DOL Starter Single Line Diagram

DOL Starter Single Line Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable dol starter single line diagram: download the diagram as SVG or open it and print to paper or PDF.

A DOL starter single line diagram represents a direct-on-line motor starting circuit in simplified single-line form, showing protection, switching, and control elements from supply to motor.

A direct-on-line (DOL) starter applies full supply voltage to the motor terminals at the moment of starting. It is the simplest, lowest-cost starting method and is the standard choice for motors up to 11 kW (some codes permit up to 15 kW) on networks where the utility or generator can tolerate the inrush current.

The single-line diagram (SLD) is a simplified representation that uses a single line to represent the three-phase (or single-phase) power conductors. It shows the sequence of protection and switching elements from the incoming supply to the motor without drawing each phase separately. This makes it easier to read and understand the protection philosophy of the circuit at a glance.

A typical DOL starter SLD, reading from top to bottom, shows: incoming supply bus, main isolation switch or moulded case circuit breaker (MCCB), the contactor (shown as a symbol of two contacts with a coil line), the thermal overload relay (shown as a rectangle or specific symbol), and then the motor (shown as a circle with 'M').

Above the contactor in the SLD, the short-circuit protection device (MCCB or HRC fuses) is shown. This device is sized to provide short-circuit protection only — it is not intended to provide overload protection, which is the role of the thermal overload relay downstream.

The control circuit is shown separately from the power circuit on a full schematic diagram but is summarised in the SLD by a dotted or dashed line between the contactor symbol and its coil, with the control voltage noted. Common control circuit elements — stop button, start button, holding contact, overload trip contact — are shown in the control circuit schematic alongside the SLD.

On the SLD, motor nameplate data is typically tabulated in a box next to the motor symbol: kW rating, supply voltage, full-load current (FLC), power factor, speed (rpm), insulation class, and IP rating. This data drives the selection of every protective and switching component in the starter.

How to wire dol starter single line diagram

Gather motor nameplate data Record all motor nameplate parameters: voltage, frequency, FLC (amps), kW, power factor, insulation class, and starting current if stated. These values drive the selection of every component in the starter and are tabulated on the SLD.
Select short-circuit protection Choose an MCCB or HRC fuses sized to provide short-circuit protection for the motor branch circuit. Per NEC 430.52, the maximum size is 250 % of FLC for inverse-time breakers or 175 % for dual-element time-delay fuses. Per IEC, size per the starter manufacturer's motor-contactor-overload coordination table for Type 1 or Type 2 coordination.
Select the contactor Choose a contactor rated for AC3 duty (squirrel-cage motor starting) at a current not less than the motor FLC. Confirm the contactor coil voltage matches the planned control circuit voltage.
Select the overload relay Choose a thermal or electronic overload relay with a setting range that covers the motor FLC. Set the relay to the motor nameplate FLC value after installation. Select Class 10 for standard loads or Class 20 for high-inertia loads.
Draw the power circuit SLD On the SLD, draw from top to bottom: supply bus symbol, MCCB symbol with rating annotation, contactor symbol (two lines with coil indicator), overload relay symbol, and motor symbol (circle with M). Annotate cable sizes and lengths between each element.
Draw the control circuit schematic alongside On a separate control circuit diagram (often to the right of the SLD or below it), show the control circuit: control fuse, stop NC button, overload trip NC contact, start NO button, contactor coil, and holding NO auxiliary contact in parallel with the start button. Note control circuit voltage.
Add motor data table and protective settings Add a data table adjacent to the motor symbol listing all nameplate values. Add a settings table listing the overload relay set current, MCCB trip setting, and cable sizes. This completes the documentation needed for installation, commissioning, and maintenance.

Specifications

Starting method	Direct-on-line (DOL) — full voltage applied at start
Motor inrush current (typical)	4–8 × FLC (locked-rotor current per motor class)
Overload relay class (standard loads)	Class 10 per IEC 60947-4-1
Maximum DOL motor size (indicative)	Up to 11–15 kW (network operator dependent; always confirm)
Short-circuit protection sizing (NEC 430.52)	Maximum 250 % of FLC for inverse-time breaker; 175 % for time-delay fuse
Contactor contact duty	AC3 — squirrel-cage motor starting and running
SLD diagram standard	IEC 60617 (graphical symbols for diagrams)
Applicable standards	IEC 60947-4-1, NEC/NFPA 70 Article 430, BS 7671, AS/NZS 3000

Safety warnings

Isolate and lock out the supply at the MCCB before working on any part of the starter or motor terminals. Verify all conductors, including the motor terminals (which may hold residual energy from the run capacitor), are de-energised before touching.
DOL starting generates inrush currents of 4–8 times FLC. This mechanical torque impulse can damage coupled mechanical equipment or injury nearby personnel if the machine starts unexpectedly. Always ensure mechanical coupling guards are in place before commissioning.
The MCCB in the DOL starter SLD is sized for short-circuit protection, not overload protection. Do not rely on the MCCB alone to protect the motor from overload. The thermal overload relay must be present, correctly set, and proven to trip before commissioning.
DOL starter installation for motors above 0.75 kW (1 HP) must comply with NEC/NFPA 70 Article 430, BS 7671, AS/NZS 3000, or IEC 60364. Work must be carried out by a qualified electrician and inspected per local authority requirements.
This single-line diagram is for illustrative and reference purposes only. Actual designs must account for specific motor data, cable runs, fault levels, and local code requirements.

Tools needed

True-RMS clamp-type ammeter (for measuring actual starting and running currents)
Calibrated voltmeter (CAT III, for verifying supply and control voltages)
Megohm tester (for insulation resistance testing of motor windings and cables)
Phase-rotation meter
Insulated screwdrivers and torque wrench for terminal connections
Lockout/tagout set

Common mistakes

Setting the overload relay to match the MCCB rating rather than the motor nameplate FLC — the motor receives no effective overload protection and will burn out.
Using the MCCB as the routine start/stop switch instead of the contactor, rapidly eroding the MCCB's contact life because it is not designed for repeated motor inrush switching.
Omitting the holding contact wiring, resulting in a pushbutton-hold-to-run circuit where the motor stops the moment the start button is released.
Not checking the motor's direction of rotation before coupling it to driven equipment — connecting three-phase supply without verifying phase rotation can drive the machine in reverse, damaging pumps, compressors, or conveyors.
Selecting a contactor with insufficient short-circuit withstand (Iq) for the available fault level at the installation point — always match the contactor's rated ultimate short-circuit current (Icu) to the calculated prospective short-circuit current.

Troubleshooting

Motor starts but overload relay trips within seconds under no load: Cause: Overload relay set too low, motor running on two phases (single-phasing due to blown fuse or open contactor contact), or motor winding fault causing excess current Fix: Measure current in all three phases with a clamp meter at the motor terminals. If one phase reads zero, a fuse has blown or a contactor contact has failed. If all three phases are present but current is above FLC, verify the overload relay set current matches the nameplate exactly. If current is high under no load, suspect a winding fault and have the motor tested.
Motor hums loudly and fails to accelerate (stalls) at starting: Cause: Motor receiving single-phase supply only (one phase missing), or mechanical load too heavy for direct-on-line starting torque Fix: Immediately stop the motor to prevent winding damage — a stalled motor draws locked-rotor current (LRC) continuously, which is 4–8 × FLC. Check all three-phase fuses and contactor contacts for continuity. Verify the mechanical load is within the motor's rated capacity. For high-inertia loads, consider soft-starter or star-delta starting.
MCCB trips immediately every time the start button is pressed: Cause: MCCB instantaneous trip setting too low for motor inrush, or a fault on the motor cable or winding Fix: Check the MCCB's instantaneous setting — for motor circuits it must be set to allow the starting inrush (typically 8 × FLC for short duration). Disconnect the motor cable and test insulation resistance on the cable and motor windings. If insulation is good, adjust the MCCB trip curve per NEC 430.52 or IEC coordination tables.

Frequently asked questions

What does 'direct-on-line' mean in motor starting?

Direct-on-line means the full supply voltage is applied to the motor terminals at the instant of starting, without any impedance, reduced voltage, or electronic control in the power path. This produces maximum starting torque but also generates a high inrush current — typically 4–8 times the motor's full-load current — lasting for the duration of the acceleration period.

What is the maximum motor size typically permitted for DOL starting?

Maximum permitted motor size for DOL starting varies by jurisdiction and network operator. IEC and many European networks typically permit DOL up to 4–11 kW depending on the utility agreement. Some industrial sites with strong networks permit higher. NEC does not specify a DOL size limit but supply authority requirements apply. Always confirm with the local network operator.

What is the difference between the MCCB/fuse and the overload relay on a DOL starter SLD?

The MCCB or fuse provides short-circuit protection — it interrupts very high fault currents quickly to protect wiring and equipment from damage. The thermal overload relay provides overload protection — it trips on sustained overcurrent of 110–150 % of the set current, protecting the motor windings from damage due to mechanical overload or a stalled rotor.

Why does a DOL starter SLD show the contactor separately from the MCCB?

The contactor and the MCCB serve different functions: the MCCB provides fault current interruption capacity and cannot safely make or break motor inrush current repeatedly. The contactor is designed for repeated making and breaking of motor starting current. Using the MCCB for routine switching would damage it rapidly.

What information is typically shown in the motor data table on a DOL starter SLD?

A motor data box on the SLD typically lists: motor kW or HP rating, supply voltage and frequency, full-load current (FLC) in amperes, power factor (cos φ), synchronous and/or full-load speed (rpm), insulation class (e.g. Class F), IP rating, and motor frame reference. This data is used to select and set every protective element in the starter.

Full written guides

Related diagrams

Free electrical calculators

Edit this diagram free in the online editor