Contactors are repeatedly used to close and interrupt an electrical circuit. An electromechanical contactor circuit closes when voltage is applied to a coil inside it. The coil acts as a magnet and rapidly draws the main contacts together. When voltage to the coil is shut off, springs pull the main contacts apart again to open the main circuit. In addition to the coil circuit and main circuit contacts, there may be auxiliary switches. These are single sets of small contacts that can be wired to pilot lights, for example. Electronic contactors act similarly but interrupt the circuits mainly in power electronics.
Connect the main terminals. The contactor will have three terminals marked L1, L2 and L3. These must connect to the power supply. In an electrical panel, these wires normally come from the main circuit breaker. The contactor will have three more main terminals marked T1, T2 and T3. Connect the wires leading to the load to these terminals. If the contactor is being used on a low voltage, 120- to 240-volt circuit, it will have only two terminals but the connection details are the same.
Connect the low-voltage control terminals. While the main terminals may operate at 480 or 600 volts, the control circuit usually is low voltage, commonly 120 volts. There will be two terminals for connecting the coil voltage, and they will be marked at 120 volts, 50/60 hertz or a similar low voltage. The wires normally come from a control transformer, but the low-voltage control circuit also includes "start/stop" buttons or switches.
Connect the auxiliary contacts. It is important to know whether the contactor is open or closed and small lights called pilot lights are used for this. The auxiliary contacts close and open at the same time as the main contacts. They will be labelled with a symbol resembling two capital Ts butting together and have two terminals each. One terminal must be connected to the control power supply. The other terminal must be connected to the pilot light. Sometimes there is an auxuliary contact with a line through the double T symbol. This means the contact normally is closed when the main contacts are open and open when the main contacts are closed. This auxiliary contact often is connected to the control's power supply on one terminal and a pilot light on the other terminal. This light signals that the power is on but the contactor is open and the load is not being supplied.
Turn off the main circuit breaker before attempting any wiring. The control power is supposed to be drawn after the main circuit breaker, but this rule sometimes is not observed. Check for control voltage before wiring the coil and contacts.