MPCB Selection Calculator

A Motor Protection Circuit Breaker combines two functions that would otherwise need separate devices: an adjustable thermal element for sustained overload, and a fixed magnetic element for short circuits. Because the thermal element is adjustable, setting it correctly matters as much as choosing the model.

Formula: Dial Setting (A) = Motor Full Load Current × Service Factor. FLC is the rated running current from the nameplate; service factor — commonly 1.0, or 1.15 on many general-purpose motors — is how far above it the motor may run continuously within its temperature limits.

Set close to FLC, never padded upward. Fuses and MCBs are sized at 175–300% of motor current because they have no motor-specific thermal characteristic. An MPCB does, and rides through normal starting at its rated setting. Padding the dial throws away the protection you paid for.

Only the thermal element is adjustable. On most standard models the magnetic threshold is fixed at the factory, typically 12–14× the dial setting for Class 10 devices under IEC 60947-4-1.

Trip class is a separate choice. Class 10 trips in about 10 seconds at 7.2× setting; Class 20 and 30 tolerate longer overload, suiting high-inertia loads. A correct dial with too fast a class is a common cause of starting trips.

The dial value is only half the decision. An MPCB is ordered as a model covering a specific adjustable band, and a setting that technically falls inside a band is not one that sits comfortably within it. Taking the 18 A setting from the example above:

A band whose minimum sits above the value is unusable. One whose maximum sits far above leaves the pointer near the bottom stop, where graduations are coarsest. A band ending exactly at the value works but leaves no reserve for adjustment or a replacement motor reading higher. Where two bands contain the value, order the one that brackets it.

Nameplate current beats a power rating

Two motors of the same kW or hp rating draw different currents, because current depends on rated voltage, efficiency class, power factor and pole count — a 7.5 kW motor has one current at 380 V and another at 415 V.

Where the nameplate is not available yet, estimate with I ≈ P ÷ (√3 × V × η × PF), P in watts and V line-to-line — provisional only, to be re-checked at commissioning.

Where the nameplate lists two voltages and two currents, use the pair matching the actual supply and winding connection — reading the wrong column puts the setting out by a factor of √3.

A breaker matched perfectly to a motor’s running current can still be the wrong device, because current range says nothing about fault performance. The MPCB must also interrupt the prospective short-circuit current where it is installed — a figure set by the supply transformer, the cable run and the system configuration, not by the motor.

Both ratings are declared at a stated operational voltage and fall as voltage rises, so a figure quoted at 400 V does not carry to a 480 V or 690 V installation.

Where available fault current exceeds what the device can interrupt alone, backup protection is required — an upstream fuse or breaker the manufacturer has tested in that combination. A fuse of the same nominal rating from another family is not the same device.

The related choice is coordination class. Type 1 accepts contactor damage from a full-rated fault provided nothing hazardous escapes the enclosure; Type 2 requires both devices to stay serviceable. Type 2 is the sensible default wherever a fault means downtime, and can only be claimed from tested combination tables.

The physics is the same everywhere. The vocabulary, the governing standard and the sizing basis are not.

Same device, different names

“MPCB” is IEC shorthand. The same product is sold as a manual motor starter, manual motor protector or manual motor controller, and in North America a version marked for use without a separate upstream device is a self-protected combination motor controller.

The sizing multiplier is not universal

Under IEC practice the thermal element is set at rated nameplate current, with the service factor applied where above unity. The trip characteristic already absorbs normal starting, so no further multiplier is added unless the manufacturer calls for one.

Where the US National Electrical Code applies, overload protection for a continuous-duty motor above 1 hp is sized as a percentage of nameplate full-load amperes — commonly 125% where the motor is marked 1.15 service factor or 40°C rise, and 115% otherwise. The code also separates those nameplate amperes from the table values used for conductor and short-circuit protection.

This calculator multiplies FLC by whatever factor you enter, so it serves both: the actual service factor for IEC practice, or 1.25 / 1.15 for the NEC basis. The edition in force locally and the manufacturer’s instructions govern.

The same motor output also gives a different full-load current on every common LV system, which is why the nameplate matched to the actual supply is the only safe input:

Frequency does not change the dial arithmetic. It changes the motor — synchronous speed, and so the current a load demands — so a motor moved between 50 Hz and 60 Hz needs its nameplate re-read, not its old setting reused.

Thermal elements are calibrated at a reference ambient, so a hot plant room trips earlier than the dial suggests. Check for a temperature compensation table or an ambient-compensated variant where conditions are extreme.

1. Padding the dial above actual FLC to stop nuisance tripping. This removes real overload margin — an MPCB already handles normal starting at its rated setting.

2. Ignoring a service factor above 1.0. A 1.15 motor runs continuously at 115% of nameplate FLC; setting the dial to bare FLC trips it under conditions it is rated for.

3. Using a generic FLC table instead of the nameplate. Table values are approximations and can differ from the specific motor installed.

4. Not checking position in a star-delta starter. Main line and in-delta positions carry different currents; the wrong basis puts the setting out by a factor of √3.

5. Assuming the instantaneous trip is adjustable. On most models only the thermal dial is field-adjustable — the magnetic threshold is fixed at manufacture.

6. Checking current range but never breaking capacity. Verify Icu and Ics at the actual operational voltage, and add tested backup protection where the prospective fault current exceeds them.

7. Setting an MPCB on a VFD input to the motor FLC. It carries the drive’s input current; motor overload protection comes from the drive.

8. Reusing the old setting after a motor swap. A nominally equivalent replacement can have a different nameplate current — re-read and re-set.

9. Picking a band that only just contains the setting. A value at the extreme end of a model’s range leaves no room for adjustment or a slightly different motor later.