor current = (HP × 1000  x (KVA/HP))/(√3 V.Cosφ) = 102A

Hence, Contactor must rating must be AC-3 duty with a current rating more than 12.28A and making breaking current more than or equal to 102A.

Selection of thermal overload

Min. Thermal Overload  Relay setting = 70%x Full Load Current(Phase)
Min. Thermal Overload Relay setting = 70%x12.28 = 8.6A
Max. Thermal Overload  Relay setting = 120%x Full Load Current(Phase)
Max. Thermal Overload Relay setting = 120%x12.28 = 14.7A

Hence, Overload must be chosen closer to the range of 8.6-14.7.

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