Electric Motor in Enclosed Spaces: Ambient Temperature, Ventilation and Derating

The rated power of an electric motor, the value printed on its nameplate, is valid at a specific reference ambient temperature: usually 40 degrees according to the standards. When you run the motor in an environment above this temperature, it can no longer deliver its full nameplate power; because the temperature difference needed for cooling shrinks and the winding reaches a higher temperature at the same load. When you run the motor inside a panel, cabinet or an enclosed space the problem grows further: the heat produced by the motor and the other equipment around it builds up in the small volume, the room temperature rises, and the motor actually starts to operate in a far hotter environment than its nameplate assumes. The result is an unnoticed overheating and a shortened insulation life.

In this article we cover, from a practitioner's view, the effect of ambient temperature on power derating in motors running in enclosed spaces, panels and cabinets: the power-correction table above 40 degrees, ventilation and forced cooling, the accumulated heat problem, the IC cooling codes and correct sizing for ambient temperature. The aim is to help you predict the real operating temperature and choose the right power before placing your motor in a small, hot volume.

The Relationship Between Ambient Temperature and Rated Power

A motor's power is limited by its ability to reject the heat it produces to the surroundings. The winding can heat up to a maximum temperature allowed by its insulation class; the difference between this temperature and the ambient temperature is the measure of the heat the motor can reject. The reference ambient is taken as 40 degrees, and at this temperature the motor delivers its full nameplate power. As the ambient temperature rises, the "temperature margin" the winding can use up to its allowed maximum shrinks; so the motor must produce less heat, that is carry less load. This means a direct power derating.

Conversely, if the ambient temperature is below 40 degrees the motor can run slightly above its nameplate power; but this upper side also has bearing, shaft and mechanical limits and must not be overdone. In practice the real risk hides on the hot side, above 40 degrees, because the overload there quietly ages the winding.

Power-Correction Table Above 40 Degrees

The table below shows the approximate power-correction (derating) factor to apply versus ambient temperature. These values reflect the typical trend in the standards; for the exact value the motor manufacturer's data should be used.

As shown, in a 60-degree environment a motor rated 30 kW can safely carry only about 24 kW. If your machine requires 30 kW, in such an environment you must either select a motor one size larger or improve the cooling.

Enclosed Space, Panel and Cabinet: The Accumulated Heat Problem

When you place a motor inside a panel, cabinet or a small machine room, the real danger is that the ambient temperature is higher than what you measure. Because:

• The motor's own losses turn into heat and build up in the enclosed volume,
• If the same cabinet holds other heat sources such as a drive, transformer or brake resistor, their heat is added too,
• If air circulation is poor in the tight volume, a pocket of hot air forms around the motor,
• Even if the outside environment is temperate, the cabinet interior can be 15-25 degrees hotter than outside.

So the derating calculation must be based not on the outside ambient temperature but on the actual temperature inside the cabinet where the motor sits. Many failures arise from sizing a motor on the assumption that it is in an "air-conditioned facility" when it is in fact continuously overloaded inside a hot cabinet.

IC Cooling Codes and Cooling Method

The motor's cooling method is defined by IC (International Cooling) codes. The most common are:

• IC411: Cooling from the frame surface by the airflow produced by the shaft-end fan. This is the method of standard TEFC (totally enclosed, fan-cooled) motors. In an enclosed space this fan uses the surrounding hot air, so cooling efficiency drops.
• IC416: Forced cooling by an independent external fan. It provides constant-flow air independent of the motor's own speed; it secures cooling at low speed or in a hot environment.
• IC418: Direct blowing with outside ambient air; used in clean environments free of dust and dirt.

If IC411 is insufficient in an enclosed and hot space, IC416 forced cooling or a ventilation solution that brings fresh air to the cabinet is needed. The temperature of the cooling air directly governs the power the motor can carry.

Correct Sizing and Cooling Solutions

The logic for correctly sizing a motor in an enclosed space is:

• First measure the real temperature inside the cabinet/space or predict it with a heat-load calculation. Include all heat sources (motor, drive, other equipment).
• Find the power-correction factor corresponding to this temperature from the table.
• Calculate the motor power to select by dividing the power required by the machine by this factor. For example, if 27 kW is needed at 50 degrees, 27 / 0.90 = a 30 kW rated motor is required.
• Alternatively, improve the cooling: ventilation, a forced cooling fan (IC416), adding a fan/cooler to the cabinet or moving the motor out of the cabinet.

Often the most economical solution is to add simple ventilation to lower the cabinet temperature; this can be cheaper than stepping the motor up a size and also protects the other equipment in the same cabinet.

Frequently Asked Questions

At what ambient temperature is the nameplate power valid?

According to the standards it is usually valid for a 40-degree reference ambient and an altitude of 1000 metres. Above these conditions (a hotter environment or a higher altitude) the motor cannot deliver its full nameplate power and derating is required.

Which temperature should I use for a motor inside a panel?

Not the outside ambient temperature but the temperature inside the panel/cabinet where the motor actually sits. This temperature can be markedly higher than outside because of the heat that builds up in the enclosed volume; for a correct value, measure inside the cabinet at a point close to the motor.

Is it possible to improve cooling instead of derating?

Yes, and it is often more economical. Adding ventilation to the cabinet, using a forced cooling fan (IC416) or lowering the cabinet temperature lets you use the nameplate power without stepping the motor up a size. Which solution is more suitable is judged by the cabinet structure and the total heat load.

In motors running inside enclosed spaces, panels and cabinets, the right decision starts with a power correction based on the real cabinet temperature, not the outside ambient. As the ambient temperature rises above 40 degrees the motor carries less load; this must be compensated either by selecting a larger motor or by improving the cooling. HEM Motor delivers electric motors across a wide power range from stock and helps you define options such as hot-environment rating and forced cooling (IC416) from the start; share the temperature conditions of your space and let us determine the right power and cooling solution together and prepare a tailored quotation.

Predicting and Measuring the Cabinet Interior Temperature

The basis of correct derating is knowing the real temperature inside the cabinet or enclosed space. You can do this in two ways: measurement and calculation. Measurement is the most reliable; while the motor runs continuously at full load, the steady-state temperature is recorded with a sensor placed inside the cabinet close to the motor. It is important to take the measurement after the motor has finished heating up, usually after several hours of continuous running; the low values in the first minutes are misleading.

In a system not yet built, a heat-load calculation is made. The losses of all equipment in the cabinet (heat in kilowatts) are summed; this heat is balanced against the amount that can be rejected by natural convection from the cabinet surface and by ventilation if present. If the heat rejected is less than the heat produced, the cabinet interior temperature rises above the outside ambient and this difference can be calculated. This calculation depends on the cabinet surface area, material, outside ambient temperature and ventilation flow. The result gives the real ambient temperature the motor will see, and derating is done to this value. In practice, an interior temperature 15-25 degrees above the outside is a common situation in a closed metal cabinet without ventilation.

Effect of Accumulated Heat on Other Equipment

The heat that builds up in an enclosed space affects not only the motor but all equipment in the same volume. The permitted ambient temperature of drives (VFDs) is usually lower than that of motors; most drives apply their own derating above 40 degrees and markedly cut their output current at 50-55 degrees. So when the motor and drive share the same hot cabinet, it is often the drive that limits the system power. The current-carrying capacities of elements such as contactors, fuses and cables also drop at high temperature. So heat management in enclosed-space design must be treated holistically: cooling should be planned by considering not only the motor but the most temperature-sensitive component in the cabinet. Often the right solution is ventilation that brings enough fresh air to the cabinet or a cabinet cooler that controls the interior temperature; this protects the motor, the drive and the auxiliary equipment together and lets the system use its nameplate power.

Insulation Class, Temperature Margin and Life

To fully grasp the importance of ambient temperature, one must look at the relationship between insulation class and temperature rise. The winding insulation has a thermal class such as F or H; this class sets the maximum temperature the winding can withstand. The total winding temperature is the sum of the ambient temperature and the motor's own temperature rise. The temperature rise of a motor designed for the 40-degree reference is chosen so that this sum stays below the insulation limit. When the ambient rises above 40 degrees, the same temperature rise pushes the total to or above the insulation limit; derating is the way to keep this total below the limit by lowering the temperature rise, that is the load. Insulation life is related exponentially to temperature; as a common rule, every 10 degrees of sustained over-temperature roughly halves insulation life. So a motor running without derating in a hot cabinet, even if its nameplate promises ten years of life, can fail far earlier due to unnoticed over-temperature. In some applications, where the ambient temperature is known to be high, selecting a motor with a higher insulation class (e.g. H) is an effective solution that reduces or removes the need for derating. This is part of selecting the right motor for a hot environment from the start.

Related reading: cooling methods IC411 and IC416, high-altitude derating and power reduction, high ambient temperature derating and selection, temperature monitoring with PT100 and PTC and external forced cooling fan.