Single-Phase Direct-On-Line (DOL) Starter Wiring Diagram
This is a free printable single phase dol starter diagram: download the diagram as SVG or open it and print to paper or PDF.
A complete reference for single-phase direct-on-line motor starter circuits covering contactor, overload relay, push-button control, and protective device wiring with safety compliance notes.
A direct-on-line (DOL) starter connects a motor directly to the supply voltage at the moment of starting, resulting in a full-voltage start with the highest possible starting torque and the highest possible starting current — typically 6–8 times the motor's full-load current. For single-phase motors, DOL starting is the simplest and most common method and is suitable for motors where the supply and local infrastructure can tolerate the start surge.
The power circuit of a single-phase DOL starter consists of the supply (Line and Neutral for single-phase, or two phases for a split-phase 230 V draw from a three-phase system), a main isolator or moulded-case circuit breaker (MCCB), a thermal or electronic overload relay, and the main contactor. The main contactor is a heavy-duty switching device controlled by its electromagnetic coil, which is itself switched by the control circuit.
The control circuit is typically wired at the same voltage as the supply (230 V AC for single-phase systems in most countries, or stepped down to a safe control voltage via a control transformer in some designs). The control circuit includes a start push button (normally open, momentary), a stop push button (normally closed, momentary), a hold-on auxiliary contact on the contactor (for latching), and the overload relay's normally closed (NC) contact in series with the coil circuit. When the stop button is pressed or the overload trips, the coil de-energises, the contactor opens, and power is removed from the motor.
The overload relay is set to a current value slightly above (typically 105–110% of) the motor's full-load current (FLC) as shown on the motor nameplate. Setting the overload too high removes protection; setting it too low causes nuisance tripping during normal start cycles.
For compliance, all single-phase motor starter installations must meet the requirements of the applicable wiring standard: IEC 60947-4-1 (contactors and motor starters), NEC Article 430 in North America, or the relevant clauses of BS 7671, AS/NZS 3000, or local equivalent. A qualified electrical engineer or licensed electrician must design and certify industrial motor starters.
How to wire single phase dol starter diagram
- Install the main isolator and short-circuit protection Mount a suitably rated isolator switch or MCCB as the first device downstream of the supply. The short-circuit interrupting capacity must exceed the prospective fault current at the installation point. This device must be accessible and lockable for safe isolation of the starter during maintenance.
- Wire the power circuit through the contactor Connect the Line and Neutral (or two-phase conductors) from the isolator through the main contacts of the contactor (L1/T1 and L2/T2 terminals), then through the overload relay, and finally to the motor terminals. Use cable sized for the motor's full-load current plus a service factor per the applicable standard.
- Wire the overload relay to the motor Connect the overload relay output terminals to the motor supply leads. Set the overload relay current adjustment to the motor nameplate full-load current. Confirm the relay's reset mode (manual or automatic) is appropriate — automatic reset may allow a motor to restart after a thermal trip without operator intervention, which is potentially hazardous.
- Wire the control circuit Wire the control circuit from Line (or control transformer secondary) through the normally closed overload relay contact (95/96), through the normally closed stop push button (SB1), through the normally open start push button (SB2) in parallel with the hold-on auxiliary contact (13/14 on the contactor), to the contactor coil (A1), then return to Neutral (or control transformer secondary return, A2).
- Connect the hold-on auxiliary contact Connect a wire from the junction between the stop button and start button to one terminal of the auxiliary contact (13) on the contactor. Connect the other terminal (14) to the junction between the start button and the coil terminal (A1). This ensures the contactor remains energised after the start button is released.
- Earth (ground) all metalwork Connect a continuous protective earth (PE) conductor from the main earth point through the enclosure metalwork, contactor frame, motor frame, and all exposed conductive parts. Earth conductor sizing must comply with the applicable standard. Test earth continuity with a multimeter before energising.
- Test starting and stopping sequences After inspection by a qualified person, energise the starter with no motor connected first. Operate the start button and verify the contactor closes. Operate the stop button and verify the contactor opens. Then connect the motor, verify correct direction of rotation, and measure running current against the nameplate FLC. Adjust overload relay if measured current differs significantly from nameplate.
Specifications
| Supply voltage (typical single-phase) | 230 V AC, 50 Hz (Europe/UK/Australia) or 120/240 V AC, 60 Hz (North America) |
|---|---|
| DOL starting current (typical) | 6–8 × motor full-load current (FLC) |
| Overload relay setting | 100–105% of motor nameplate FLC |
| Contactor utilisation category (motor loads) | AC3 (squirrel-cage induction motor starting) |
| Control circuit voltage (typical) | 230 V AC (same as supply) or stepped down to 24 V AC/DC via control transformer |
| Enclosure ingress protection (outdoor/wet areas) | Minimum IP54 per IEC 60529 |
| Applicable standard — motor starters | IEC 60947-4-1 / NEC Article 430 / BS 7671 / AS/NZS 3000 |
| Earth conductor sizing | Per Table 54.7 of BS 7671 or NEC 250.122 as applicable |
Safety warnings
- All single-phase motor starter installations must comply with IEC 60947-4-1, NEC Article 430, BS 7671, AS/NZS 3000, or the applicable local standard. Design and certification must be performed by a licensed electrical engineer or electrician.
- The main isolator must be in an accessible, lockable position so that the starter can be safely isolated before maintenance. Implement a lockout/tagout procedure whenever working on the starter or connected motor.
- Set the overload relay to the motor nameplate full-load current. An incorrectly set overload relay can allow the motor to run in an overloaded condition, causing overheating, insulation breakdown, and fire.
- Never use automatic reset on overload relays in applications where an unattended motor restart could cause injury or damage to connected plant. Manual reset should be the default for most applications.
- Ensure all enclosures, contactors, and control devices are rated for the installation environment. Motor starters in wet, dusty, or chemically aggressive locations require appropriate IP-rated enclosures (minimum IP54 for outdoor or wet areas).
Tools needed
- Digital multimeter with clamp function
- Insulation resistance tester (Megger)
- Torque screwdriver (for terminal tightening to manufacturer specification)
- Wire stripper and cable crimp tool
- Flatblade and Phillips screwdrivers
- DIN rail cutter (for mounting rail trimming)
- Phase rotation meter (if three-phase supply involved)
- Non-contact voltage tester
Common mistakes
- Wiring the hold-on auxiliary contact in parallel with the stop button instead of the start button — causes the motor to be unstoppable.
- Setting the overload relay significantly above the motor's nameplate FLC — removes overload protection and risks motor burnout.
- Using automatic overload reset in applications where the motor drives unattended machinery — allows automatic restart after a protective trip without operator awareness.
- Failing to check the contactor's AC3 utilisation category rating against the motor load — a contactor undersized for the starting duty will have a shortened contact life.
- Omitting earth continuity testing after installation — a broken earth path leaves motor and enclosure metalwork at supply potential in the event of an insulation fault.
- Wiring the control circuit from Neutral instead of from Line — creates a circuit where the stop button de-energises correctly but the components upstream remain live, misleading service personnel.
Troubleshooting
- Contactor closes momentarily then immediately opens
- Cause: Hold-on auxiliary contact not wired or open-circuit; or overload relay has tripped and NC contact is open Fix: Check that the hold-on auxiliary contact (13/14) is connected correctly in parallel with the start button. Check overload relay — if tripped, reset it and investigate the cause of the overload. Measure motor running current against nameplate FLC.
- Motor does not start and contactor does not close
- Cause: Open circuit in control circuit — broken stop button contact, overload trip, or broken wire Fix: With the isolator on and supply confirmed, measure voltage at each point in the control circuit in sequence: supply rail > overload relay NC contact (95/96) > stop button > start button > coil terminal (A1). The point where voltage disappears indicates the open circuit.
- Motor runs but trips overload frequently
- Cause: Overload relay set too low, motor mechanically overloaded, or motor supply voltage too low Fix: Measure running current with a clamp meter. If current matches nameplate FLC, verify overload relay setting. If current exceeds FLC, investigate mechanical load. If voltage at motor terminals is more than 10% below nameplate voltage, investigate supply and wiring.
- Motor starts but cannot be stopped with stop button
- Cause: Stop button contacts welded or bypassed, or hold-on contact wired across stop button instead of start button Fix: Isolate the circuit immediately using the main isolator — do not attempt to repair a live starter. Verify stop button contacts with a multimeter in continuity mode (must be normally closed). Verify hold-on contact is in parallel with start button, not stop button.
- Motor rotates in the wrong direction
- Cause: Supply phase connections to motor terminals transposed (relevant when drawing from two phases of a three-phase supply) Fix: Isolate and lock out the starter. Swap the connections on two of the motor supply terminals (or swap L and N on the supply input if operating from two phases of a three-phase supply). Restart and verify rotation.
Frequently asked questions
What is the purpose of the hold-on auxiliary contact in a DOL starter?
The hold-on contact (also called a sealing or latching contact) is a normally-open auxiliary contact on the main contactor that closes when the contactor energises. It bridges the start push button so the circuit remains energised after the operator releases the start button. Without it, the motor would only run for as long as the start button is held down.
Why does a DOL starter use a normally closed stop button and a normally open start button?
The normally closed (NC) stop button ensures that a wiring fault, broken wire, or disconnected stop button de-energises the contactor coil, stopping the motor — this is called fail-safe design. If the stop button were normally open, a broken wire would prevent the motor from being stopped. The normally open start button prevents the motor from starting automatically on power restoration after a supply interruption.
What is the difference between a thermal overload relay and an electronic overload relay?
A thermal overload relay uses a bimetal element heated by motor current. Overload current causes the element to deflect and trip a contact, removing power from the contactor coil. Response time is inversely proportional to overload magnitude and is affected by ambient temperature. An electronic overload relay measures current electronically and implements a thermal model in firmware, providing more accurate protection and often including phase-loss and earth-fault detection, independent of ambient temperature.
What protection does a DOL starter not provide?
A standard DOL starter with an overload relay does not protect against short circuits in the motor winding — that protection requires a separate fuse or circuit breaker upstream. It also does not provide earth-fault protection unless the overload relay includes this function. Some designs add a separate earth-leakage circuit breaker (ELCB/RCD) for earth fault protection.
How do I set the overload relay on a single-phase DOL starter?
Read the motor full-load current (FLC) from the motor nameplate in amperes for the supply voltage being used. Set the overload relay adjustment dial to this value. Most overload relays have a range of approximately 50–100% of their nominal rating range, adjustable via a calibrated dial. Do not set above the nameplate FLC unless advised by the motor manufacturer for a specific duty cycle.
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