DOL Starter: What It Is, Wiring, and How to Size One
A DOL (Direct-On-Line) starter connects a motor straight to full mains voltage using a magnetic contactor, with a thermal overload relay for protection and start/stop buttons for control. It is the simplest and cheapest way to start a three-phase motor — suitable up to roughly 5–7.5 kW, above which the full starting current (6–8 × running current) dips the supply too hard and you move to star-delta or a soft starter.
DOL is the default starter across Pakistani workshops, tube wells and small pumping sets — because it works, and because it is three components you can buy off the shelf. Here is exactly what those components are, how the control circuit latches, and how to size each part to your motor.
What a DOL starter actually is
A DOL starter is not one device. It is an assembly of three functional parts working together:
- A magnetic contactor — the power switch that connects and disconnects the motor from the three-phase supply.
- A thermal overload relay — sits in series with the motor and trips the contactor if the motor draws sustained overcurrent.
- Start and stop pushbuttons — the operator control, wired so the motor stays running after you release the start button.
Upstream of all three sits short-circuit protection — an MCB or, better, a motor-protection circuit breaker (MPCB). The contactor makes and breaks the load; the overload relay protects against slow overloads; the MCB/MPCB protects against short circuits. Three different jobs, three different devices.
How a DOL starter works: the latch circuit
The clever part of a DOL starter is the control circuit, which lets a momentary press of the start button keep the motor running.
The hold-in (latch) contact
When you press Start, current flows to the contactor coil and it pulls in — closing the main power contacts to the motor, and simultaneously closing a normally-open auxiliary contact. That auxiliary contact is wired in parallel with the start button.
So when you release the start button, current keeps reaching the coil through the auxiliary contact. The contactor holds itself in. This is the hold-in, seal-in or latch circuit — the single most important idea in motor control wiring.
Stopping and tripping
Pressing Stop — a normally-closed button in series with the coil — breaks the coil current. The contactor drops out, the main contacts open, and because the auxiliary contact opens too, the latch is gone. The motor stays off until Start is pressed again.
The thermal overload relay works the same way: its normally-closed contact is wired in series with the coil. On a sustained overload it opens that contact, dropping the contactor exactly as if you had pressed Stop — but automatically, before the motor windings cook.
DOL starter components at a glance
| Component | Job | Sized to |
|---|---|---|
| Magnetic contactor | Switches motor on/off; carries running current | AC-3 rating ≥ motor full-load current |
| Thermal overload relay | Trips on sustained overcurrent | Set dial to motor nameplate FLC |
| Start / Stop buttons | Operator control + latch | Control voltage (230 V or 24 V) |
| Auxiliary NO contact | Holds the contactor in after Start released | Often built into the contactor |
| MCB / MPCB | Short-circuit protection | Type 2 coordination with contactor |
DOL starter wiring: power and control
Power circuit. Three-phase supply → MCB/MPCB → contactor main contacts (L1/L2/L3) → through the overload relay → to the motor terminals (U/V/W). All three phases pass through both the contactor and the overload relay.
Control circuit. One phase (or a control transformer output) → Stop button (NC) → Start button (NO), with the contactor's auxiliary NO contact wired across the Start button → contactor coil → through the overload relay's NC contact → back to neutral. That single loop gives you start, latch, stop and auto-trip in one circuit.
Always confirm the coil voltage of your contactor matches your control voltage. A 230 V control circuit needs a 230 V coil; a 24 V control circuit needs a 24 V coil. Mismatched coils are a common cause of a starter that will not pull in — or one that burns out.
When to use a DOL starter — and when not to
A DOL starter throws the motor straight onto full voltage, so it draws its full locked-rotor current at the instant of starting — typically 6 to 8 times the running current. For a small motor on a strong supply that is fine. For a large motor, or a weak rural feeder, that inrush dips the voltage enough to dim lights, disturb other equipment, and in bad cases stall the start.
| Motor size | Recommended starting method |
|---|---|
| Up to ~5 kW (7.5 hp) | DOL — simple and adequate |
| ~5.5 kW to ~30 kW | Star-delta, to cut starting current to a third |
| Large / frequent starts / soft-start needed | Soft starter or VFD |
The 5 kW line is a guide, not a law — a strong urban 3-phase supply may start a larger motor DOL without trouble, while a long rural feeder may struggle with a smaller one. If starting DOL noticeably dims the lights, the supply is telling you to move up to star-delta starting.
How to size a DOL starter in 4 steps
Step 1 — Find the motor full-load current (FLC)
Always read the motor nameplate first. The FLC printed there is the real figure to size against. The values below are typical for standard 4-pole 400 V three-phase motors and are useful only as a sanity check.
| Motor | Typical FLC at 400 V, 3-phase |
|---|---|
| 0.75 kW (1 hp) | ~2 A |
| 1.5 kW (2 hp) | ~3.5 A |
| 2.2 kW (3 hp) | ~5 A |
| 3.7 kW (5 hp) | ~8 A |
| 5.5 kW (7.5 hp) | ~11 A |
Single-phase motors of the same power draw considerably more current than the three-phase figures above, so size from the actual single-phase nameplate rating.
Step 2 — Choose the contactor by its AC-3 rating
Motor switching is AC-3 duty under IEC 60947-4-1. Select a contactor whose AC-3 current rating (at your voltage) is equal to or greater than the motor FLC. Do not size from the contactor's thermal current (Ith) or its model number — those are usually larger, and using them leads to an undersized contactor for motor duty. This is exactly why utilization categories exist; the full explanation is in AC-1 vs AC-3 vs AC-6a: Contactor Utilization Categories.
Step 3 — Set the thermal overload relay to the motor FLC
Pick an overload relay whose adjustable range brackets the motor FLC, then set the dial to the nameplate FLC. Match the trip class to how hard the motor starts:
| Trip class | Trip time at 7.2 × setting | Use for |
|---|---|---|
| Class 10 | 4–10 s | Standard motors, normal starts (most cases) |
| Class 20 | 6–20 s | Higher-inertia loads, longer starts |
| Class 30 | 9–30 s | Heavy loads with long run-up (large fans, loaded conveyors) |
A relay set below the FLC nuisance-trips on every start; set above it, the winding is unprotected. Set it at the nameplate figure.
Step 4 — Add short-circuit protection
The overload relay does not clear short circuits — it is far too slow. Upstream you need an MCB or MPCB rated to break the prospective fault current, coordinated with the contactor so that after a fault the contactor survives (Type 2 coordination). A D-curve MCB or a dedicated MPCB is usual for motor circuits, because they tolerate the starting inrush without nuisance-tripping.
Common DOL starter problems
Nearly every DOL fault traces back to the contactor or its coil circuit:
- The contactor buzzes or rattles instead of pulling in cleanly — usually low coil voltage or a control-circuit fault. See What Causes a Contactor to Chatter?
- The contactor coil keeps failing — almost always chattering or undervoltage keeping the coil at inrush current. See Why Do Contactor Coils Keep Burning Out?
- The overload trips on every start — relay set below FLC, or trip class too fast for the load's run-up time.
- The motor will not stay running after Start is released — the auxiliary hold-in contact is not wired across the Start button.
- Panel electronics misbehave when the starter switches — fit coil surge suppression; see RC Snubber vs Varistor vs Diode.
Frequently asked questions
What does DOL stand for?
Direct-On-Line. The motor is connected directly across the full mains voltage at start, with no reduction in voltage or current.
What is the maximum motor size for a DOL starter?
As a rule of thumb, up to about 5 kW (7.5 hp) on a normal three-phase supply. The real limit is whether your supply can tolerate the starting inrush — a strong supply allows larger, a weak one less.
What is the difference between a DOL starter and a contactor?
A contactor is one component — the switch. A DOL starter is the complete assembly: contactor plus thermal overload relay plus start/stop control. The contactor is the heart of it, but a bare contactor is not a starter.
Why does a DOL starter draw such high current at start?
Because the motor is thrown onto full voltage while stationary. A stationary motor behaves almost like a short circuit until it speeds up, drawing 6–8 times its running current for a second or two.
Can I use a DOL starter for a single-phase motor?
Yes. The principle is identical — contactor, overload relay, start/stop latch — just with single-phase wiring. Size the contactor and overload to the single-phase FLC, which is higher than the three-phase figure for the same power.
Do I still need an MCB if I have an overload relay?
Yes. They protect against different faults. The overload relay handles slow overloads over seconds to minutes; it cannot clear a short circuit. The MCB or MPCB handles the short circuit in milliseconds. You need both.
What should I set the overload relay to?
The motor's full-load current from the nameplate. Not the contactor rating, not the MCB rating — the motor FLC.
DOL or star-delta for a water pump?
Small pumps up to a few kW are usually fine on DOL. Larger submersible or booster sets, especially on a rural supply, benefit from star-delta to soften the starting dip — provided the motor is delta-connectable and the load allows a reduced-torque start.
Size to the nameplate, match the contactor's AC-3 rating, set the overload to FLC, and coordinate the MCB. Get those four right and a DOL starter will run for years.
Related guides
- Star-Delta Starter: How It Works and When You Need One
- AC-1 vs AC-3 vs AC-6a: Contactor Utilization Categories Explained
- Why Do Contactor Coils Keep Burning Out?
CNC Electric supplies IEC 60947-4-1 magnetic contactors and thermal overload relays for DOL starters across Pakistan, with free delivery and cash on delivery. Browse the magnetic contactors collection or WhatsApp +92 326 1111 376 and we will match the contactor and overload to your motor.