Contactor Wiring Diagram: Coil, Power Poles, and Auxiliary Contacts

A contactor is not a relay. The distinction matters when you are wiring a motor circuit. Relays are rated for control-level currents -- a few amps at most. Contactors handle power loads: motors, heaters, lighting banks, capacitor banks. They are built for repetitive switching, with arc chambers to extinguish the arcs that kill ordinary relay contacts. Understanding the terminal layout is the foundation of any contactor wiring job.

Contactor Terminal Layout

Every AC contactor has three groups of terminals:

Coil Terminals: A1 and A2

The A1 and A2 terminals connect to the electromagnetic coil. Apply the rated coil voltage across these two terminals and the contactor closes its main contacts. Remove voltage and a return spring opens them.

Coil voltage varies by application:

The coil voltage must match your control circuit. Getting this wrong does not usually damage the coil immediately -- a 24V coil on 120V will burn out fast; a 120V coil on 24V will not close at all. Check the label on the contactor face or side.

Always install a coil suppressor in parallel with the coil:

Without suppression, the collapsing coil field generates voltage spikes that kill PLC output modules and sensitive relay contacts.

Power (Main Contact) Terminals: L1-L2-L3 and T1-T2-T3

The L1, L2, L3 terminals are the line-side inputs -- they connect to the incoming three-phase supply. The T1, T2, T3 terminals are the load-side outputs -- they feed the motor or other load.

When the coil is energized, the main contacts close and connect L1 to T1, L2 to T2, L3 to T3. Open the coil circuit and all three phases disconnect simultaneously.

For single-phase applications, use L1 and L2 (or just one pole if the contactor is rated for it). Two-pole contactors exist specifically for single-phase motor switching.

Power terminal torque matters. Under-torqued connections cause resistance heating that eventually produces loose terminals, arcing, and weld failures. Always torque to the manufacturer's specification -- typically printed on the terminal block or in the datasheet. For a 32A contactor, 4 Nm is a common figure; for a 95A frame, 12 Nm is typical.

Auxiliary Contact Terminals: 13/14 (NO) and 21/22 (NC)

Auxiliary contacts are low-current contacts (typically 10A maximum) used in the control circuit. Terminal numbering follows the IEC convention:

Additional auxiliary contact blocks can snap onto the side of most contactors (Schneider TeSys, ABB A-series, Siemens 3RT, Eaton XTCE, etc.). These provide additional NO and NC contacts for interlocking, pilot lights, or PLC feedback.

The NO auxiliary (13/14) wired in parallel with the start pushbutton creates the seal-in or hold-in circuit -- it keeps the contactor energized after the start button is released. This is fundamental to any start/stop station wiring.

Wiring a Contactor with a Start/Stop Station and Overload Relay

The complete circuit consists of two parts: the power circuit and the control circuit.

Power Circuit

  1. Three-phase supply L1, L2, L3 to contactor line terminals L1, L2, L3.
  2. Contactor load terminals T1, T2, T3 to the overload relay input terminals (also labeled T1, T2, T3 or 1T, 3T, 5T depending on the manufacturer).
  3. Overload relay output terminals (2T, 4T, 6T) to motor terminals U, V, W.
  4. Each phase should be protected by a short-circuit device (MCB or fuses) upstream of the contactor.

Control Circuit

For a 120VAC control circuit with a maintained contactor:

  1. L1 (hot leg) to Stop pushbutton (NC) terminal.
  2. Stop pushbutton second terminal to Start pushbutton (NO) terminal.
  3. Start pushbutton second terminal to A1 (coil terminal) AND to contactor auxiliary NO contact terminal 13.
  4. Contactor auxiliary NO contact terminal 14 connects back to the junction between Stop and Start -- this is the seal-in connection.
  5. A2 (coil return) connects through the overload relay NC contact to L2 (neutral or second leg).
  6. Overload relay NC contact carries the designation 95/96 on Schneider and ABB devices (or similar -- check the model).

This creates the classic DOL control loop:

Safety Note

Working on contactors connected to 3-phase mains is hazardous. Voltages of 208V, 240V, 400V, or 480V are present at L1/L2/L3 terminals when the supply is live -- even with the contactor de-energized, the line terminals remain at full potential. Isolate and lock out/tag out the upstream disconnect before touching any terminals. Verify absence of voltage with a calibrated tester at all three line terminals and the earth terminal before working.

Adding Mechanical and Electrical Interlocks

In a reversing or changeover circuit, two contactors are used. They must never close simultaneously. Protection has two layers:

Always use both forms of interlocking for reversing contactors. Mechanical interlock alone can fail if the mechanism wears. Electrical interlock alone relies on proper wiring integrity.

Contactor Sizing

Select the contactor AC-3 duty rating at or above the motor full-load current (FLC) for squirrel-cage motors. AC-3 is the standard duty rating for motor starting with inductive loads. For resistive loads (heaters, lighting), use the AC-1 rating, which is usually higher.

Common frame sizes and approximate AC-3 ratings:

For star-delta starters, the star contactor only carries 58% of the line current in normal operation, so it can be one frame size smaller than the main and delta contactors.

Drawing and Verifying Contactor Circuits

CircuitDiagramMaker includes dedicated contactor symbols -- coil, NO main contacts, NC and NO auxiliary contacts -- so you can draw both the power circuit and the control ladder side by side. Simulate the control circuit to confirm that the seal-in path works and that an overload trip correctly drops the coil. Catching a wiring error on screen is far less frustrating than finding it at commissioning with a 55kW compressor running late on a project.

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Key Takeaways