Sizing for magnetic contactor

When we design or replaced the magnetic contactor for motor starter or for others application,the important point is how to sizing the size of magnetic contactor.This is critical point because if we not sizing properly,it can cause a serious damage and our application maybe failure to perform in perfect condition.

This time i want  shared some information and basic technique how to sizing the magnetic contactor with properly method and suitable with our application needed.It is to ensure our application design is correctly function and have a good resulted.

Rating of  Magnetic Contactor

  • NEMA Rating

NEMA ratings are based on maximum horsepower ratings as specified in the National Electrical Manufacturers Association standards.Usually contactor are selected according to the table below :-

  • IEC Rating

The Main Contact have a rating to allowed some of current value ( ampere ) to through it.The current rating depend on the type of application.

Normally we follow the IEC rated maximum operation current such as below :-

1) AC1 – Non-inductive or slightly inductive rows

2) AC2 – Starting of slip-ring motors

3) AC3 – Starting of squirrel-cage motors and switching off only after the motor is up to speed. (Make Locked Rotor Amps (LRA), Break Full Load Amps (FLA))

4) AC4 – Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop. (Make and Break LRA)

5) AC11 – Auxiliary (control) circuits

* For more detail,please refer to the manufacturing manual or catalog


How to sizing the magnetic contactor?

From my experience,i usually  follow this basic method to choose the right magnetic contactor for my motor control application :

1) What does application required?Make a decision for the application

  • Basic contactor only?
  • Traditional stater?
  • Combination stater?

2) CONSIDER this factor when we sizing the magnetic contactor:-

  • Load Type : AC-1 or Inductive ( AC-3 ) *Refer IEC rating table
  • Duty Cycle : One direction, reversing, plugging  * Refer manual instruction
  • Horse Power (hp)
  • Full Load Ampere ( FLA ) * Refer to the motor data plate information

3) Decide the power supply for the coil of magnetic contactor.It very important because we not have a problem when choose the power supply for control circuit.Normally if we want control using PLC,we can use 24 VDC power supply or we can use 110VAC /240 VAC if we want conventional motor control.

* Please refer the manufacturing manual or catalog for details information.Usually it completed with table for choose the right magnetic contactor for our application.


  1. haitham hashim says:

    Useful information this site Best sites

  2. Under AC4 duty,like elevator, will the MCB/OL trip prematurely?. Should we increase the setting?.

  3. papa jun says:


  4. remhenson says:

    if i have a 100 hp motor, 3-phase operating at 400V, what size should be my magnetic contactor?

    thank you so much.

  5. shrish says:

    i just want to confirm In DOL when motor change into star to delta then what changes occur in rpm of motor…please explain

    • Hi,
      Let me explain.. In DOL starter, motor running in full speed without change from star to delta. DOL means Direct Online starter.
      If motor running in star to delta..that STAR DELTA STARTER.

  6. your in electrical engineering formula and calculation

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