What size motor can a DOL starter be used with?

In practice, DOL starting is used for motors up to approximately 5.5 kW on a standard 400 V three-phase supply, though some installations allow up to 7.5 kW where the supply impedance is low and the DNO permits the starting current. Above this rating, the starting inrush (typically 6–8 times FLC) causes voltage dips that affect other equipment on the same supply. The network operator or supply authority may impose lower limits — always check before installing DOL above 4 kW.

What is the starting current reduction achieved by a star-delta starter?

A star-delta starter reduces starting current to approximately one-third (33%) of the DOL starting current. Starting torque is also reduced to one-third, so star-delta is only suitable for loads that can start unloaded or under light load. The transition from star to delta must occur when the motor has reached approximately 70–80% of its running speed; a timer relay is used to control this transition, typically set between 5 and 15 seconds depending on the motor and load.

When should a VFD be specified instead of a star-delta starter?

A VFD should be specified when you need variable speed control (pumps, fans, conveyors), when starting torque must be controlled precisely, when the load cannot tolerate the torque step at star-delta transition, when energy savings from speed reduction are required (the affinity laws mean halving fan speed cuts power consumption by approximately 87.5%), or when the supply is sensitive to starting current inrush. VFDs also provide motor protection functions including overcurrent, overvoltage, and phase loss detection.

What overload protection is required for motor circuits?

BS 7671 Regulation 552.1 requires overload protection for motor circuits unless the motor is continuously supervised or the supply authority permits omission. In practice, all motor starters include an overload relay set to the motor full load current (FLC) as stated on the motor nameplate. Bi-metallic overload relays are standard in DOL and star-delta starters. Electronic motor protection relays (MPRs) are used in more critical applications, providing phase loss detection, stall protection, and thermistor inputs for the motor winding temperature sensor.

What cable type and size should be used for motor circuits?

Motor circuits typically use multicore armoured cable (SWA) to BS 5467 or BS 6724 for fixed installation, or flexible conduit to the motor terminal box. Cable size is selected based on the motor FLC with a diversity factor of 1.0 (no diversity for motors). The cable must also satisfy volt drop limits (typically 4% under running conditions for motor circuits) and withstand the starting current thermally. Aluminium wire armour provides both mechanical protection and an earth path; a separate earth conductor should be run if the armour continuity cannot be guaranteed.

Do motor starters need to be installed in a specific enclosure?

Motor starters must be housed in enclosures with an IP rating appropriate to the environment — IP54 minimum for dusty or damp industrial locations, IP65 or higher for washdown areas. The enclosure must provide mechanical protection, be lockable for safe isolation, and allow safe access for maintenance. All enclosures must be clearly labelled with the motor circuit designation, voltage, and maximum current. Where multiple starters are installed in a panel, busbars and internal wiring must comply with BS EN 61439 (Low-voltage switchgear and controlgear assemblies).

What documentation is required after motor starter installation?

At minimum, you must provide: circuit diagrams (power and control), cable schedules, device settings (overload relay setting, timer relay setting for star-delta, VFD parameter list), test and commissioning records, and an Electrical Installation Certificate (EIC) for the new circuit. For VFDs, the parameter list should be recorded and a copy stored both with the panel and in the asset management system. Operation and maintenance instructions for the starter manufacturer should be left with the end user.

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Motor Starter Installation UK: DOL, Star-Delta & VFD Guide

Everything UK electricians need to know about motor starter installation — DOL starters for small motors, star-delta for medium motors, VFDs for speed control, overload protection, forward/reverse control circuits, and commissioning documentation.

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14 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1DOL starters are suitable for motors up to 5.5 kW (some up to 7.5 kW) and draw 6–8 times full load current on starting — only acceptable where the supply can handle the inrush.
2Star-delta starters reduce starting current to approximately one-third of DOL, making them the standard choice for motors from 7.5 kW to 45 kW in industrial premises.
3Variable Frequency Drives (VFDs) provide fully controlled acceleration, speed control, and the lowest starting current — essential for pumps, fans, compressors, and conveyor systems.
4All motor circuits require overload protection set to the motor full load current (FLC) — typically a bi-metallic overload relay or electronic motor protection relay.
5Control circuits (24 VDC or 110 VAC SELV) must be segregated from power circuits, and all conductors must be labelled in accordance with BS 7671 and the installation drawings.
6Forward/reverse contactors must be mechanically and electrically interlocked to prevent simultaneous energisation, which would cause a phase-to-phase short circuit.

01 · Industrial Guide

Types of Motor Starters

Motor starters control the starting, stopping, and protection of electric motors. Choosing the correct starter type is essential — an undersized or incorrectly specified starter damages equipment, trips protection devices, and can cause supply voltage dips that affect neighbouring plant. The three principal starter types used in UK industrial installations are direct on-line (DOL), star-delta, and variable frequency drives (VFDs). Each has distinct characteristics, cost profiles, and application ranges.

DOL (Direct On-Line) — simplest and lowest cost. Motor connected directly to supply voltage. Starting current 6–8 × FLC. Suitable for motors up to approximately 5.5 kW where supply can absorb the inrush.
Star-Delta — motor windings connected in star during starting (reduces voltage per winding to 58%), then switched to delta for running. Reduces starting current to ~33% of DOL. Suitable for motors 7.5 kW to 45 kW.
VFD / Inverter Drive — converts AC to DC then back to variable frequency AC. Fully controlled acceleration ramp, speed control, and lowest starting current. Suitable for all motor sizes where speed control or soft-start is required.

Soft starters (electronic reduced voltage starters) are a fourth option for applications where speed control is not required but a smooth torque ramp is needed, particularly for conveyors and compressors. They reduce the voltage electronically during starting but run at fixed speed.

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02 · Industrial Guide

Direct On-Line (DOL) Starters

A DOL starter consists of a main contactor, an overload relay, and a control transformer or 24 VDC power supply. On pressing the start button, the control circuit energises the contactor coil, closing the main contacts and connecting the motor directly to the three-phase supply. The overload relay monitors current continuously; if current exceeds the set point for longer than the trip time characteristic, the relay opens its contact in the control circuit, de-energising the contactor.

Main contactor selection — rated by AC utilisation category. AC-3 (squirrel cage motors) is standard for DOL applications. The contactor must be rated for the motor kW rating and the associated making and breaking duty.
Overload relay setting — set to motor nameplate FLC (full load current in amps). Allow for Class 10 trip class for standard motors, Class 20 for high-inertia loads. The dial on bi-metallic relays is marked in amps; set precisely.
Short-circuit protection — a motor circuit breaker (MCB Type D or MCCB) upstream of the contactor provides short-circuit protection. Motor circuit breakers combine the MCB and overload relay function in a single device.
Control circuit — typically 24 VDC from a PELV power supply, or 110 VAC SELV from a control transformer. Start/stop pushbuttons, pilot lamps, and auxiliary contacts for interlocking wired in the control circuit.

03 · Industrial Guide

Star-Delta Starters

Star-delta starting is the most widely used reduced-voltage starting method for motors between 7.5 kW and 45 kW in UK industrial premises. The motor must have six terminals accessible (both ends of each winding) — motors wound for star-only connection cannot be used with star-delta starters.

Three contactors required — main contactor (KM1), star contactor (KM3, closed during starting), and delta contactor (KM2, closed during running). Mechanical interlock between star and delta contactors is mandatory.
Timer relay — an off-delay timer relay (typically 5–15 seconds) controls the star-to-delta transition. Setting too short causes excessive current spike at transition; setting too long causes the motor to run in star (drawing high current at load) before switching to delta.
Transition current spike — the open-circuit transition from star to delta causes a current transient that can exceed DOL starting current. Closed transition star-delta (with a resistor bank during switching) eliminates this but is more complex and expensive.
Overload relay position — fitted in the delta circuit (between KM2 contacts and motor terminals), not in the main line. This means the overload relay sees delta current (57.7% of line current). Set the overload relay dial to 58% of motor FLC, or use a relay with star-delta compensation.

Star-delta starters are not suitable for loads that require significant torque during starting (e.g., loaded conveyors, positive displacement pumps) because starting torque is reduced to one-third of DOL torque. For high-inertia or high-torque starting requirements, specify a VFD or soft starter.

04 · Industrial Guide

VFD / Inverter Drives

Variable Frequency Drives (VFDs), also called inverter drives or variable speed drives (VSDs), are now the preferred solution for motor control in most new industrial installations. They convert the incoming 50 Hz AC supply to DC (rectifier stage), then synthesise a variable frequency, variable voltage AC output (inverter stage) using pulse-width modulation (PWM).

Starting current — typically 100–150% of motor FLC during acceleration, compared to 600–800% for DOL. Acceleration ramp time is adjustable (typically 0–60 seconds) to match the load inertia and torque requirements.
Energy savings — for centrifugal loads (pumps, fans), reducing speed by 20% reduces power consumption by approximately 49% (affinity laws). VFDs pay for themselves rapidly in systems that run below full speed for significant periods.
Harmonic distortion — VFDs generate harmonic currents (primarily 5th and 7th harmonics) that can cause overheating in transformers and cables, and interfere with other equipment. Mitigate with line reactors, DC link reactors, or active front-end drives in large installations.
Motor cable length — PWM switching causes voltage reflections on long cables. Use screened motor cable and install an output reactor or dV/dt filter if cable length exceeds the VFD manufacturer's recommendation (typically 50–100 m without filtering).
Earthing the VFD — the VFD screen (EMC gland) must be bonded to the panel earth bar at both ends. The motor cable screen must be bonded at the VFD and at the motor terminal box. This is essential for EMC compliance and for protection against bearing currents in larger motors.

05 · Industrial Guide

Wiring and Control Circuits

Motor control panels are complex electrical assemblies that must comply with both BS 7671 (wiring of the fixed installation) and BS EN 61439 (the LV switchgear and controlgear assembly standard). Control circuit wiring requires particular care to ensure safe operation and to facilitate fault-finding.

Power and control segregation — run power conductors (400 V, motor leads) and control conductors (24 VDC or 110 VAC) in separate cable trays or conduit runs, physically separated by at least 50 mm or a metallic barrier. This prevents induced noise on control circuits.
Conductor identification — all conductors in the panel must be identified at every termination with ferrules marked to match the circuit diagram. Use the IEC colour code: brown/black/grey for three-phase, blue for neutral, green and yellow for earth. Control circuit conductors commonly use black or grey with numbered ferrules.
DIN rail layout — arrange devices logically from top to bottom: incoming isolator, MCBs/MCCBs, contactors, overload relays, control power supply, control terminal strip. Allow at least 25 mm clearance between device rows and provide sufficient cable duct capacity for the conductors.
Terminal blocks — use DIN rail-mounted terminal blocks for all external connections. Label terminal blocks with the circuit diagram reference. Provide separate terminal blocks for power (PE, L1, L2, L3, N) and control (common, start, stop, run indication, fault indication).

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06 · Industrial Guide

Overload Protection Requirements

BS 7671 Regulation 552.1 requires that motors are protected against overload unless overload cannot occur (e.g., the motor is protected by its load characteristic) or where overload protection could cause a greater hazard than the overload itself (e.g., a fire pump that must run until it fails). In virtually all industrial installations, overload relays are required.

Bi-metallic overload relays — the standard solution for DOL and star-delta starters. Current through bi-metallic strips heats and deflects them, tripping the relay after a time that depends on the degree of overload. Set dial to motor nameplate FLC. Always test operation during commissioning.
Electronic motor protection relays — microprocessor-based relays that measure true RMS current on all three phases. Provide phase loss detection (single phasing protection), phase imbalance alarm, motor thermistor input (PTC), stall detection, and ground fault detection. Essential for pumps and compressors in unmanned locations.
Thermistor protection — motors in arduous duty (high ambient temperature, frequent starting) should be fitted with PTC thermistors in the stator windings. Connect the thermistor output to the motor protection relay for direct winding temperature monitoring independent of current measurement.
Resetting — after an overload trip, investigate and rectify the cause before resetting. Never manually force a contactor closed or bypass the overload relay. Repeated tripping without investigation leads to motor winding failure.

07 · Industrial Guide

Forward/Reverse Control Circuits

Many industrial applications require motors to run in both directions — lathes, hoists, conveyors, and valve actuators being common examples. Forward/reverse control requires two contactors wired to swap two of the three supply phases (swapping any two phases reverses a three-phase induction motor). The critical safety requirement is that both contactors must never be energised simultaneously.

Mechanical interlock — a physical bar or lever mechanism that prevents both contactors from closing simultaneously. This is a hard-wired mechanical safety measure, not dependent on the control circuit logic. Always use mechanically interlocked contactor pairs for forward/reverse.
Electrical interlock — auxiliary normally-closed (NC) contacts from each contactor wired in series with the opposing contactor coil circuit. If the forward contactor is energised, its NC auxiliary contact opens the reverse contactor circuit, providing a second layer of protection.
Stop before direction change — the control logic should require the motor to be stopped (and ideally allow time to decelerate) before selecting the opposite direction. Plugging (reversing a running motor) causes very high current and mechanical stress and should be avoided unless specifically designed for.
Limit switches — travel limit switches (position limit switches) should be wired to de-energise the relevant contactor when the mechanical limit of travel is reached, preventing damage to machinery. Use NC contacts wired in series with the contactor coil.

08 · Industrial Guide

Typical Applications by Starter Type

Correct starter selection depends not only on motor size but also on the load characteristics, duty cycle, and whether speed control or soft starting is required. The following guidance covers the most common UK industrial applications.

DOL starters — small workshop machinery (bench drills, grinders, lathes), small air compressors (up to 4 kW), roller shutter doors, small conveyor drives, exhaust fans, and sump pumps where motor size is within DOL limits.
Star-delta starters — medium air compressors (7.5–45 kW), large fans started unloaded, centrifugal pumps started against a closed valve, machine tools with large spindle motors, and industrial refrigeration compressors.
VFD drives — HVAC supply and extract fans (all sizes), chilled water pumps, cooling tower fans, variable-pressure hydraulic systems, conveyor systems requiring speed matching, extruder drives, mixers, and centrifuges.
Soft starters — loaded conveyor belts where VFD speed control is not required, large air compressors on fixed-speed systems, applications where smooth torque ramp is needed but speed variation is not required, and situations where VFD harmonics are a concern on isolated generators.

When specifying motor starters for new installations, document the load type, required starting torque, duty cycle (continuous, intermittent, or periodic), and ambient temperature. This information is needed to select the correct contactor AC utilisation category, overload relay trip class, and VFD derating if applicable.

09 · Industrial Guide

For Electricians: Motor Starter Commissioning and Documentation

Motor starter installation and commissioning is typically notifiable work under Part P of the Building Regulations for domestic premises, and requires an Electrical Installation Certificate (EIC) for all new motor circuits in commercial and industrial premises under BS 7671. Thorough commissioning records protect you and your client.

Issue the EIC On Site

Use the Elec-Mate EIC app to complete the Electrical Installation Certificate on your phone during commissioning. Record the overload relay setting, motor FLC, circuit protective conductor size, and test results — then export the PDF before you leave the site.

Quote VFD Upgrades on the Day

When servicing star-delta or DOL starters, calculate the energy savings from a VFD upgrade and quote immediately using the Elec-Mate quoting app. A VFD upgrade on a 15 kW pump running at 80% speed typically pays back in under two years — an easy sell to a facilities manager.

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Frequently Asked Questions About Motor Starter Installation

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Motor Starter Installation UK: DOL, Star-Delta & VFD Guide

Key Takeaways

Types of Motor Starters

Get the BS 7671 A4:2026 Cheat Sheet — free

Direct On-Line (DOL) Starters

Star-Delta Starters

VFD / Inverter Drives

Wiring and Control Circuits

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Overload Protection Requirements

Forward/Reverse Control Circuits

Typical Applications by Starter Type

For Electricians: Motor Starter Commissioning and Documentation

Issue the EIC On Site

Quote VFD Upgrades on the Day

Motor circuit certification and quoting with Elec-Mate

Frequently Asked Questions About Motor Starter Installation

What size motor can a DOL starter be used with?

What is the starting current reduction achieved by a star-delta starter?

When should a VFD be specified instead of a star-delta starter?

What overload protection is required for motor circuits?

What cable type and size should be used for motor circuits?

Do motor starters need to be installed in a specific enclosure?

What documentation is required after motor starter installation?

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