MPCB Plus Contactor: Building a Motor Starter
What do you get when you pair an MPCB with a contactor? A motor starter: the MPCB supplies an adjustable thermal overload element dialed to the motor's full-load current (FLC) per IEC 60947-4-1 plus a fixed magnetic short-circuit trip around 12-13x In, and the contactor supplies the high-cycle switching the MPCB's manual mechanism was never built for. Skip the manufacturer's coordination table when combining the two and a fault that should clear in milliseconds welds the contactor's contacts instead. This article covers the two common builds (thermal-magnetic MPCB, and magnetic-only MPCB plus a separate overload relay), contactor sizing against the MPCB's setting range, Type 1 vs Type 2 coordination, link-module mounting, and where each build earns its place on a panel.
Why an MPCB Needs a Contactor
An MPCB switches manually and is rated for isolation, not for thousands of on/off cycles a day. Its internal contacts are sized for occasional operation plus fault interruption, not for the electrical and mechanical wear of routine motor starts and stops. A contactor is built for exactly that: AC-3 rated contacts designed for repeated make-and-break under load, driven by a coil that a PLC output, a float switch, or a push-button can energize from a distance. Put the two together and the MPCB becomes the protection and isolation device while the contactor becomes the switching device, each doing the job it is rated for.
This division of labor is not optional once a motor needs remote or automatic control. A standalone MPCB with no contactor works fine for a bench tool an operator switches by hand at the panel. Add a level sensor, a building-automation signal, or a start/stop station 30 meters away, and the loop needs a contactor coil in the circuit. What we see in the field: panel builders sometimes try to skip the contactor on "simple" pumps to save a component, then get called back when the customer asks for remote start six months later.
Two Ways to Build the Starter
There are two accepted architectures, and the choice changes what protects the motor from overload.
Thermal-Magnetic MPCB + Contactor
The MPCB (Schneider GV2ME/GV2P, ABB MS132, Siemens SIRIUS 3RV2011) carries both the adjustable bimetal overload and the fixed magnetic trip in one housing. The contactor downstream does nothing but switch; all protection lives in the MPCB. This is the default build for the majority of manual motor starters up to roughly 100 A, and it is what most panel builders reach for first because it needs one fewer device and one fewer set of wiring terminals than the alternative.
Magnetic-Only MPCB + Overload Relay + Contactor
Here the MPCB (Schneider GV2L, ABB MO132/MO165) trips on short circuit only — no thermal element inside it at all. A separate electronic or bimetal thermal overload relay mounted on or beside the contactor handles the overload function. This build costs one more component but buys finer overload protection: electronic overload relays offer ground-fault sensing, current imbalance detection, and communication (Modbus, IO-Link) that a mechanical MPCB dial cannot match. It is the standard choice where the overload relay needs to report status to a PLC, or where an OEM standardizes on one overload relay family across multiple motor sizes and swaps only the magnetic MPCB and contactor for frame size.
Sizing the Contactor to the MPCB
The contactor's AC-3 rated operational current must cover the motor's full-load current across the MPCB's whole setting range, not just the nameplate value of the motor on the bench today. A GV2ME06 set to 4 A needs a contactor rated for at least that current at the supply voltage and duty; oversizing the contactor one frame is common practice so the same combination starter covers a small range of motor sizes without a contactor change. Undersize it and the contacts erode fast under repeated starts; oversize it too far and the physical footprint and cost climb for no protection benefit.
Formula: MPCB magnetic trip threshold vs contactor duty — Source: IEC 60947-4-1, manufacturer coordination tables
Im = k × In
| Symbol | Description | Unit |
|---|---|---|
| Im | Magnetic (instantaneous) trip current of the MPCB | A |
| In | MPCB rated/dial-set current (motor FLC) | A |
| k | Magnetic trip multiplier, typically 12-13 for MPCBs (vs 5-10 for an MCB C-curve) | dimensionless |
The contactor must survive whatever current flows during that magnetic trip window without the contacts opening under load — that is the entire point of the coordination table. This depends on the motor's start current profile too: a motor with a long, heavy inrush stresses the same contactor harder than a light-duty pump, even at identical FLC.
Coordination: Type 1 vs Type 2
IEC 60947-4-1 defines two coordination outcomes for a starter after a short-circuit fault, and the difference decides how much a fault costs the customer in downtime and parts.
Type 1: Damage Allowed
After the fault, the MPCB and contactor may be damaged; they must not endanger the operator, but replacement parts may be needed before the starter runs again. Type 1 combinations are typically cheaper and still fully compliant, common on cost-sensitive OEM machines with in-house spares.
Type 2: No Damage, Light Welding Only
The starter survives the fault with, at most, light contact welding that separates easily without replacing parts — the equipment is back in service after the fault clears. Type 2 is the usual specification for process plants and MCC applications where an unplanned parts swap means unplanned downtime. Manufacturers publish coordination tables pairing specific MPCB models, specific contactor models, and specific short-circuit current levels to a declared Type; deviate from the table (wrong contactor model, wrong frame) and the Type 2 declaration no longer applies, even if every individual component is rated correctly on its own.
Read the full mechanics of this classification in our guide to Type 1 vs Type 2 coordination.
Wiring and Mounting: Link Modules vs Separate Enclosures
Siemens SIRIUS pairs a 3RV2 MPCB to a 3RT2 contactor with a snap-on link module that carries the power connection between them without external wiring — the two devices bolt together on the DIN rail as one mechanical unit. Schneider TeSys and ABB MS-series offer similar direct-mount or short-jumper connections between the MPCB and the matched contactor. Direct mounting saves panel width, cuts wiring time, and removes a set of screw terminals that could loosen in service. It also locks the builder into that manufacturer's matched contactor for that MPCB frame size — mixing brands means running separate power cables between the two devices instead, which still works electrically but gives up the space and labor savings.
Some panel builders wire MPCB and contactor as fully separate DIN-rail devices even when a link module is available, usually to keep spare-parts stock brand-agnostic across a mixed fleet of equipment. That is a legitimate trade-off, not a shortcut — check that the coordination table still applies to the specific separate-wiring configuration, since some published tables assume the manufacturer's own link module and its exact conductor length.
Where Each Build Fits
Thermal-magnetic MPCB + contactor covers most single motors on a machine or skid: pumps, fans, small conveyors, compressors under roughly 30-50 kW where nameplate FLC rarely changes once the motor is selected. Magnetic-only MPCB + overload relay + contactor earns its extra cost in motor control center panels with dozens of feeders, where a plant standardizes on one overload relay platform for reporting and spares, and swaps only the magnetic MPCB and contactor per motor size. A third option — fuse plus a separate overload relay instead of an MPCB — trades the MPCB's reset-and-reuse convenience for a fuse's often higher interrupting rating; it is worth comparing on a fault-heavy feeder.
| Criteria | Thermal-Magnetic MPCB + Contactor | Magnetic-Only MPCB + Overload Relay + Contactor | Fuse + Overload Relay + Contactor |
|---|---|---|---|
| Component count | 2 devices | 3 devices | 3 devices (plus fuse holder) |
| Overload protection | Bimetal, dial-set to FLC | Separate relay: bimetal or electronic | Separate relay: bimetal or electronic |
| Communication/reporting | None on standard models | Available on electronic relays (Modbus, IO-Link) | Available on electronic relays |
| Reset after trip | Manual toggle on the MPCB | Reset on the overload relay | Fuse replacement required after clearing |
| Typical use | Standalone machine, skid, single motor | MCC with standardized relay platform | Fault-heavy feeder needing high interrupting rating |
Before finalizing a contactor choice for either build, run it against a proper contactor selection checklist, and confirm the pairing against the standard both devices are certified to — see our overview of IEC 60947-4-1 for contactors and motor starters. For the MPCB side of the pairing on its own, start with what makes it tick in what an MPCB is and how it works, or browse the full range of motor protection circuit breakers and matched contactors.
Frequently Asked Questions
Can I use an MPCB without a contactor?
Yes, for a motor an operator switches locally by hand at the panel. Add remote start, a PLC signal, or an automatic control loop and a contactor becomes necessary, since the MPCB's manual mechanism is not rated for that duty cycle.
Do I need the same brand for the MPCB and contactor?
Not electrically, but the published Type 1 or Type 2 coordination table almost always pairs a specific MPCB model with a specific contactor model from the same manufacturer. Mixing brands means the coordination declaration no longer applies unless the manufacturer has separately tested that combination.
What size contactor do I need for a given MPCB?
Match the contactor's AC-3 rated operational current to the top end of the MPCB's dial setting range, not just today's motor FLC, and confirm the pairing against the manufacturer's coordination table rather than sizing on rated current alone.
Is a link module required, or can I wire the MPCB and contactor separately?
A link module is not required electrically. It saves panel space and wiring labor and is common with matched-brand pairs. Separate wiring works too, provided the specific configuration is still covered by the manufacturer's coordination table.
Why does my magnetic-only MPCB need a separate overload relay?
A magnetic-only MPCB (such as Schneider GV2L or ABB MO132) trips on short circuit only; it has no thermal element inside. Without a separate overload relay, the motor has no protection against a sustained overload short of a short circuit.
Conclusion
An MPCB alone protects a motor branch but cannot switch it thousands of times a week; a contactor alone switches but does not protect. Together, sized against each other and checked against the manufacturer's coordination table, they form the motor starter that shows up in nearly every industrial panel. Pick thermal-magnetic MPCB + contactor for a simple standalone build, move to magnetic-only MPCB + overload relay + contactor when the application needs communication or a standardized relay platform, and never assume a coordination Type carries over once the contactor model, MPCB model, or wiring configuration changes from what the table specifies. Reference the full MPCB engineering guide for the wider picture across sizing, trip classes, and standards.
