Magnetic Contactor Wiring: Power Contacts, Coil Circuit, and Auxiliary Contact Diagrams

Magnetic Contactor Wiring: Power Contacts, Coil Circuit, and Auxiliary Contact Diagrams — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable magnetic contactor diagram: download the diagram as SVG or open it and print to paper or PDF.

Learn how to wire a magnetic contactor for motor control, including main power contacts, coil voltage selection, auxiliary contacts for self-latching and interlocking, and overload relay integration.

A magnetic contactor is an electromagnetically operated switch designed for repeated switching of electrical power circuits, particularly motor loads. Unlike a relay, which is designed for control-circuit current levels, a contactor is rated for high power — main contacts typically handle tens to hundreds of amperes at full line voltage. Contactors are the fundamental building block of Direct-On-Line (DOL) motor starters, star-delta starters, reversing starters, and changeover circuits.

A contactor consists of two electrically separate circuits: the main (power) circuit and the coil (control) circuit. The main circuit passes through three pairs of main contacts (for three-phase applications) designated L1/T1, L2/T2, and L3/T3 in IEC notation (or L1-T1, L2-T2, L3-T3). When the coil is energised, the electromagnet pulls in the armature, closing all three main contacts simultaneously and connecting the three-phase supply to the motor terminals.

The coil circuit operates at a much lower power level than the main circuit and can be designed for virtually any AC or DC control voltage (24 V DC, 24 V AC, 110 V AC, 230 V AC being the most common). The coil is connected between terminals labelled A1 and A2. The control voltage is always specified when ordering a contactor — it is not field-adjustable.

Auxiliary contacts are low-current contacts mechanically linked to the main contact armature. They switch simultaneously with the main contacts but are rated only for control-circuit currents (typically 6–10 A). Auxiliary contacts are designated using the IEC convention: the first digit indicates the contact position in the device (1 through 4 for the first four pairs), the second digit indicates the type: 1 for NC (normally closed) and 2 for NO (normally open) — so a contact designated '13/14' is the first auxiliary pair, normally open, and '21/22' is the second auxiliary pair, also normally open. NC contacts are designated '11/12' (first pair, normally closed).

Self-latching (holding circuit): an NO auxiliary contact wired in parallel with the start button ensures the contactor remains energised after the start button is released. This is the standard DOL starter circuit. The stop button (NC) breaks the coil circuit to de-energise the contactor.

Overload relay: always wire a thermal or electronic overload relay in series with the motor and contactor. The overload relay protects the motor from damage due to sustained overcurrent by interrupting the coil circuit through its NC contact when the thermal element trips.

How to wire magnetic contactor diagram

Select the contactor for the application Specify the contactor by AC utilisation category, rated operational current, and coil voltage. For standard squirrel-cage induction motors use utilisation category AC-3 (starting and switching motors during running). The contactor rated current must equal or exceed the motor full-load current. Coil voltage must match the available control supply voltage. Confirm the available auxiliary contact configuration suits the circuit requirements.
Mount the contactor and overload relay on the DIN rail Mount the contactor on a 35 mm DIN rail in the motor control centre or panel. Attach the thermal overload relay directly below the contactor — most manufacturers offer clip-on overload relays that mate mechanically with their contactor range. Mounting them together simplifies wiring (main contacts pass directly into overload relay input terminals) and allows the overload relay to trip a mechanical interlock as well as the electrical contact.
Wire the main power circuit Connect the three-phase supply lines to contactor input terminals L1, L2, and L3. Connect the output terminals T1, T2, and T3 to the overload relay input terminals. From the overload relay output terminals (T1, T2, T3 of the relay), connect to the motor terminal box (U, V, W). Ensure cable cross-section is rated for the motor full-load current. Apply correct cable ferrules on all stranded conductors.
Wire the coil circuit and start/stop buttons Connect one control voltage supply terminal to the stop button NC contact input. Wire the stop button NC output to the normally open start button input. Wire the start button NO output to coil terminal A1. Wire coil terminal A2 to the control voltage neutral or common. For AC coil contactors, A1/A2 orientation is arbitrary. For DC coil types, observe polarity.
Add the self-latching auxiliary contact Wire contactor auxiliary NO contact terminals 13 and 14 in parallel with the start button contacts. This means terminal 13 connects to the same point as one side of the start button, and terminal 14 connects to the same point as the other side. When the contactor energises, contact 13/14 closes and holds the coil energised independently of the start button.
Wire the overload relay NC contact into the coil circuit Connect the overload relay's NC trip contact in series with the coil circuit — typically between the stop button and A1, or between the control supply and the stop button. When the overload trips, its NC contact opens, interrupting the coil circuit and dropping out the contactor. The overload relay must be reset (manually or automatically per the relay setting) before the motor can restart.
Verify, test and commission Before energising the power circuit, verify all wiring against the schematic. Energise the control circuit only and test start/stop button operation — confirm contactor pulls in on start, holds in on release of start button, and drops out on stop. Energise the power circuit and verify correct motor rotation direction. Test the overload trip function by simulating an overload trip and confirming the contactor drops out.

Specifications

Standard IEC main contact designation	L1/T1, L2/T2, L3/T3 (three-phase)
Standard IEC coil terminals	A1 (coil supply), A2 (coil return/neutral)
Standard IEC auxiliary contact designation (NO)	13/14, 23/24, 33/34, 43/44 (odd number = input, even = output; last digit 3/4 = NO)
Standard IEC auxiliary contact designation (NC)	11/12, 21/22, 31/32 (last digit 1/2 = NC)
Motor utilisation category (standard squirrel cage)	AC-3 (IEC 60947-4-1)
Applicable standard	IEC 60947-4-1 (low-voltage switchgear — contactors and motor starters)
Overload relay coordination (with contactor)	Per IEC 60947-4-1 Type 2 coordination — no damage or permanent deformation under rated short-circuit condition

Safety warnings

Contactor and motor control wiring involves potentially lethal three-phase voltages. All installation, wiring, and commissioning work must be performed by a qualified electrician holding the appropriate licence under applicable regulations (IEC 60364, NEC/NFPA 70, BS 7671, AS/NZS 3000, or local equivalent).
Always isolate and verify dead — switch off and lock out the main supply before working on the contactor or motor circuit. A contactor can re-energise immediately if the power is not locked out. Verify with a calibrated phase-sequence tester that all three phases are dead before touching any conductors.
Never bypass or omit the overload relay. Running a motor starter circuit without overload protection allows a stalled or overloaded motor to draw locked-rotor current indefinitely, causing winding insulation failure, fire, and motor destruction.
In a reversing starter, the electrical interlock between the forward and reverse contactor coil circuits is mandatory. Without the interlock, energising both contactors simultaneously creates a three-phase short circuit with catastrophic results.
Short-circuit protection (fuses or MCCB) must be coordinated with the contactor and overload relay as a tested combination per IEC 60947-4-1. Using un-coordinated protection can result in contactor damage during a fault that leaves the circuit without protection.

Tools needed

Calibrated three-phase voltage tester and phase sequence meter
Digital multimeter
Insulated screwdrivers (VDE rated to 1000 V)
Crimping tool for bootlace ferrules
Torque screwdriver (for terminal torque)
Cable markers and marker pen
Lock-out/tag-out kit
Test lead probes with shrouded connectors

Common mistakes

Wiring the overload relay NC trip contact in parallel with (rather than in series with) the coil circuit — the overload relay trip then has no effect on the contactor and provides no motor protection.
Omitting the self-latching auxiliary contact parallel to the start button — the motor only runs while the start button is held down (jog mode rather than run mode).
Setting the overload relay current above the motor's full-load current to prevent nuisance tripping — this defeats the overload protection and exposes the motor to thermal damage.
Using the wrong auxiliary contact type — wiring an NC contact where an NO was required causes the contactor to drop out immediately on energisation (NC opens as contactor pulls in).
Failing to wire the electrical interlock in a reversing starter — the absence of interlocking means both forward and reverse contactors can be energised simultaneously, causing a three-phase short circuit.
Connecting control and power cables in the same conduit without maintaining segregation — induced voltages from the power circuit can cause false coil energisation or noise in control signals.

Troubleshooting

Contactor does not pull in when start button is pressed: Cause: No control voltage at coil, stop button NC contact open, overload relay in tripped state (NC contact open), or faulty coil. Fix: Measure voltage at coil terminals A1–A2 while pressing start. If voltage is present and contactor does not pull in, coil is faulty — replace contactor. If no voltage: check control supply fuse, measure stop button NC contact continuity, and check overload relay reset status.
Contactor pulls in but drops out immediately when start button is released: Cause: Self-latching auxiliary contact (13/14) is not wired or its connection is open. Fix: Verify terminal 13/14 wiring. Measure continuity across 13/14 while contactor is manually held in — if contact is open while the main contacts are closed, the auxiliary contact or its wiring is faulty. Re-make connection or replace auxiliary contact block.
Motor trips overload relay frequently at normal load: Cause: Overload relay current setting below motor full-load current, single phasing on the supply (causing remaining phases to draw higher current), or motor mechanically overloaded. Fix: Verify overload relay current setting matches the nameplate full-load current. Measure current on all three phases with a clamp meter — uneven phase currents indicate a supply or winding fault. Check mechanical load on the motor shaft.

Frequently asked questions

What is the difference between a contactor and a relay?

A contactor is designed for switching high-power loads — motor circuits, heaters, and power distribution — with main contacts rated for tens to hundreds of amperes. A relay is designed for control-circuit switching, typically under 10 A. Contactors also have a more robust arc-quenching mechanism for the arcing that occurs when interrupting high inductive (motor) loads.

What do A1 and A2 terminals on a contactor mean?

A1 and A2 are the coil terminals of the contactor — the connection points for the control voltage that energises the electromagnet. The coil voltage (e.g. 24 V AC, 110 V AC, 230 V AC) must match the contactor's rated coil voltage exactly. For AC coils, A1 and A2 are not polarity-sensitive. For DC coil contactors, observe the polarity marked on the device.

How does a self-latching (holding) circuit work with a contactor?

In a Direct-On-Line (DOL) starter, a normally open auxiliary contact (e.g. 13/14) is wired in parallel with the start button. When the start button is pressed, current flows through the coil and the contactor energises. The auxiliary contact simultaneously closes, providing an alternative current path that bypasses the start button. When the button is released, current continues through the auxiliary contact, keeping the contactor energised. The stop button (normally closed, in series with the coil circuit) breaks this path when pressed.

What is an overload relay and why is it always used with a contactor?

A thermal or electronic overload relay protects a motor from sustained overcurrent — typically caused by mechanical overload, loss of a phase, or stalled conditions. It is wired in series between the contactor main contacts and the motor terminals, sensing current flow. When the overload trips, its normally closed contact in the coil circuit opens, de-energising the contactor and disconnecting the motor. It is a mandatory protection device in any motor starter; a fuse alone is not sufficient for motor overload protection.

How do I wire a reversing contactor circuit?

A reversing starter uses two contactors (forward and reverse). The main contacts of the forward contactor connect the motor in one phase sequence (L1-T1, L2-T2, L3-T3). The reverse contactor swaps two phases to the motor (L1-T3, L2-T2, L3-T1), reversing rotation. An electrical interlock — an NC auxiliary contact of each contactor wired into the coil circuit of the other — prevents both contactors from energising simultaneously, which would cause a three-phase short circuit.

Related diagrams

Free electrical calculators

Edit this diagram free in the online editor