DOL Starter Wiring Diagram: Direct-On-Line Motor Starter Explained

Direct-on-line (DOL) starting connects a three-phase motor directly across the supply at full voltage. It is the simplest motor starting method -- no reduced voltage, no autotransformer, no timing gymnastics. The motor sees full line voltage the moment the contactor closes, draws its full locked-rotor current (typically 6 to 8 times FLC), and accelerates to rated speed. For motors up to 7.5 kW (10 HP) on most utility supplies, DOL starting is the standard approach.

This guide covers the complete power circuit and control circuit of a DOL starter -- contactor sizing, overload relay setup, start/stop pushbutton wiring, and the holding (seal-in) contact that keeps the motor running after you release the start button.

How a DOL Starter Works

A DOL starter has two separate circuits that need to be understood independently before looking at the combined diagram.

Power circuit: Carries the full motor current. Three-phase supply (L1, L2, L3) flows through the main contactor and then through the overload relay thermal elements to the motor terminals (U, V, W).

Control circuit: Carries a small current -- typically 1 to 5A -- that operates the contactor coil. The control voltage can be the same as the supply (e.g., 400V across two phases) or stepped down via a control transformer to 110V or 24V for safety.

Key Components

• Main contactor (KM): The heavy-duty switch that connects the motor to the supply. Rated in amperes at AC3 duty (motor starting/stopping).
• Overload relay (F1 or OL): Thermal or electronic device wired in the power circuit. Its normally-closed (NC) contact sits in the control circuit and trips the contactor when motor current exceeds the set threshold.
• Stop button (S1): NC pushbutton in series with the control circuit. Pressing it breaks the circuit and de-energizes the contactor.
• Start button (S2): NO pushbutton that momentarily energizes the contactor coil.
• Holding (seal-in) contact: A NO auxiliary contact on the contactor, wired in parallel with S2. When the contactor energizes, this contact closes and keeps the coil energized after you release the start button.

Power Circuit Wiring

The power circuit is straightforward. Follow this connection sequence:

1. Three-phase supply enters through an isolator or main circuit breaker (MCCB) -- size this at 125% of motor FLC per NEC 430.52, or use a Type D MCB rated for motor starting.
2. Incoming phases L1, L2, L3 connect to the top (line) terminals of the main contactor KM.
3. Bottom (load) terminals of KM connect to the input terminals of the overload relay (T1, T2, T3 or 2, 4, 6 depending on manufacturer).
4. Output terminals of the overload relay connect to motor terminals U, V, W.
5. Motor frame must be bonded to earth (ground) at the motor terminal box.

Safety note: The power circuit carries full motor current at line voltage. Use cables sized to 125% of motor FLC. For a 5.5 kW motor at 400V (FLC approximately 11A), use 2.5 mm² copper minimum. Always isolate and verify dead before working on the power circuit.

Contactor Sizing

Contactors are rated by AC utilization category. Motor loads require AC3 rating (squirrel cage motors, starting and plugging). For a 5.5 kW / 400V motor with 11A FLC, a 16A AC3 contactor is the minimum -- common choices are Schneider LC1D09 (9A AC3) for smaller motors and LC1D18 (18A AC3) for this range.

Overload relay setting: Set the overload to 100% of motor FLC (not 125%). The overload relay is a slow-trip device designed to trip on prolonged overcurrent, not on normal starting inrush. Set it at nameplate amps.

Control Circuit Wiring

This is where the DOL starter gets interesting. The control circuit must latch (hold) after the start button is released, and it must unlatch when the stop button is pressed or the overload trips.

Schematic -- top to bottom

Control supply (L1 or 110V)
       |
    [S1 NC Stop]
       |
    [F1 NC Overload]
       |
    [S2 NO Start] --- parallel with [KM NO holding contact]
       |
    [KM Coil]
       |
   Neutral / L2

Wiring Step-by-Step

1. Take the control supply from phase L1 (or the secondary of a control transformer).
2. Run through the stop button S1 (NC terminal). The wire continues when S1 is not pressed.
3. Continue through the overload relay NC contact (terminals 95 and 96 on most Telemecanique/Schneider overloads, or 97-98 for manual reset).
4. Now split into two parallel paths:
   • The start button S2 (NO terminals) -- momentary path
   • The holding contact (auxiliary NO contact on KM) -- latching path
5. Both paths rejoin and feed the KM coil terminals A1 (+) and A2 (−/neutral).
6. Complete the circuit back to neutral or L2.

How the Seal-in Works

When you press S2, current flows through S2 and energizes the KM coil. KM pulls in and three things happen simultaneously:

• Main contacts close, connecting the motor to the supply.
• The auxiliary NO holding contact closes.
• The holding contact now carries the coil current, so releasing S2 does not break the circuit.

The contactor stays latched until the stop button breaks the series path, or the overload relay trips its NC contact open.

NO and NC Contact Assignments

Understanding which contacts are NO and NC is critical for correct wiring.

Indicator Lamps (Optional)

Many panels add indicator lamps to the control circuit:

• Green "Running" lamp: Connect in parallel with the KM coil (A1 to A2). Lights when motor runs.
• Red "Stopped/Tripped" lamp: Connect across the overload NC contact output so it lights when OL has tripped.
• Amber "Power On" lamp: Connect across the control supply.

Lamp voltage must match control voltage. Use LED panel lamps for long life -- 22 mm panel-mount types (e.g., Schneider XB5AV or equivalent).

Reversing Starter (Extension of DOL)

A reversing DOL starter uses two contactors -- Forward (KM1) and Reverse (KM2) -- wired so that two supply phases are swapped when the reverse contactor is energized (swapping any two of L1/L2/L3 reverses a three-phase motor).

Interlocking is mandatory: The NC auxiliary of KM1 goes in series with the KM2 coil, and vice versa. Energizing both simultaneously causes a three-phase short circuit.

Common DOL Starter Wiring Mistakes

• No holding contact: The motor runs only while the start button is held. This works but is not a latched starter -- add the auxiliary contact.
• Overload set too high: Setting the OL above motor FLC defeats its protection. Set it to nameplate amps.
• Wrong overload contact polarity: Using the NO contact of the overload (terminals 97-98 on manual reset) instead of the NC contact means the contactor never energizes.
• Control circuit tied to unprotected phase: If a phase is lost, the control circuit may still have voltage from another phase, masking the fault. Use a control transformer or phase-loss relay for sensitive applications.
• No earth continuity to motor frame: NEC 430.12 requires the motor frame to be bonded to the equipment grounding conductor.

Create Your Own DOL Starter Diagram

Laying out both the power circuit and control circuit before you start panel wiring prevents expensive rework. With CircuitDiagramMaker, you can:

• Draw the power circuit (contactor, overload relay, motor) and control circuit on separate layers
• Label every terminal with its actual terminal number (A1, A2, 95, 96, T1-T3)
• Add pushbutton symbols with correct NO/NC designation
• Wire the holding contact in parallel with the start button and verify the logic visually
• Export the completed diagram as a PDF for panel documentation

Create your own DOL starter diagram -- free

Key Takeaways

• A DOL starter connects the motor directly to full supply voltage via a contactor -- no reduced voltage, highest starting current.
• The power circuit (KM main contacts + overload relay) is separate from the control circuit (coil, pushbuttons).
• The seal-in (holding) contact is a NO auxiliary contact on the contactor, wired in parallel with the start button.
• The stop button is NC and the overload relay NC contact are both in series -- either one breaks the control circuit and drops the contactor.
• Set the overload relay to 100% of motor nameplate FLC, not 125%.
• Reversing DOL starters require mandatory electrical and mechanical interlocking between the forward and reverse contactors.
• Always size the main circuit breaker or fuse for motor starting inrush (NEC 430.52 or local equivalent).