MPCB Derating for Temperature and Altitude
What is MPCB derating for temperature and altitude? Derating is the correction applied to a motor protection circuit breaker's usable current rating once ambient temperature moves outside the bimetal's compensated band (roughly -25°C to +40°C, extending to +55°C on some frames) or once installation altitude exceeds the standard reference of about 2000 m used in IEC 60947-1. Skip the correction and the thermal element reads hot even though the motor draws its normal full-load current, or the breaker's dielectric and interrupting margin thins out, so nuisance tripping on one end and reduced fault-clearing confidence on the other are both on the table. This article covers the compensated range, the temperature and altitude correction factors, how enclosure heat stacks on top of ambient, and how to fold all of it into the dial setting and frame choice.
Why Ambient Conditions Change What an MPCB Can Carry
The thermal element in an MPCB is a bimetal strip. It bends in proportion to the heat generated by current flowing through it, and that bending trips the mechanism once the deflection crosses a threshold calibrated to the dial's FLC setting. The bimetal does not know the difference between heat from motor current and heat from the surrounding air. Raise the ambient a few degrees and the strip starts closer to the trip threshold before the motor draws a single extra amp. Manufacturers build in ambient compensation, a second bimetal or a mechanical linkage that cancels out ambient drift within a stated temperature window, so the dial reading stays accurate across normal panel conditions. Outside that window, compensation runs out and the correction has to come from the person sizing the breaker, not the device itself.
Altitude works through a different mechanism but lands on the same conclusion: less margin than the nameplate implies. Air density drops as altitude rises, and thinner air convects heat away from the breaker's frame and busbar connections less effectively. It also has lower dielectric strength, which narrows the gap between rated operating voltage and the voltage at which arcing or flashover becomes a real risk. Both effects are quantified by the frame manufacturer for its own construction; neither one is optional to ignore once the installation sits meaningfully above sea level.
The Compensated Range: Where No Derating Is Needed
Inside the compensated band, published for each frame in its catalog, a motor circuit at 35°C ambient trips at the same current as one at 20°C ambient. That is the entire point of the feature. Panel builders who install MPCBs in a conditioned electrical room, an outdoor kiosk in a mild climate, or a factory floor with normal ventilation rarely need to think about temperature derating at all. What we see in the field: the boundary gets missed most often in enclosures with poor ventilation next to a VFD or a transformer, where the local air around the breaker sits well above the room's stated ambient. Measure at the device, not at the room thermostat.
Calculating the Temperature Derating Factor
Once ambient at the device exceeds the compensated band, the frame's continuous current capacity has to be pulled back. The correction is manufacturer-specific and published as a derating curve or table for each frame size, but the underlying relationship used across the industry follows the same form.
Formula: Current derating for ambient temperature and altitude — Source: IEC 60947-1 standard service conditions; manufacturer derating curves
Iderated = In × ktemp × kalt
| Symbol | Description | Unit |
|---|---|---|
| In | Frame's rated current at standard reference conditions | A |
| Iderated | Usable continuous current after correction | A |
| ktemp | Temperature correction factor, less than 1 above the compensated band, typically stepped down a few percent per 5°C increment | — |
| kalt | Altitude correction factor, less than 1 above roughly 2000 m, stepped down per 500-1000 m increment | — |
Read the result the right way around: it is not that the motor's FLC changes. It is that the frame you picked has less headroom left over, so a breaker that was already dialed near its top setting range in a hot enclosure may need the next frame size up, not just a lower dial position. Applying ktemp and kalt to the frame's continuous rating, then checking that the derated value still clears the motor's nameplate FLC with dial range to spare, is the actual selection step. A dial set at its maximum position with no margin left is a warning sign, not a solution.
Altitude: Thinner Air, Less Cooling, Less Dielectric Margin
Above 2000 m, two things move at once. Continuous current capacity falls because convective cooling weakens with air density, and the breaker's insulation and clearance/creepage margins tighten because thinner air breaks down at a lower voltage than air at sea level. A frame rated 690 V at sea level does not automatically hold that margin at 3000 m. High-altitude installations, mine sites, mountain processing plants, high-elevation water pumping stations, need both corrections applied together, not just the current one. Manufacturers publish combined tables precisely because the two effects do not cancel or average out; each has to be read off its own curve and multiplied through.
Enclosure Temperature Rise Stacks on Top of Ambient
Site ambient is only the starting number. A sealed IP66 enclosure in direct sun, packed with VFDs, contactors, and busbar runs, commonly runs 10-20°C above the outside air once everything inside is loaded and running. That internal rise adds directly to whatever correction the site's outdoor or room temperature already required. An MPCB that looks fine on paper at a 35°C site ambient can sit inside a 55°C enclosure microclimate by mid-afternoon, well past the compensated band, with no altitude involved at all.
This is where undersized ventilation, not the breaker, causes the nuisance trip that gets blamed on the device. Forced ventilation, larger enclosure volume, or spacing breakers apart to reduce mutual heating are the fixes; derating the dial down is a workaround, not a cure, because it also reduces the margin against a genuine overload.
Applying Derating When You Select and Setting an MPCB
Start from the motor's nameplate FLC, the same starting point used in general MPCB selection and dial-setting work. Then work through the site conditions in order: measured or estimated ambient at the device, expected internal enclosure rise under full load, and site altitude if it exceeds roughly 2000 m. Pull the correction factors from the frame manufacturer's published curve, multiply them against the frame's continuous rating, and confirm the derated figure still covers the motor's FLC with dial range left over. If it does not, move up a frame size rather than accepting a dial parked at its ceiling.
Trip class selection interacts with this too. A motor with a long start time under a hot, high-altitude condition needs the trip class matched to actual start duration, since derated thermal capacity combined with an under-matched class compounds the nuisance-trip risk rather than isolating it. The same logic carries over to star-delta and soft-start sizing, where inrush duration already eats into thermal margin before temperature or altitude correction is even applied, and to high-inertia, frequent-start applications, where cumulative heating from repeated starts leaves less spare capacity for an already-derated frame.
Frequently Asked Questions
Does every MPCB need altitude derating below 2000 m?
No. Standard reference conditions in IEC 60947-1 already assume an altitude ceiling around 2000 m, so ratings published without qualification hold up to that point. Correction only becomes necessary once the installation sits above the manufacturer's stated threshold.
What happens if temperature derating is ignored in a hot enclosure?
The bimetal reads the extra ambient heat as additional motor current, so the breaker trips below the motor's actual FLC. The result reads as a nuisance trip on start or under normal running load, even though nothing is wrong with the motor or the wiring.
Does the magnetic trip need derating too, or just the thermal element?
The magnetic short-circuit trip is a fixed instantaneous setting and is far less sensitive to ambient temperature than the bimetal. Altitude affects it indirectly through dielectric margin at higher voltages, but the day-to-day derating conversation is almost entirely about the thermal element.
How much correction is typical above 2000 m altitude?
It varies by frame and manufacturer, and the only reliable number is the one on that frame's published curve. As a general pattern, both current capacity and voltage/dielectric margin step down in increments as altitude climbs past the standard reference point, rather than falling off a single cliff at exactly 2000 m.
Can oversizing the frame avoid the derating calculation entirely?
Oversizing buys margin but does not remove the need to check the numbers. A larger frame still has its own compensated band and its own derating curve, and setting the dial correctly to the motor's FLC still depends on confirming the derated rating clears that FLC at site conditions.
Is enclosure internal temperature or outdoor ambient the number to use?
Use the temperature at the breaker's actual mounting location inside the running enclosure, not the outdoor or room ambient. A well-loaded, poorly ventilated cabinet routinely runs well above the surrounding air temperature.
Conclusion
Temperature and altitude derating exist because the bimetal and the frame's insulation both respond to the physical environment, not just to motor current. Inside the compensated band and below roughly 2000 m, most installations need no correction at all. Outside either boundary, pull the manufacturer's derating curve, apply the temperature and altitude factors to the frame's rated current, and confirm the result still covers the motor's FLC with room on the dial. Treating a nuisance trip as a device fault, when it is really an uncorrected enclosure microclimate or an unaccounted altitude, wastes time that a five-minute check against the derating table would have saved.
