Selecting the right contactors and relays for use in motor control and other industrial circuits is extremely important. It is important that the contactors are chosen keeping in mind the equipment which is to be connected to it and the current it needs to interrupt.
A contactor which is chosen for a heater circuit cannot be used in a motor circuit of the same current rating.
The International Electrotechnical Commission (IEC) has categorized contactors into the following categories.| IEC Categories | Applications |
| AC1 | Non-inductive or slightly inductive rows |
| AC2 | Starting of slip-ring motor |
| AC3 | Starting of squirrel-cage motors and switching off only after the motor is up to speed. This contactor is designed to make Locked Rotor Current and Break the Full Load Current. |
| AC3 | Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop. (Make and Break Locked Rotor Current) |
| AC11 | For use in Auxiliary (control) circuits |