Contactor Relay Wiring Diagram

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A contactor uses a low-current coil signal to switch high-current main contacts, making it the standard method for remotely controlling motors, heaters, and large loads from a control circuit.

A contactor is an electrically operated switching device designed for making and breaking circuits under load, with a high number of operating cycles. It differs from a circuit breaker in that it is not designed to provide fault protection; it differs from a relay primarily in its current and voltage ratings and the physical separation between its control circuit and its power circuit.

The construction is straightforward: a coil wound around an iron core creates an electromagnet when energised. The electromagnet attracts the moving armature, which carries the main contacts. When the coil is de-energised, a return spring pushes the armature back and opens the contacts.

**Main (power) contacts** switch the load circuit — a three-phase motor, single-phase heater, or other high-current device. A standard three-phase contactor has three main contacts labelled 1L1/2T1, 3L2/4T2, 5L3/6T3 (IEC nomenclature) or sometimes L1/T1, L2/T2, L3/T3. Even numbers are outgoing (load) terminals; odd numbers are incoming (line) terminals.

**Auxiliary contacts** are smaller contacts mechanically linked to the armature. They switch at the same time as the main contacts and are used for control circuit logic. Normally open (NO) auxiliary contacts (e.g., 13/14) close when the coil is energised; normally closed (NC) auxiliary contacts (e.g., 21/22) open. Common uses include: - **Self-hold (seal-in) circuit:** an NO auxiliary in parallel with the momentary start button holds the coil energised after the button is released. - **Interlock:** an NC auxiliary from contactor K1 in the coil circuit of contactor K2 prevents both contactors from energising simultaneously — essential in forward/reverse or star-delta starters. - **Signal feedback:** an auxiliary contact wired to a pilot light or PLC input confirms contactor state.

**Coil voltage** must match the control supply. Common ratings are 24 V AC, 110 V AC, 230 V AC, 24 V DC, and 48 V DC. AC coils have higher inrush current on pickup; DC coils have a more controlled current profile. Energising an AC coil with DC (or vice versa) will damage it.

A **thermal overload relay** is typically mounted directly onto the contactor below the main contacts, forming a motor starter combination. The overload relay samples phase current through bimetallic elements or electronic sensors and trips if current exceeds the motor's full-load ampere rating for a defined time. Its NC stop contact is wired in series with the contactor coil circuit.

This content is a generic educational reference. All contactor installations must comply with IEC 60947-4-1, NEC NFPA 70, BS 7671, AS/NZS 3000, or the applicable local installation code and must be carried out by a competent person.

Contactors and relays share the same fundamental operating principle — an electromagnetic coil closes a set of contacts — but they are designed for very different current levels and applications. Understanding where each is appropriate, and how to combine them in a control circuit, is central to industrial motor control and power distribution design. You can draw relay and contactor control circuits, including ladder logic representations, free in the browser at CircuitDiagramMaker.com.

How to wire contactor relay wiring diagram

Determine load requirements and select the contactor Identify the load type (motor, resistive heater, capacitor bank), rated current, supply voltage (single or three-phase), and required switching duty. Select a contactor with an AC-3 (motor duty) or AC-1 (resistive duty) current rating that meets or exceeds the load current with an appropriate derating factor.
Select control circuit supply voltage and coil Choose the control voltage (24 V AC/DC for lower risk, 230 V AC is also common). Ensure the contactor coil is rated for this voltage. Low-voltage control circuits (24 V) reduce shock risk to operators interacting with control panels.
Wire the main power contacts Connect the three-phase supply to terminals 1L1, 3L2, 5L3. Connect the load (motor or other device) to terminals 2T1, 4T2, 6T3. Use cable rated for the full-load and starting current, properly sized and terminated in accordance with the installation code. Torque all terminal connections to the manufacturer's specification.
Wire the thermal overload relay Mount the overload relay onto the contactor. Connect the overload relay output terminals to the motor terminal box through appropriately rated cable. Set the overload relay adjustment dial to the motor's nameplate full-load ampere (FLA) rating.
Wire the control circuit — stop button In series with the contactor coil, wire an NC momentary stop pushbutton. Any open in this series chain will drop the contactor out. Also wire the overload relay's NC contact (95/96 or 97/98) in series here.
Wire the control circuit — start button and seal-in contact Wire an NO momentary start pushbutton in series between the stop button / overload contact chain and the contactor coil terminal. Wire the contactor's NO auxiliary contact (13/14) in parallel with the start button to create the self-holding circuit.
Test the control circuit before energising the main circuit With the main power isolated but control supply active, press the start button. The contactor should pick up and hold in after releasing the button. Press the stop button — the contactor should drop out. Verify the overload relay test button also drops the contactor. Only then energise the main power circuit.

Specifications

Main contact duty (motor loads)	IEC AC-3 (starting and stopping squirrel-cage motors); AC-1 for resistive loads
Standard current ratings (3-phase)	9 A, 12 A, 18 A, 25 A, 32 A, 40 A, 65 A, 80 A, 95 A (IEC frame sizes vary by manufacturer)
Common coil voltages	24 V AC/DC, 110 V AC, 230 V AC, 48 V DC
Auxiliary contact rating (typical)	10 A AC at 250 V
IEC terminal numbering — main contacts	Line: 1L1, 3L2, 5L3 / Load: 2T1, 4T2, 6T3
IEC terminal numbering — aux contacts (NO)	13/14
IEC terminal numbering — aux contacts (NC)	21/22
Mechanical endurance (typical)	≥ 10 million operating cycles (no-load); 1–3 million cycles at rated AC-3 current

Safety warnings

Contactors switching motor or heater loads carry mains voltage on main contact terminals at all times when the supply is connected, even when the contactor is open. Always isolate the upstream circuit breaker and verify all main terminals are dead with a voltage tester before working inside the panel. Follow IEC 60364, NEC NFPA 70, BS 7671, or your local code.
Never energise an AC coil from a DC supply or vice versa. Mismatched coil supply will immediately damage or destroy the coil.
Do not bypass or remove the thermal overload relay from the motor starter circuit. The overload relay protects the motor windings from damage due to sustained overcurrent, mechanical overload, single-phasing, and prolonged starting conditions.
The NC stop button and the overload NC contact must be wired in series with the coil, not in parallel. Wiring them in parallel would mean neither could stop the motor independently.
All electrical work on motor control panels must be performed by a suitably qualified and licensed electrician in accordance with the local electrical installation code and workplace health and safety requirements.

Tools needed

Digital multimeter (voltage, continuity, resistance)
Non-contact voltage tester
Insulated screwdrivers (flat blade in multiple sizes for terminal screws)
Torque screwdriver or driver with settings matching contactor terminal torque specifications
Wire stripper and crimper
Ferrule crimping tool (for stranded wire in terminal blocks)
Cable loom and cable ties
DIN rail cutter

Common mistakes

Wiring the start button's NO contact in series instead of parallel with the seal-in auxiliary contact — the contactor will only energise while the button is held, dropping out the moment it is released.
Wiring the stop button as NO instead of NC — the coil will not receive power at all, and pressing the stop button will briefly close the circuit instead of opening it.
Setting the overload relay to the circuit breaker rating rather than the motor FLA, providing no meaningful protection against sustained motor overload.
Using a control wire gauge that is too small for the terminal block, causing the ferrule to pull out under vibration and intermittently dropping the coil circuit.
Omitting the electrical interlock between two contactors in a reversing or star-delta starter, allowing both contactors to energise simultaneously and creating a three-phase short circuit.
Connecting the overload relay reset incorrectly (e.g., auto-reset when manual-reset is required) so the motor automatically restarts after a thermal trip, potentially in a dangerous condition.

Troubleshooting

Contactor will not pick up when start button is pressed: Cause: No voltage on coil terminals (open circuit in the control chain), wrong coil voltage, or the NC contacts of the stop button or overload relay are open Fix: Measure voltage across the coil terminals (A1/A2) while pressing the start button. If no voltage: trace back through the control circuit with a voltmeter, checking across each series component (overload NC, stop NC, start NO). The component across which you measure the full control voltage is the open element. If voltage is present at the coil but contactor will not pick up, the coil is open-circuit — replace the contactor.
Contactor picks up when start is pressed but drops out immediately when released: Cause: Seal-in auxiliary contact is not wired in parallel with the start button, or the auxiliary contact (13/14) is open-circuit (failed or not making up) Fix: With the control supply on, press the start button and probe terminal 13 and 14 — they should show closed circuit (0 V across them) when the contactor is picked up. If they show open circuit, the auxiliary block contact is not closing — check for mechanical interference or a failed auxiliary block.
Motor trips the overload relay repeatedly: Cause: Overload set too low relative to motor FLA, motor drawing excess current due to mechanical overload, single-phasing (one phase lost), or ambient temperature causing bimetallic calibration drift Fix: Verify the overload setting matches the motor nameplate FLA exactly. Measure all three phase currents with a clamp meter — if one phase is significantly higher or one phase has no current (single-phasing), investigate the supply and cable connections. Check for mechanical issues on the driven load.

Frequently asked questions

What is the difference between a contactor and a relay?

Both use a coil to operate contacts, but contactors are rated for higher currents (from around 9 A to thousands of amperes), have main contacts rated for inductive loads like motors, and are physically constructed to handle the arcing that occurs when switching heavy currents. Relays are generally smaller, rated for lower currents, and used in control circuits.

What does a self-hold or seal-in circuit do in a contactor diagram?

A seal-in circuit uses the contactor's own normally open auxiliary contact (13/14) wired in parallel with the momentary start pushbutton. When the start button is pressed and the coil energises, the auxiliary contact closes and maintains the coil circuit after the button is released. The stop button (an NC contact in series) breaks this seal-in circuit to de-energise the coil.

Why is the overload relay's NC contact wired in series with the contactor coil?

The overload relay's NC contact is normally closed — it passes current and allows the coil to stay energised. If the overload relay trips due to overcurrent, its NC contact opens, interrupting the coil circuit and dropping out the contactor, cutting power to the motor. This is a fail-safe arrangement: any break in the coil circuit stops the motor.

What happens if the coil voltage does not match the supply?

An AC coil supplied with DC will draw continuous high current (AC coils rely on impedance, which requires the AC frequency; on DC, only resistance limits current) and burn out within minutes. A DC coil supplied with AC will likely fail immediately due to the wrong impedance characteristics. Always verify coil voltage rating before connecting.

How do I interlock two contactors to prevent simultaneous energisation?

Wire the NC auxiliary contact (21/22) of contactor K1 in series with the coil circuit of contactor K2, and vice versa. When K1 is energised, its NC auxiliary opens the coil circuit of K2 so K2 cannot energise simultaneously. This electrical interlock is essential in forward/reverse motor starters and star-delta starters.

What is the difference between a relay and a contactor in a wiring diagram?

A relay is a small electromechanical switch, typically rated up to 10–15 A, used to switch control-voltage signals or low-current loads; it usually has multiple sets of contacts (NO and NC) for logic functions. A contactor is a heavy-duty version designed for switching high-current motor and power circuits — ratings from 9 A to hundreds of amps — with main power contacts and auxiliary contacts handled separately. In a combined wiring diagram the relay often appears in the control circuit (e.g., timer output or PLC output) to pilot the contactor coil, which in turn switches the motor supply, keeping low-voltage logic circuits isolated from high-current power circuits.

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