However safely an IE4 motor runs at a sea-level plant, running that same motor at the same power at a mine or highland facility at 1500 metres may not always be possible. The reason is not the motor’s quality but physics: as altitude rises, the air thins, cooling weakens and the power the motor can draw drops. This phenomenon is called “derating” (power reduction). In this article we cover the cause of derating at high altitude in IE4 motors, how it is calculated above 1000 m, the combined effect of altitude and ambient temperature, the frame upsizing solution and correct sizing for high-altitude facilities.

Derating and power reduction curve at high altitude for an IE4 motor

The Standard Reference: 1000 Metres and 40 Degrees

The rated power on an electric motor’s nameplate is given for a standard ambient condition: usually up to 1000 metres altitude and up to 40 °C ambient temperature. Under these conditions the motor can deliver its nameplate power continuously without exceeding its temperature class. When conditions fall outside this reference — higher altitude or hotter ambient — the continuous power the motor can deliver decreases. The ambient-temperature side of derating is covered in derating at high ambient temperature, and the overview that treats both together in high-altitude and hot-environment motor selection.

What Changes as Altitude Rises? Air Thinning

As you go higher, atmospheric pressure and air density fall. The motor’s cooling is largely provided by air passing over the frame (IC411-type surface cooling). When the air thins:

  • the same volume of air carries less heat,
  • the cooling effect of the air blown by the frame fan decreases,
  • the motor heats up more at the same load,
  • power must be reduced to avoid exceeding the insulation class temperature limit.

So at high altitude the problem is not “the motor loses power” but “the motor cannot cool enough at the same power”. Cooling methods are explained in electric motor cooling methods (IC411/IC416), and the effect of fan design on efficiency in cooling and fan design in IE4 motors. For the insulation class’s relation to temperature, see insulation class and cast iron.

How Is Power Reduction Calculated Above 1000 m?

The general engineering rule is: no derating is needed up to 1000 metres. Above 1000 metres, a gradual reduction in the continuous power the motor can deliver is applied for each additional 100 metres. A practical approach is to assume a small percentage loss for each 100 metres above 1000 m and total this loss for the chosen altitude. At high altitudes such as 2000-3000 m, for example, this loss reaches significant levels and may require moving the motor up to a higher power or a larger frame. The exact value is taken from the motor manufacturer’s derating table; therefore the altitude must always be shared at the quotation stage on high-altitude projects. The articles at what load to run the motor and motor power calculation help you manage the remaining power margin after derating.

Altitude and Ambient Temperature: A Combined Factor

High altitude rarely comes alone; at the same site the ambient temperature may also differ from the reference. The two factors are evaluated together with a multiplying effect:

  • High altitude + hot ambient: both deratings apply together, total power reduction increases.
  • High altitude + cold ambient: low ambient temperature can offset part of the altitude loss (stated in manufacturer tables).

Therefore, in places like high mountain plants, mines on high plateaus and mountain-top pump stations, both factors must be taken into account at once. For the effect of cold ambient on the motor, see cold-environment fan and compressor motors, and for hot-dusty environments rolling mill and foundry heavy duty.

Compensating derating with frame upsizing for an IE4 motor at a high-altitude facility

The Solution: Frame Upsizing and Correct Sizing

The most common way to compensate for derating is to meet the required shaft power with a larger frame (a higher power class). The motor then delivers the required continuous power despite the altitude power reduction, without exceeding its temperature limit. In practice there are two approaches:

  • Selecting a higher power/frame: the motor is chosen so that the derated power for the site altitude stays above the requirement. For the IE4 frame-power relationship, see frame size and power matching.
  • Strengthening cooling: in some applications an external forced cooling fan helps (especially at low speed). See external forced cooling fan.

The low-loss advantage of IE4 super premium motors helps here too: producing less heat at the same power, an IE4 motor loses less power margin after derating. Where IE4 efficiency losses are reduced is explained in efficiency losses in IE4 motors.

High-Altitude Facilities and IE4 Selection

Typical facilities requiring high altitude: mountain and highland mines, quarries on high plateaus, mountain-top water pumping stations, high-altitude ski and tourism facilities, greenhouses and farms in mountainous regions. At these facilities the IE4 motor’s efficiency advantage both lowers energy cost and eases derating management through lower heat production. Which application requires IE4 is covered in the IE4 threshold in pumps, fans and compressors, and sector priorities in which facility should move to IE4 first. For mining specifically, stone quarry and mine motor protection and for irrigation irrigation and agricultural pump motors are useful.

Supply and Project Planning

Since derating at high altitude can move the motor up to a higher power, lead-time, shipping and commissioning planning become important. For high-power and project supply, see IE4 premium motor supply and high-power supply above 90 kW. For a wider range, see our IE4 electric motors category and our home page for all models.

Insulation Class and Temperature Rise: Where Is the Derating Limit?

The real limit behind derating is the temperature the motor’s insulation class can withstand. As the motor runs, heat forms in the windings, adding a “temperature rise” (delta T) on top of the ambient temperature. The insulation class (e.g. class F) defines a certain total temperature limit; a safety margin is usually left in the design. When cooling weakens at high altitude, this temperature rise grows, and power is reduced to avoid exceeding the limit. Therefore, on high-altitude projects, a higher-temperature insulation (e.g. class H) or a design with lower temperature rise can reduce the derating margin. The effect of insulation class on life and durability is detailed in winding and insulation class (F/H), and temperature rise in temperature rise class (delta T 80K). To see the risk early by monitoring winding temperature, see temperature monitoring with PT100 and PTC thermistor.

Running at Low Speed with a VFD: An Additional Source of Derating

High altitude is not the only source of derating. If the motor is driven at low speed by a variable frequency drive, the cooling air turned by the frame fan decreases, and the motor cools less while delivering the same torque at low speed. So when high altitude + low-speed operation come together, the total derating increases. In that case an external forced cooling fan becomes critical; see external forced cooling fan. The constant/variable torque distinction in variable-speed applications is explained in motor selection in variable-speed applications, and general VFD use in VFD with an asynchronous motor.

A Common Mistake: Not Stating Altitude at the Quotation Stage

The most common problem on high-altitude projects is not sharing the altitude at order time and shipping a motor sized for sea level. Such a motor heats up more than expected at high altitude, protection trips or its life shortens. The way to avoid this mistake is to clearly state the facility’s altitude and maximum ambient temperature at the quotation stage. The information needed for a correct quote is listed in information to provide when requesting a quote, and common purchasing mistakes in mistakes made when buying an electric motor. To prevent the wrong motor from arriving, also see nameplate matching.

Frequently Asked Questions

At exactly what altitude does derating begin?

The generally accepted reference is 1000 metres. Up to 1000 metres the standard condition is assumed and no derating is applied; above this height, a gradual reduction in the continuous power the motor can deliver is taken into account for each additional 100 metres. The exact value comes from the manufacturer’s derating table.

If the altitude is high but the ambient is cold, do I still need to reduce power?

A cold ambient can offset part of the altitude loss, because as ambient temperature falls the motor’s heating margin grows. The two factors are evaluated together and the net derating is determined from the manufacturer’s table. So the best practice is to share both altitude and ambient temperature at the quotation stage.

Instead of derating, could I just strengthen the cooling?

In some applications an external forced cooling fan can help, but in most cases the safest and most permanent solution is to move the motor up to a higher power/frame. The best approach is to determine the sizing for altitude and temperature together with your supplier.

A Practical Example: A Pump Motor on a High Plateau

Consider a scenario: an IE4 motor is needed for a water pumping station at about 2500 metres altitude, and the shaft power the application requires is known. At this altitude, when the gradual power reduction foreseen for each 100 metres above 1000 m is totalled, the continuous power the motor can deliver decreases meaningfully compared with sea level. If the ambient temperature is also above the reference, the two deratings apply together and the total loss grows further. In this case the safest solution is to move the motor up to a higher power/frame so that it comfortably meets the required power even after derating. Conversely, if the ambient is cold at the same altitude (e.g. a high mountain climate), the low temperature can offset part of the altitude loss and a smaller frame may suffice. The exact value is always taken from the manufacturer’s derating table. This example makes concrete why altitude and temperature must be reported together and at the quotation stage. Pump power calculation is completed by motor power calculation, the deep-well application by deep-well pump motor selection, and the irrigation side by irrigation and agricultural pump motors. The effect of efficiency-class choice on payback is covered in IE4 or stay with IE3.

Get a Quote

Share your facility’s altitude and ambient temperature; we will calculate the derating and recommend the IE4 motor with the right frame to safely deliver the continuous power required at high altitude. For a fast quote, call +90 (532) 345 49 86 or write to us via our contact page.

Purchasing and Selection Checklist

  • Has the facility altitude (metres) been determined?
  • Has the ambient temperature (maximum °C) been determined?
  • Has the combined altitude + temperature derating been calculated?
  • Does the remaining power after derating meet the requirement?
  • If needed, has a higher power/frame been selected?
  • Is the cooling method (IC411/IC416, external fan) suitable?
  • Have the insulation class and temperature limit been checked?
  • Have lead-time, shipping and commissioning been planned for high altitude?