How to Select and Set an MPCB for a Motor
How do you select and set an MPCB for a motor? Match the frame's setting range to the motor's full-load current (FLC) from the nameplate, dial the thermal element to that FLC, verify the magnetic trip (fixed near 12-13x In) clears the motor's inrush without tripping on start, and confirm the trip class and Type 1/Type 2 coordination rating against the paired contactor per IEC 60947-4-1. Get any one of these wrong and the motor either nuisance-trips on every start or runs unprotected against a stalled-rotor overload. This guide covers the nameplate data to pull, dial setting math, magnetic trip verification, trip class selection by start duration, short-circuit coordination, and the choice between thermal-magnetic and magnetic-only MPCBs.
Start With the Motor Nameplate, Not the Panel Drawing
The panel drawing usually lists a motor's kW or hp rating. The MPCB dial is set in amps, not kilowatts, so pull the full-load current (FLC) directly off the motor nameplate at the supply voltage actually used. A 7.5 kW, 400 V three-phase motor with a nameplate FLC of 15.2 A needs the dial at 15.2 A, not at a rounded "15 kW breaker" value pulled from a catalog table sized for a different voltage or efficiency class.
Check three more nameplate fields before opening the MPCB's setting cover: service factor (a 1.15 SF motor can run continuously above FLC), ambient temperature rating (bimetal elements are temperature-compensated over a stated range, typically -25°C to +40°C or +60°C), and the motor's start method. A star-delta or soft-start motor draws less line current during acceleration than a direct-on-line unit of the same size, which changes which trip class fits.
Setting the Thermal Dial to Full-Load Current
The thermal element handles the everyday protection job: it trips on sustained overload, locked rotor, or a bearing dragging the motor into overcurrent. Set the dial to the nameplate FLC, not the next round number up. Rounding up "to be safe" removes protection margin exactly where the motor needs it. A stalled rotor at 6x FLC takes longer to trip a dial set 10% high than one set correctly, and on a Class 10 device that gap can mean minutes instead of seconds against a hot winding.
Formula: Thermal dial setting — Source: IEC 60947-4-1, Annex C guidance
Iset = FLCnameplate
| Symbol | Description | Unit |
|---|---|---|
| Iset | Thermal dial setting on the MPCB | A |
| FLCnameplate | Full-load current from the motor nameplate at supply voltage | A |
Some specifiers set the dial mid-range in the frame's adjustment band instead of at the motor's actual FLC, reasoning that a mid-band setting leaves room to adjust later without swapping the device. That thinking backfires: the dial has to sit at FLC from commissioning, and any later change should follow a nameplate change, not a guess. What we see in the field is dials left at the factory default because nobody checked them against the nameplate after commissioning; it is worth a five-minute check on every panel handover.
Checking the Magnetic Trip Against Starting Current
The magnetic element is fixed, not adjustable, and it is set high on purpose, typically around 12-13x the dial's In, so it does not react to the motor's own inrush. A direct-on-line motor draws 6-8x FLC for the first several hundred milliseconds of a start; the magnetic trip sits above that with margin by design. The selection task is not to adjust the magnetic trip (you cannot) but to confirm the frame picked has a magnetic trip point compatible with the motor's actual starting current profile.
High-inertia loads, large fans, centrifuges, some conveyor drives, pull a heavy start current for longer than a pump or small compressor. If start current stays above roughly 8-10x FLC for more than a second or two, check the manufacturer's trip-class curve before committing to a standard frame. This is where trip class selection does the real work, not the magnetic setting.
Matching Trip Class to the Motor's Start Time
Trip class defines how long the thermal element tolerates an overload before tripping, tested at 7.2x the set current from cold per IEC 60947-4-1. Class 10 trips within 10 seconds and covers most direct-on-line motors with start times under 5-6 seconds. Class 10A trips faster, tightening protection where cable or winding thermal margin is thin. Class 20 and Class 30 tolerate 20 and 30 seconds, sized for high-inertia loads with start times that would nuisance-trip a Class 10 device.
Picking the wrong class produces one of two failures. Too fast a class, Class 10 on a high-inertia fan, nuisance-trips on every cold start. Too slow a class, Class 30 on a small pump, leaves the winding exposed to a stalled-rotor condition for 20-plus extra seconds it does not need. Read the full breakdown in the MPCB trip class guide for trip-time curves by class.
Confirming Short-Circuit Rating and Coordination Type
Two numbers matter beyond the dial: the MPCB's rated breaking capacity (Icu) at the installation's prospective short-circuit current, and the coordination type declared with the downstream contactor. Icu has to clear the available fault current at that point in the panel. A 50 kA-rated frame on a 65 kA bus is a paper protection scheme, not a real one. Manufacturers publish Icu by frame and upstream device combination; check the panel's actual fault-current study, not a generic assumption.
Coordination type governs what survives a fault. Type 1 accepts damage to the starter (contactor, overload relay) after a short circuit, as long as no hazard to people results; parts get replaced. Type 2 limits damage to light, separable contact welding, and the starter keeps working after the fault clears. Manufacturers publish coordination tables pairing a specific MPCB with a specific contactor and, if used, overload relay for a declared Type at a declared fault current. Selecting components off the same table is the only way to claim the rating. See the Type 1 vs Type 2 coordination guide for how to read these tables, and the IEC 60947-4-1 standards overview for how the standard defines both types.
Thermal-Magnetic vs Magnetic-Only: Which to Select
Most panel builds use a thermal-magnetic MPCB, overload and short-circuit protection in one device, paired with a contactor for switching. Some builds instead use a magnetic-only MPCB (short-circuit trip only, no thermal element) paired with a separate electronic overload relay and contactor. The magnetic-only route costs more in parts count but gives finer overload protection; class selection, phase-loss sensitivity, and sometimes diagnostics live on the overload relay instead of a fixed bimetal.
| Criteria | Thermal-Magnetic MPCB | Magnetic-Only MPCB + Overload Relay | When to select |
|---|---|---|---|
| Overload protection | Built-in bimetal, dial-set to FLC | Separate electronic overload relay | Magnetic-only where finer class tuning or diagnostics matter |
| Parts count | Lower (2 devices: MPCB + contactor) | Higher (3 devices: MPCB + relay + contactor) | Thermal-magnetic for simple, cost-sensitive feeders |
| Phase-loss sensitivity | Bimetal-based, model-dependent | Electronic, typically faster and configurable | Magnetic-only where phase-loss speed is critical |
| Example series | Schneider TeSys GV2ME/GV2P, ABB MS132, Siemens SIRIUS 3RV2 thermal-magnetic | Schneider TeSys GV2L, ABB MO132/MO165, Siemens 3RV2 magnetic-only variants | Match to the overload relay already standardized on the panel line |
Both routes are documented as combination starters in each brand's family; pick components from the same manufacturer's coordination table so the declared Type and Icu carry through. Browse the motor protection circuit breakers collection and the thermal overload relays collection to compare current parts on hand. For the switching side, pair either build with a contactor sized per the contactor selection checklist.
Common Selection Mistakes
Rounding the dial up "for safety" is the most frequent error, and it is backwards; it delays the trip on a genuine overload. Sizing off a kW table instead of the actual nameplate FLC is the second most common, especially on retrofits where the original motor was swapped for a different-efficiency replacement carrying a different FLC at the same power rating. A third: pairing a thermal-magnetic MPCB from one brand with a contactor from another and assuming the Type 2 rating holds. Coordination tables are brand- and model-specific, and cross-brand pairing usually means no declared coordination at all.
A fourth mistake shows up on high-inertia loads: leaving the trip class at 10 because that is what shipped on the previous panel, then wondering why the fan motor trips on every cold morning start. Set it wrong and it nuisance-trips. Check the start time, then check the class curve, not the other way around.
Frequently Asked Questions
What FLC value do I set the MPCB dial to?
Set it to the exact full-load current on the motor nameplate at the actual supply voltage used, not a rounded value from a kW-to-amp table. If the motor has a service factor above 1.0, confirm with the manufacturer whether the dial should track the SF current or the base FLC.
Can I adjust the magnetic trip setting on an MPCB?
No. The magnetic trip is fixed at the factory, typically around 12-13x the rated current (In), specifically so it clears normal motor inrush without adjustment. Selection means picking a frame whose fixed magnetic trip and thermal setting range fit the motor, not tuning the magnetic element itself.
How do I know which trip class my motor needs?
Estimate or measure the motor's start time. Under 5-6 seconds typically fits Class 10; longer starts on high-inertia loads such as large fans or centrifuges need Class 20 or Class 30. Manufacturer trip-class curves give the exact trip time at 7.2x the set current for each class.
Does the MPCB need to match the contactor brand for Type 2 coordination?
In practice, yes. Coordination tables are published per manufacturer for specific MPCB-plus-contactor (and overload relay, if used) combinations at a declared fault current. Mixing brands without a published table means the Type 2 rating is not verified, even if both devices meet the standard individually.
What happens if the MPCB's Icu is lower than the available fault current?
The device may not clear the fault safely, risking damage beyond the declared coordination type or a failure to interrupt the fault at all. Check the panel's fault-current study against the MPCB's rated Icu at the installation voltage before finalizing the selection.
Conclusion
Selecting and setting an MPCB comes down to five checks: dial the thermal element to nameplate FLC, confirm the fixed magnetic trip clears the motor's actual inrush, match trip class to start time, verify Icu against the panel's fault-current study, and pull MPCB, contactor, and any overload relay from the same manufacturer's coordination table. Skip one and the protection scheme looks complete on paper but fails at the one moment it has to work. For the full protection-scheme background, see the MPCB engineering guide, and for the switching side pick from the manual motor starters or contactors collections.
