MPCB Selection Calculator
Calculate Smarter. Work Faster.
Free MPCB selection calculator — thermal dial setting (Ir) from the motor nameplate current, an illustrative adjustment band, the typical fixed magnetic trip, a starting-inrush check and trip-class screening from starting method and acceleration time.
MPCB Selection Details
Enter the motor nameplate current, where the MPCB sits in the starter, and how the motor starts.
Use the rated current printed on the motor nameplate for the actual supply voltage and winding connection — not a generic kW/HP table value.
VFD-fed motors are not covered: an MPCB on a drive input carries the drive’s input current, and motor overload protection comes from the drive itself.
Locked-rotor current from the motor nameplate code letter or catalogue (commonly about 5–8× FLA; often higher on IE3/IE4 motors). Acceleration time is the run-up time to full speed on the real load.
Enter values and hit calculate
Enter values and hit calculate to see the checks.
Enter values above to see a breakdown.
How MPCB Dial Setting Is Determined
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 (IEC practice): Thermal Setting Ir (A) = motor nameplate rated current for the actual supply voltage and connection. The IEC 60947-4-1 thermal characteristic is built around that setting — the element must not trip within 2 hours at 1.05× Ir from cold and must trip within 2 hours at 1.2× Ir from hot — so no extra multiplier is added. Where the MPCB sits inside the delta branch of a star-delta starter, it carries winding current and Ir = FLA ÷ √3.
Service factor is not an IEC setting multiplier. Service factor is a NEMA motor rating: a reserve the motor can tolerate occasionally, at higher temperature and shorter insulation life, not a normal operating point. Most IEC-rated motors carry no service factor at all. Where the US NEC applies, 430.32 sets a maximum overload setting of 125% of nameplate FLA for motors marked with a service factor of 1.15 or more or a temperature rise of 40 °C or less, and 115% for all others — the calculator shows that basis separately.
Set close to FLC, never padded upward. In NEC-style motor branch circuits, short-circuit fuses and breakers are commonly permitted at roughly 175–300% of motor current because they are not the overload device and 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 around 13× the maximum of the model’s adjustment band (often quoted as 12–14× depending on manufacturer). It does not move with the dial: turning the dial down to the bottom of a band leaves the magnetic trip at the same amperes, which is a higher multiple of the setting.
Trip class is a separate choice. IEC 60947-4-1 defines trip classes by the trip time at 7.2× Ir from cold: Class 10A between 2 and 10 s, Class 10 between 4 and 10 s, Class 20 between 6 and 20 s, and Class 30 between 9 and 30 s. Most MPCB models are Class 10 or 10A; a high-inertia load with a long run-up may need a Class 20 device, or a contactor with a separate Class 20/30 overload relay. A correct dial with too fast a class is a common cause of starting trips.
How the trip-class screening works. The calculator takes the starting current the thermal element actually sees, as a multiple M of Ir, and scales each class’s minimum trip time at 7.2× with an I²t approximation: tmin(M) ≈ tmin(7.2) × (7.2 ÷ M)². The lowest class whose estimated minimum trip time is comfortably above the acceleration time (run-up no more than 70% of it) is reported; if only a tighter fit exists, that class is shown and flagged as marginal. This is a cold-start screening estimate only — a hot restart trips sooner, and the manufacturer’s published cold and hot curves govern.
Worked Example
Motor nameplate FLA 18 A, DOL start, locked-rotor current 7× FLA, run-up 2 s. IEC basis: Ir = 18 A. Illustrative band 17–23 A (18 A sits clear of both stops). Magnetic trip ≈ 13 × 23 = 299 A (about 276–322 A). Starting current 18 × 7 = 126 A, so M = 126 ÷ 18 = 7.0×. Screening allowance for asymmetric first-cycle inrush: 2 × 126 = 252 A, below the 276 A lower magnetic figure — acceptable. Class 10 minimum trip time at 7.0× ≈ 4 × (7.2 ÷ 7.0)² ≈ 4.2 s, about twice the 2 s run-up — Class 10 passes the screen. With a 3 s run-up the run-up would reach about 70% of that time, and the screening result moves to Class 20 to keep margin for a hot restart. Had the 13–18 A band been chosen instead, its magnetic trip of about 13 × 18 = 234 A (216–252 A) would sit at or below the 252 A inrush allowance — a nuisance-trip risk. NEC basis for the same motor marked SF 1.15: maximum overload setting 1.25 × 18 = 22.5 A.
Available dial graduations vary by manufacturer and model line. This calculator gives the setting value, an illustrative band and typical magnetic figures; the specific model whose range brackets that value must be chosen from the manufacturer’s catalogue, and its magnetic trip, trip class, breaking capacity and coordination verified against the device documentation.
Reading an MPCB Trip Curve
Every figure this calculator returns lands in one region of the breaker’s time–current characteristic.
The sloping blue line is the thermal element: inverse-time, so the further current rises above the setting, the faster it operates. Moving the dial slides the whole curve sideways.
The amber band is a direct-on-line start: 5–8× FLC for a second or two. The curve passes above it, so a healthy start finishes before the thermal element accumulates enough heat to trip.
The red vertical line is the fixed magnetic element, clearing a short circuit in milliseconds. Because it is set at about 13× the band maximum rather than 13× the dial, its position on this dial-based axis depends on where the dial sits in the band — at the band maximum it is about 13× the setting, and it moves further right as the dial is turned down.
Diagnosing from the curve: a trip after seconds of running points at the sloping region — overload, phase imbalance, a stalled machine, or a dial below actual FLC. A trip at the instant of switching points at the vertical region — a short circuit, a cable or winding fault, or too low a magnetic threshold for that inrush.
Choosing the Right MPCB Adjustment Range
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.
MPCB Selection by Starting Method
The dial setting depends on what the breaker actually carries, which depends on where it sits. The same motor and device can need two different settings purely because of position in the starter.
Sizing a complete star-delta starter?
The Star Delta Starter Calculator gives the main, star and delta contactor currents, the overload relay setting and the short-circuit device from motor kW, HP or FLC. It also carries the full power circuit diagram — supply, KM1/KM2/KM3 and the cross-connected winding ends — and the control circuit diagram with timer changeover, start hold-in and the interlock between the star and delta coils.
Both are worth reading before fixing a dial value: they show why the basis differs, with the short-circuit device in the incoming line carrying line current while the overload element in the main contactor branch carries winding current — about 0.58 × line FLC in delta running.
| Starting Method | Dial Setting Basis | What to Watch |
|---|---|---|
| Direct-on-line | Nameplate line FLC (see setting basis) | Highest inrush of any method; confirm the trip class suits the acceleration time |
| Star-delta, MPCB in main line | Nameplate line FLC (see setting basis) | Breaker sees line current in both star and delta stages |
| Star-delta, MPCB inside the delta branch | About 0.58 × line FLC (line FLC ÷ √3) | Easy to set from the wrong column; verify against the actual starter schematic |
| Soft starter, MPCB upstream | Nameplate line FLC, then check the soft starter maker's coordination table | Reduced inrush but longer ramp; extended acceleration still heats the motor |
| Variable frequency drive, MPCB on the drive input | Drive input current rating, not the motor FLC | Protects the feeder and serves as an isolating device — it gives the motor no overload protection |
The drive case is most often got wrong. An MPCB ahead of a VFD carries the rectifier’s input current, so setting it to motor FLC is meaningless — overload protection comes from the drive’s own thermal model.
Soft starters need one caution: limiting starting current does not limit starting heat. A motor ramped over twenty seconds can absorb more thermal energy than the same motor thrown across the line for three.
If it trips during starting, do not turn the dial up
That hides the symptom and keeps the fault. Check trip class against actual acceleration time, supply sag under inrush, a stiff or jammed driven machine, phase balance, starts per hour, and whether the magnetic threshold suits this inrush.
Breaking Capacity: What Current Range Does Not Cover
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.
| Rating | Meaning | Practical Reading |
|---|---|---|
| Icu | Ultimate breaking capacity | The device clears the fault safely, but may not be fit for further service afterwards |
| Ics | Service breaking capacity | The device clears the fault and remains serviceable; often quoted as a percentage of Icu |
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.
MPCB vs Separate Overload Relay vs Fuse-Based Motor Protection
| Protection Scheme | Devices Needed | Notes |
|---|---|---|
| MPCB | 1 device | Combined overload + short-circuit, adjustable dial, common with a contactor for remote switching |
| MCCB/MCB + separate overload relay | 2 devices | Breaker for short-circuit, relay for overload; more flexible overload curve options in some cases |
| Fuse + contactor + overload relay | 3 devices (fuse, contactor, relay) | Traditional scheme, still common, generally most economical per device but more components to coordinate |
MPCBs consolidate two or three devices into one, simplifying panel design and wiring and reducing spares. The trade-off is a higher per-device cost than a basic fuse, usually offset by panel space, labour and simpler coordination.
When extending an existing panel, matching the scheme already used across the facility usually outweighs a theoretically better alternative for one circuit, for spares and maintenance familiarity.
MPCB Selection Across Standards and Regions
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 the rated nameplate current. The IEC 60947-4-1 characteristic already provides the 1.05× no-trip / 1.2× trip band and absorbs normal starting, so no further multiplier is added unless the manufacturer calls for one. Service factor is a NEMA motor concept and is not applied as a setting multiplier under IEC practice.
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 — a maximum of 125% where the motor is marked with a service factor of 1.15 or more or a temperature rise of 40°C or less, and 115% otherwise. Where that setting cannot carry the starting current, NEC 430.32(C) permits the next step up, to not more than 140% or 130% respectively. The code also separates those nameplate amperes from the table values used for conductor and short-circuit protection.
This calculator keeps the two bases separate: the IEC basis returns the nameplate current itself, and the NEC basis returns the 430.32 maximum from the service factor and temperature-rise marking you enter. The NEC figure is an upper limit, not a target — setting nearer the nameplate current still gives closer protection. 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:
| Region | Typical LV Motor Supply | Frequency | Usual Framework |
|---|---|---|---|
| India | 415 V three-phase | 50 Hz | IEC-aligned national standards |
| UK & Europe | 400 V three-phase | 50 Hz | IEC / EN |
| USA | 480 V (208 V in some facilities) | 60 Hz | NEC Article 430, UL product standards |
| Canada | 600 V (208 V in some facilities) | 60 Hz | Canadian Electrical Code |
| Australia & New Zealand | 400 V three-phase | 50 Hz | IEC-aligned national standards |
| Gulf states | 400 V three-phase | 50 Hz | IEC-aligned |
| Saudi Arabia | 400 V (380 V in older installations) | 60 Hz | IEC-aligned, 60 Hz equipment |
| South Africa | 400 V three-phase | 50 Hz | IEC-aligned national standards |
| Japan | 200 V three-phase | 50 Hz east / 60 Hz west | JIS, broadly IEC-aligned |
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.
Common Mistakes When Selecting an MPCB
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. Treating service factor as a normal running point. A 1.15 service factor is a reserve for occasional use at higher winding temperature, not a licence to run continuously at 115% of FLA. Under IEC practice the dial is set to nameplate current; under the NEC, 125% is a maximum permitted setting, not a required one. If a motor genuinely runs in its service-factor region, the load or the motor size needs attention first.
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, or that it follows the dial. On most models only the thermal dial is field-adjustable — the magnetic threshold is fixed at manufacture, at a multiple of the band maximum. A high-inrush IE3/IE4 motor near the top of a small band can trip magnetically on switch-on even with a correct dial.
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.
Frequently Asked Questions
What is an MPCB and how is it different from a standard MCB or MCCB? +
An MPCB combines adjustable thermal overload and magnetic short-circuit protection in one device built for motor circuits. Its dial lets one model cover a band of motor currents, unlike an MCB or MCCB with a fixed rating and no motor-specific thermal characteristic.
How do I set an MPCB's dial for my motor? +
Under IEC practice, set the thermal dial to the motor's nameplate rated current for the actual supply voltage and winding connection. If the MPCB sits inside the delta branch of a star-delta starter, set it to nameplate current divided by √3 (about 0.58×). Where the US NEC applies, 430.32 gives a maximum setting of 125% or 115% of nameplate current depending on the motor marking.
What is motor service factor and why does it affect the dial setting? +
Service factor is a NEMA motor rating — commonly 1.0 or 1.15 — showing a reserve the motor can tolerate occasionally, at higher temperature and reduced insulation life. It is not a normal operating point and is not an IEC setting multiplier. It matters under the US NEC, where a service factor of 1.15 or more (or a marked temperature rise of 40 °C or less) raises the maximum permitted overload setting from 115% to 125% of nameplate current.
Does the MPCB dial setting change for star-delta starting? +
Yes, depending on position. In the main incoming line it sees full line current and is set to nameplate line FLC. Inside the delta branch it carries winding current — line FLC divided by √3, about 0.58 times. Confirm against the starter schematic.
What is the typical instantaneous (short-circuit) trip threshold for an MPCB? +
On most standard models it is fixed at the factory at about 13 times the maximum of the model's adjustment band — often quoted as 12–14× depending on the manufacturer. It does not follow the dial. A useful screening check is that the lower magnetic figure should sit comfortably above the motor's starting current, with allowance for the asymmetric first-cycle inrush; the manufacturer's data governs.
Which MPCB trip class do I need for my motor? +
It depends on how long the motor takes to reach full speed. Under IEC 60947-4-1, at 7.2× the setting from cold, Class 10A trips in 2–10 s, Class 10 in 4–10 s, Class 20 in 6–20 s and Class 30 in 9–30 s. If the run-up time approaches the minimum trip time of the chosen class at the actual starting current, the device can trip during starting — choose a slower class, or a contactor with a separate Class 20/30 overload relay. The calculator's class result is a cold-start screening estimate.
Why does my MPCB trip instantly when a high-efficiency motor starts? +
IE3 and IE4 motors often draw a higher locked-rotor current and a sharper first-cycle inrush than older designs. If the MPCB band is small relative to the motor, its fixed magnetic trip — about 13× the band maximum — may sit too close to that inrush. Choosing a band whose maximum is higher, so the setting sits lower in the band, raises the magnetic threshold in amperes; confirm with the manufacturer's selection table for IE3/IE4 motors rather than turning the thermal dial up.
Can one MPCB model cover multiple different motor sizes? +
Yes — that is the point of the adjustable dial. One model covers a band of currents, so choose a band that brackets the calculated setting rather than one that only just reaches it.
Does an MPCB eliminate the need for a separate motor overload relay? +
Generally yes for the overload function, since the thermal element does the job a separate relay would. A contactor is still needed where the motor requires remote, automatic or frequent switching.
Should I use the motor's FLC from its nameplate or from a general reference table? +
Always the nameplate. Generic kW or hp tables are approximations and can differ meaningfully from a specific motor's tested rating, which varies with voltage, efficiency class, power factor and pole count.
What happens if the MPCB dial is set too high or too low for the actual motor? +
Too low causes nuisance tripping under normal load. Too high leaves the motor unprotected against a sustained overload that can overheat the windings. Set it as close to actual FLC as the model's graduations allow.
Is an MPCB sized the same way under IEC practice and the US National Electrical Code? +
No. IEC practice sets the thermal element at the nameplate rated current, with no service-factor multiplier. Under the US National Electrical Code, 430.32 sets a maximum overload setting for a continuous-duty motor above 1 hp of 125% of nameplate full-load amperes where the motor is marked with a service factor of 1.15 or more or a temperature rise of 40°C or less, and 115% otherwise — an upper limit, not a target.
What breaking capacity does my MPCB need, and what is the difference between Icu and Ics? +
It must interrupt the prospective short-circuit current available where it is fitted, which depends on the supply transformer, cable run and system configuration. Icu is the ultimate breaking capacity, after which the device may not be reusable; Ics is the service breaking capacity, after which it stays serviceable.
Can I put an MPCB between a VFD and the motor, or set one on the drive input to the motor FLC? +
Neither, as a rule. An MPCB on the drive input carries the drive's input current and gives the motor no overload protection, which comes from the drive's own thermal model. Switching a breaker between a running drive and its motor should only be done where both manufacturers permit it.
How do I choose the MPCB adjustment range, not just the dial value? +
Choose a band that contains the calculated setting with room either side. A band whose minimum is above the value cannot be turned down far enough; one whose maximum is far above leaves the pointer near the bottom stop.