How to Read and Wire a Reversing Contactor Wiring Diagram for 3-Phase Motors

If you need to reverse the direction of a 3-phase AC motor, you must swap any two of its power leads. A reversing contactor wiring diagram achieves this safely using two electrically and mechanically interlocked contactors: one for forward rotation, one for reverse. If both contactors close simultaneously, you create a dead phase-to-phase short circuit, resulting in an arc flash and destroyed equipment. This guide traces the exact power and control paths, maps the physical terminals, and gives you a concrete bill of materials to build it right the first time.

The Core Decision: Sizing and Selecting Your Assembly

Before tracing wires, you must size the contactors to the motor's Full Load Amps (FLA) under AC-3 duty (squirrel-cage motor starting and switching off during run). Do not size based on locked-rotor amps; the overload relay handles that. Use the decision matrix below to pick your exact hardware.

Terminal Mapping and Diagram Symbol Translation

IEC wiring diagrams use specific alphanumeric codes for terminals. If you are looking at a NEMA diagram, the numbering will differ, but the physical logic remains identical. Here is how the symbols on your reversing contactor wiring diagram translate to the physical screw terminals on an IEC contactor like the TeSys D series.

Symbol Translation Note: In IEC schematics, a coil is drawn as a simple rectangle or circle with 'KM1' (Forward) or 'KM2' (Reverse) inside. Auxiliary contacts are drawn as two parallel lines with a diagonal slash (NO) or a solid bridge (NC), tagged with the contactor name and terminal numbers (e.g., KM1/13-14).

Node-by-Node Wiring Trace: Power and Control Circuits

Let's trace the circuit from the source to the load, assuming a 480V 3-phase power supply and a 120VAC control circuit derived from a step-down transformer. Always de-energize and lock out the main disconnect before terminating wires.

1. The Power Circuit (The Phase Swap)

1. Source to Disconnect: 480V 3-phase (L1, L2, L3) enters the main fused disconnect switch.
2. Disconnect to Contactors: Load side of the disconnect feeds the top terminals (L1, L2, L3) of both the Forward (Q1) and Reverse (Q2) contactors in parallel.
3. Forward Path (Q1): Bottom terminals T1, T2, T3 route straight through to the thermal overload relay (F1) inputs 1, 3, 5. Outputs 2, 4, 6 go directly to the motor terminals U, V, W.
4. Reverse Path (Q2 - The Swap): This is where the magic happens. Route Q2's T1 to overload input 1 (same as Forward). Route Q2's T3 to overload input 3. Route Q2's T2 to overload input 5. By swapping the physical wires on the T2 and T3 terminals of the reverse contactor, you reverse the phase sequence reaching the motor, flipping the rotating magnetic field.

2. The Control Circuit (Interlocks and Coils)

The control circuit operates at 120VAC. We use pushbuttons: S1 (Stop - NC), S2 (Forward - NO), S3 (Reverse - NO).

1. Control Hot: 120VAC leaves the control transformer secondary (X1) and hits the Stop pushbutton (S1) terminal 11.
2. Stop to Forward: S1 terminal 12 (output) wires to the Forward pushbutton (S2) terminal 13, and jumps to the Reverse pushbutton (S3) terminal 13.
3. Forward Coil Path: Pressing S2 sends 120V out of S2 terminal 14. This wire goes to the NC auxiliary contact of the Reverse contactor (Q2 terminals 21-22). Out of Q2 terminal 22, it hits the Forward coil (Q1 terminal A1). Q1 terminal A2 returns to the transformer neutral (X2).
4. Electrical Interlock Logic: If Q1 is energized, its own NC aux contact (Q1 21-22) opens. This breaks the path to the Reverse coil (Q2 A1). Even if an operator jams both S2 and S3 simultaneously, the Reverse coil cannot receive power.
5. Holding Circuit: A wire jumps from S2 terminal 14 to Q1's NO aux contact (13). When Q1 pulls in, contact 13-14 closes, sending 120V back to the coil A1, bypassing the S2 pushbutton so the motor stays running when you release the button.

Verification: Proving the Circuit with a Multimeter

Never trust your wiring blindly. Before applying 480V power, use a digital multimeter (like a Fluke 117 or 87V) set to resistance (Ohms) and continuity to verify the assembly. Ensure the main disconnect is LOCKED OUT.

Critical Safety and Interlock Rules

The most common failure mode in DIY reversing contactor builds is relying solely on electrical interlocks (the NC auxiliary contacts). If an auxiliary contact block vibrates loose, or if the contacts weld shut due to a high inrush current fault, the electrical interlock fails. If the operator then presses 'Reverse' while the motor is running forward, both contactors will close, resulting in a catastrophic phase-to-phase short.

You must use a mechanical interlock. On the Schneider TeSys D line, this is a physical plastic and metal block (like the LA9D09970) that mounts between the two contactors. It contains a physical seesaw lever that makes it mechanically impossible for both contactor armatures to be pulled in at the same time. The electrical interlock prevents the coils from energizing simultaneously; the mechanical interlock prevents the main power contacts from closing simultaneously if a mechanical failure occurs. According to Schneider Electric's TeSys assembly guidelines, combining both is mandatory for safe reversing duty.

Finally, always install the thermal overload relay (F1) on the load side of the contactors, and wire its NC auxiliary contact (95-96) in series with the Stop pushbutton in the control circuit. If the motor overloads and the bimetallic strips trip, the overload relay breaks the control circuit, dropping out whichever contactor is currently engaged. Set the overload dial exactly to the motor nameplate FLA, never higher.