A 3 phase stop start wiring diagram—commonly known as a Direct-On-Line (DOL) starter circuit—is the foundational control topology for industrial induction motors. It uses a contactor to switch high-current 3-phase power and a low-current control circuit with pushbuttons and an auxiliary holding contact to manage the logic. If you are wiring a standard 10 HP (7.5 kW), 400V/480V 3-phase motor today, the default, battle-tested recommendation is the Schneider Electric TeSys D LC1D18 contactor paired with an LRD21 thermal overload relay. This combination handles up to 18A at 400V, provides reliable latching logic, and integrates seamlessly on standard 35mm DIN rail.

⚠️ MAINS VOLTAGE WARNING: A 3-phase DOL circuit involves lethal line-to-line voltages (400V-480V AC). Before touching any terminal, de-energize the main disconnect, apply a Lockout/Tagout (LOTO) device, and verify the circuit is dead using a CAT III or CAT IV rated multimeter. Local electrical codes (like NEC Article 430 or IEC 60364) may require this work to be performed or inspected by a licensed electrician.

Decoding the 3 Phase Stop Start Wiring Diagram Symbols

Before tracing the wires, you need to read the schematic. Industrial diagrams follow IEC 60617 or NEMA ICS standards. Here is exactly what the symbols on your drawing represent in physical hardware:

  • QF (Circuit Breaker / MCCB): The main 3-phase disconnect and short-circuit protection. Physically, this is a 3-pole molded case breaker.
  • KM (Contactor): The electromagnetic switch that actually starts the motor. It contains main power contacts, an electromagnetic coil, and auxiliary logic contacts.
  • FR (Thermal Overload Relay): Protects the motor from drawing too much current over time (which melts windings). It features a bimetallic trip mechanism and a 95/96 normally-closed (NC) auxiliary contact.
  • SB1 (Stop Pushbutton): Red, Normally-Closed (NC). Breaking this circuit drops the contactor coil.
  • SB2 (Start Pushbutton): Green, Normally-Open (NO). Momentarily completing this circuit energizes the coil.
  • M (Motor): The 3-phase induction load (typically wired in Star or Delta configuration at the terminal box).

Terminal and Pin Mapping Table

The most common mistake on the bench is miswiring the auxiliary contacts or confusing the line/load sides of the overload relay. Use this spec-sheet mapping when terminating your physical devices. This mapping assumes a standard IEC-style device like the Schneider TeSys D or Eaton XTCE series.

Component Terminal IDs Function & State Wire Size (Typical)
Contactor Main L1, L2, L3 / T1, T2, T3 3-Phase Power In / Out (NO) 10 AWG / 4 mm²
Contactor Coil A1, A2 Electromagnet Energization 14 AWG / 1.5 mm²
Contactor Aux 13, 14 Holding/Latching Circuit (NO) 14 AWG / 1.5 mm²
Overload Main 1, 3, 5 / 2, 4, 6 Thermal Sensing Path (Series) 10 AWG / 4 mm²
Overload Aux 95, 96 Trip Signal to Control Circuit (NC) 14 AWG / 1.5 mm²
Stop Pushbutton 11, 12 Master Kill Switch (NC) 16 AWG / 1.0 mm²
Start Pushbutton 13, 14 Momentary Trigger (NO) 16 AWG / 1.0 mm²

Node-by-Node Trace: Source to Load

Abstract descriptions don't wire panels. Here is the exact textual trace of the electrons from the main supply through the power and control circuits. We assume a 400V 3-phase system with a 230V control circuit derived from L1 and Neutral.

The Power Circuit (High Current)

  1. Source: 3-Phase supply (L1, L2, L3) enters the main disconnect switch or MCCB (QF).
  2. Contactor In: From QF load side, three conductors land on the contactor main line terminals (KM L1, L2, L3).
  3. Contactor Out: When the coil energizes, power flows through to KM T1, T2, T3.
  4. Overload In: Wires route directly to the thermal overload relay line terminals (FR 1, 3, 5).
  5. Overload Out: Power passes through the bimetallic heaters and exits at FR 2, 4, 6.
  6. Motor Terminals: The three phases land on the motor terminal box studs (U, V, W).
⚡ The Ground Path (PE): The Protective Earth (PE) is completely independent of the switching logic. A continuous green/yellow conductor runs from the main panel PE busbar directly to the motor terminal box PE stud and the panel backplate. Never route the ground wire through the contactor or the overload relay. If a phase faults to the motor casing, the PE path must provide an unswitched, low-impedance route back to the source to trip the upstream breaker instantly.

The Control Circuit (Low Current Logic)

  1. Control Source: 230V AC is tapped from L1 (or a control transformer secondary) to the Stop pushbutton (SB1 terminal 11).
  2. Stop Logic: Exits SB1 at terminal 12 (since it is NC, it is always closed unless pressed).
  3. Start Logic: Enters the Start pushbutton (SB2 terminal 13). When pressed, it exits at SB2 terminal 14.
  4. The Latching Split: At SB2-14, the wire splits. One path goes to the contactor coil (KM A1). The second path goes to the contactor auxiliary NO contact (KM 13).
  5. The Hold: When the coil energizes, KM 13 closes to KM 14. KM 14 is wired in parallel back to KM A1. When you release the Start button, the auxiliary contact maintains power to A1. This is the 'latching' or 'holding' circuit.
  6. Coil Return & Overload Protection: The coil neutral return exits at KM A2 and routes to the Overload Relay NC auxiliary contact (FR 95). It exits at FR 96 and returns to the Neutral bar (or L2). If the motor overloads, FR trips, breaking the 95-96 path, dropping the coil, and shutting off the motor.

Sizing Decision Tree: Picking the Right Contactor and Overload

Do not guess your contactor size based on horsepower alone; motor efficiency and power factor dictate the actual Full Load Amps (FLA). Read the motor nameplate FLA, then use this decision matrix to select your exact part numbers. We default to the Schneider TeSys D line for its global availability and integrated auxiliary contacts.

Motor Nameplate FLA Contactor Pick (400V AC-3) Overload Relay Pick (Adjustable Range) Breaker Sizing (Type D Curve)
6A to 9A LC1D09 LRD14 (7-10A) 16A MCCB
12A to 16A LC1D18 (Default Pick) LRD21 (12-18A) 25A MCCB
24A to 32A LC1D38 LRD35 (30-40A) 50A MCCB
> 50A Stop using DOL. Move to a Soft Starter (e.g., ATS22) or VFD to manage inrush current and mechanical shock.

The Concrete Default: If you are building a general-purpose bench test rig or wiring a standard 10HP compressor, buy the LC1D18 and LRD21. Set the LRD21 dial to the exact FLA printed on your motor nameplate (e.g., 14.5A). This provides precise thermal protection without nuisance tripping during the 6x inrush current spike of a standard DOL start.

Verifying Connections with a Multimeter

Before you throw the main breaker, you must verify your wiring. A miswired control circuit will result in a motor that won't latch, or worse, a dead short. Grab a reliable meter like a Fluke 87V or 117 and follow this sequence with the power LOCKED OUT.

  1. Verify the Stop Button (Continuity): Set meter to continuity (beep mode). Place probes on SB1-11 and SB1-12. It should beep. Press the button; the beep must stop.
  2. Verify the Start Button (Continuity): Probes on SB2-13 and SB2-14. No beep. Press the button; it must beep.
  3. Verify the Latching Path: Place one probe on KM-14 and the other on KM-A1. You should read continuity (near 0 ohms). This confirms your hold circuit is physically tied to the coil.
  4. Verify the Coil (Resistance): Set meter to Ohms (Ω). Measure across KM-A1 and KM-A2. A healthy 230V AC coil will typically read between 15Ω and 50Ω. If it reads OL (open), the coil is blown. If it reads 0.1Ω, it is internally shorted.
  5. Verify the Overload Trip Path: Measure across FR-95 and FR-96. It should read continuity (NC state). Press the manual 'Test/Trip' button on the overload relay with a small screwdriver; the continuity must break.
  6. Verify the Ground Path (Critical): Set meter to low-ohms. Place one probe on the motor casing (bare metal) and the other on the main panel PE busbar. The reading must be less than 1.0 ohm. If it is higher, your ground bond is compromised and the motor frame could become energized during a fault.

Once these six checks pass, remove your LOTO, clear the workbench, and energize the main breaker. Press Start. The contactor should pull in with a definitive 'clack', the motor should spin up to speed within 2-3 seconds, and releasing the Start button should keep it running. Press Stop to verify the drop-out. If it fails to latch, your auxiliary contact (13/14) is either miswired or the auxiliary block is physically loose on the contactor side-mount.