Motor derating for altitude and ambient temperature (IEC 60034-1) — why a motor on a mountain may deliver less power

The guide on replacing a motor and the guide on comparing motor starting methods assume a motor applied under normal, reference ambient conditions. This article covers what changes when those ambient conditions deviate from the standard reference for which the motor's nameplate power was determined: a higher ambient temperature, or a greater installation altitude above sea level.

The reference conditions of IEC 60034-1

IEC 60034-1 (general requirements for rotating electrical machines) specifies the nameplate rated power of a motor for two standard reference conditions:

an ambient temperature up to 40°C;
an installation altitude up to 1,000 m above sea level.

Within these limits, the motor delivers its full nameplate power without exceeding the temperature limit of its insulation class. Outside these limits — in a hotter room, or at greater altitude — nothing physically changes about the motor itself, but its ability to shed the heat generated internally does change.

Why a higher ambient temperature requires derating

A motor cools by giving off heat to its surroundings; the smaller the temperature difference between the winding and the ambient air, the less effective that heat transfer becomes. At an ambient temperature above the 40°C reference value, motor power must therefore be reduced so that, despite the hotter surroundings, the winding still stays within the temperature limit of its insulation class (for example class F or H). Without this derating, the winding insulation runs an increased risk of accelerated thermal ageing and, eventually, failure.

Why greater altitude requires derating — even though it may be cooler there

At greater altitude the air is thinner: air density decreases with altitude. A fan-cooled motor moves a smaller mass flow of air per unit time at the same speed as the air gets thinner, making cooling less effective — even though the ambient temperature at greater altitude is often actually lower than at sea level. Above the 1,000 m reference altitude, motor power must therefore also be reduced, to compensate for the reduced cooling capacity of the thinner air.

Note: the exact derating factors (in percent power per additional 100 m of altitude, or per degree Celsius above 40°C) vary by motor design, cooling method (for example self-ventilated versus separately powered ventilation) and manufacturer. For a specific design, always consult the derating tables or curves of the particular motor manufacturer, not a generic rule of thumb.

The two effects can occur together

A motor applied both at greater altitude and in a hotter environment (for example a mountainous, hot climate zone) experiences both derating effects at the same time. Manufacturers typically publish combined derating tables or curves with altitude and ambient temperature as two axes, rather than simply multiplying the two separate factors.

Practical relevance

When selecting a motor for an installation at significant altitude (for example a mountainous region) or in a structurally hot space (a boiler room, an unconditioned technical room in a hot climate, or an enclosure with limited ventilation), the required shaft power must be determined at the actual, local ambient conditions — not at the standard 40°C/1,000 m reference — and the motor selected accordingly, or, for an already installed motor, the effectively available power adjusted downward.

Common mistakes

Applying a motor's nameplate power without adjustment at a location with an ambient temperature above 40°C or an installation altitude above 1,000 m, without consulting the manufacturer's derating factor.
Accounting only for ambient temperature and forgetting altitude (or vice versa) at an installation where both effects apply.
Assuming a lower ambient temperature at greater altitude compensates for the need for derating — the reduced cooling capacity from the thinner air is a separate effect independent of the ambient temperature itself.
Attributing repeated thermal trips of a motor to a defective motor or incorrect loading, without considering the installation altitude and ambient temperature of the site as a possible underlying cause.