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:
- 24VAC or 24VDC -- common in modern PLC-controlled systems
- 120VAC -- standard North American control circuits
- 230VAC or 240VAC -- European and UK industrial practice
- 110VAC -- older industrial controls
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:
- AC coils: RC snubber (e.g., 100Ω + 47nF in series across A1/A2)
- DC coils: Freewheeling diode across A1/A2, cathode toward A1 (positive terminal)
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:
- 13/14 -- normally open (NO), closes when the contactor energizes
- 21/22 -- normally closed (NC), opens when the contactor energizes
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
- Three-phase supply L1, L2, L3 to contactor line terminals L1, L2, L3.
- 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).
- Overload relay output terminals (2T, 4T, 6T) to motor terminals U, V, W.
- 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:
- L1 (hot leg) to Stop pushbutton (NC) terminal.
- Stop pushbutton second terminal to Start pushbutton (NO) terminal.
- Start pushbutton second terminal to A1 (coil terminal) AND to contactor auxiliary NO contact terminal 13.
- Contactor auxiliary NO contact terminal 14 connects back to the junction between Stop and Start -- this is the seal-in connection.
- A2 (coil return) connects through the overload relay NC contact to L2 (neutral or second leg).
- 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:
- Press Start: current flows L1 → Stop(NC) → Start(NO) → A1/A2 → L2. Contactor closes.
- Release Start: contactor auxiliary 13/14 provides an alternate path, maintaining the coil circuit.
- Press Stop: circuit interrupts at the NC stop contact. Coil drops out. Contactor opens.
- Motor overloads: NC overload contact 95/96 opens. Coil drops out. Reset manually after cooling.
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:
- Mechanical interlock: A lever between the two contactors physically prevents both from closing. Available as an accessory for most contactor families (e.g., Schneider LAD9R10, ABB CAL5).
- Electrical interlock: Each contactor's NC auxiliary (21/22) is wired in series with the other's A1 coil terminal. If KM1 is energized, its NC contact 21/22 opens, breaking the circuit to KM2's coil and preventing KM2 from closing, regardless of any control signal.
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:
- 9A / 4kW at 400V (e.g., Schneider LC1D09, ABB A9)
- 18A / 7.5kW at 400V
- 25A / 11kW at 400V
- 32A / 15kW at 400V
- 40A / 18.5kW at 400V
- 65A / 30kW at 400V
- 95A / 45kW at 400V
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.
Testing a Contactor with a Multimeter
Before troubleshooting a contactor wiring problem or reinstalling a suspect unit, verify it with power removed and locked out.
- Isolate and verify zero energy. Lock out/tag out the upstream disconnect, then confirm no voltage at L1/L2/L3 and T1/T2/T3 with a calibrated tester.
- Check coil resistance. Set the multimeter to resistance (ohms) and measure across A1 and A2. A healthy coil reads a stable, non-zero resistance -- check the datasheet for the expected range. An open circuit (OL or infinite reading) means the coil is burned out; a reading near 0 ohms indicates a shorted coil.
- Check main contact continuity (de-energized). With the contactor de-energized, measure continuity between each line and load terminal pair (L1-T1, L2-T2, L3-T3). Each should show an open circuit -- the contacts are open when de-energized.
- Manually actuate and recheck continuity. Many contactors have a manual push-to-test button on the armature. Press it to close the contacts by hand and re-measure L1-T1, L2-T2, L3-T3 -- each should now show continuity near 0 ohms.
- Test the NO auxiliary contact (13/14). With the contactor de-energized, measure continuity across 13/14 -- it should be open. Actuate the armature and recheck -- it should close.
- Test the NC auxiliary contact (21/22). With the contactor de-energized, measure continuity across 21/22 -- it should show continuity (closed). Actuate the armature and recheck -- it should open.
Contactor Troubleshooting
| Symptom | Likely Cause | Fix |
|---|---|---|
| Contactor chatters (rapid open/close buzzing) | Low or fluctuating coil voltage, a worn shading coil on the armature, or loose control wiring | Verify coil voltage matches the rating under load; inspect or replace the shading ring; tighten control terminals |
| Coil hums but the armature does not pull in | Mechanical binding, low coil voltage, or a broken/worn shading coil | Check for mechanical obstruction, verify voltage at A1/A2 under load, replace the coil or contactor if the shading coil is damaged |
| Contacts welded closed | Repeated inrush/overcurrent, an undersized contactor for the load, or excessive short-circuit current through the contacts | Replace with a correctly sized contactor; verify upstream short-circuit protection is coordinated correctly |
| Motor does not start, coil energizes normally | Worn or pitted main contacts, or an open overload relay contact | Inspect and clean or replace main contacts; check the overload relay for a tripped condition and reset it |
| Contactor drops out under load | Coil voltage sagging under load, a loose A1/A2 connection, or overload relay nuisance tripping | Check voltage drop on the control circuit under load; tighten coil terminals; verify the overload relay is sized to the motor FLC |
| Auxiliary contact does not switch a pilot light or PLC input | Auxiliary contact block not fully seated, or wired to the wrong NO/NC terminal pair | Reseat the auxiliary block; confirm 13/14 (NO) vs 21/22 (NC) wiring matches the intended logic |
NEC Considerations for Contactor Circuits
NEC Article 430 governs motors, motor circuits, and controllers, including the contactors that switch them.
- Overload protection (430.32): Size the overload relay at no more than 125% of the motor's nameplate full-load current for motors with a marked service factor of 1.15 or higher (or a marked temperature rise of 40°C or less); use 115% of FLC for other motors.
- Disconnecting means (430.102(B)): A disconnecting means must be located within sight of the controller (the contactor), or be capable of being locked in the open position, so the controller can be serviced with confidence that power is removed.
- Controller rating: The contactor must be rated for the motor's horsepower and voltage at the applicable duty (AC-3 for standard motor starting) per NEC 430.
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.
Create Your Own Contactor Wiring Diagram
- Place a three-phase bus symbol at the top and run L1/L2/L3 down to the contactor line terminals
- Add the overload relay below T1/T2/T3 and connect output terminals to the motor symbol
- Draw the control rung: L1 (hot) → Stop NC → Start NO → coil A1; connect auxiliary 13/14 seal-in across the Start button
- Wire the overload 95/96 NC contact in series with coil A2 return to the neutral
- Label every terminal reference: A1, A2, L1-L3, T1-T3, 13/14, 21/22, 95/96
Create your own contactor wiring diagram -- free
Key Takeaways
- A1 and A2 are the coil terminals -- coil voltage must match the control circuit exactly.
- L1/L2/L3 are line-side inputs; T1/T2/T3 are load-side outputs. Power flows source-to-motor through these six terminals.
- Terminal 13/14 is the NO auxiliary; 21/22 is the NC auxiliary. IEC numbering is consistent across most manufacturers.
- The seal-in contact (NO auxiliary 13/14 across the Start button) maintains the coil circuit after the start button is released.
- Overload relay NC contact (95/96) breaks the coil circuit on overtemperature -- wire it in series with the coil return (A2 side).
- Always include coil suppression -- RC snubber for AC coils, freewheeling diode for DC coils.
- For reversing or changeover duty, use both mechanical and electrical interlocking between the two contactors.
Frequently asked questions
What happens if a contactor coil voltage does not match the control circuit?
A coil rated below the applied voltage (for example a 24V coil on a 120V circuit) will burn out quickly, often within seconds. A coil rated above the applied voltage (a 120V coil on a 24V circuit) simply will not generate enough magnetic pull to close the contacts, so the contactor never energizes. Always match coil voltage to the control circuit exactly.
Which wire goes on A1 and which goes on A2 of a contactor coil?
For most AC coils, A1 and A2 are not polarity-sensitive -- either control conductor can land on either terminal. DC coils are typically polarized, so check the datasheet; if a freewheeling suppression diode is fitted, its orientation depends on which terminal is positive.
Can I use a contactor in place of a relay for a motor circuit?
Contactors are the correct choice for motor and other power loads, not relays. Contactors are built with arc chambers and heavier contacts rated for repetitive switching of higher currents, while relays are rated for control-level currents only and will wear out quickly or fail if used to switch motor loads directly.
What size contactor do I need for a given motor?
Select a contactor with an AC-3 duty rating at or above the motor's full-load current (FLC) in amps, not just its horsepower rating, and confirm the voltage rating matches your supply. Manufacturer selection tables cross-reference horsepower, voltage, and FLC to the correct frame size.
Is it safe to work on a contactor with the coil de-energized?
No, not unless the upstream disconnect is also open and locked out. De-energizing the coil only opens the main contacts -- the line-side L1/L2/L3 terminals remain fully live as long as the upstream supply is on. Always lock out/tag out the disconnect and verify zero voltage before touching any terminal.
Why does a contactor need both mechanical and electrical interlocking in a reversing circuit?
Mechanical interlock uses a physical lever to prevent both contactors from closing at once, but the mechanism can wear or fail over time. Electrical interlock wires each contactor's NC auxiliary contact into the other's coil circuit as a backup. Using both means a single point of failure in either system still prevents a phase-to-phase short from both contactors closing together.