Transformer Temperature Rise and Cooling Methods: ONAN, ONAF and ANAF Explained

The number that decides how long a transformer lasts is not its kVA rating. It is its temperature.

Two transformers can leave the same factory with the same nameplate rating and the same measured losses, and still give very different service lives, because one runs cool and the other runs hot. Insulation does not fail because of age in years; it fails because of accumulated heat. And the rating on the nameplate is not a fixed quantity at all, it depends on how the unit is cooled, what the ambient temperature is, and how high the site is above sea level.

This article explains temperature rise, what the cooling codes on a datasheet actually mean, and how to specify both so that the transformer you buy is the one that survives your site.

Why temperature decides transformer life

A transformer’s insulation system degrades continuously, and the rate of degradation rises sharply with temperature. The classic relationship used in the industry is that insulation life is roughly halved for every six to eight degrees Celsius of additional hot-spot temperature.

That rule is worth pausing on. An increase of six degrees is small enough to be invisible on a datasheet and large enough to take years off the asset. It also works in the other direction: a transformer that runs cooler than its rated rise, because it is lightly loaded or generously cooled, can outlast its nominal design life considerably.

What matters is not the average oil temperature but the hot-spot temperature. The hottest point in the winding insulation, which is always above the measured average. Everything in the cooling and loading discussion that follows is ultimately about keeping that one number under control.

The three temperature numbers on a datasheet

Transformer temperature ratings are expressed as rise above ambient, not as absolute temperatures. This is the single most misunderstood point in the whole subject.

Top-oil temperature rise. How much hotter the oil at the top of the tank is than the surrounding air, at rated load.

Average winding temperature rise. The mean rise of the winding above ambient, usually measured by resistance.

Hot-spot winding temperature rise. The peak rise inside the winding. This is not directly measurable in service, so it is either calculated or inferred.

A common oil-immersed specification is a 60 K top-oil rise and a 65 K average winding rise at rated load, with a maximum ambient of 40 degrees Celsius. Those figures are defined by standards such as IEC 60076-2, and they belong together: a rise figure quoted without the ambient it assumes tells you very little.

The practical consequence is that a 65 K winding rise at a 40 degree ambient site implies a winding running near 105 degrees Celsius. Move the same transformer to a site at 50 degrees Celsius and the permissible rise must fall, or the insulation will run hotter than intended.

Cooling classes explained

The four-letter codes on a datasheet follow a fixed logic. The first letter is the internal cooling medium, the second is the internal circulation method, the third is the external cooling medium, and the fourth is the external circulation method.

ONAN. Oil Natural, Air Natural. Oil circulates by convection and the tank and radiators dissipate heat to the air naturally. No fans, no pumps, nothing to fail. This is the quietest and lowest-maintenance option, and the basis on which most distribution transformer ratings are quoted.

ONAF. Oil Natural, Air Forced. The same oil circulation, but fans blow air across the radiators. The unit carries more load but depends on fans and their control gear.

OFAF. Oil Forced, Air Forced. Pumps circulate the oil as well as fans blowing air, used on larger transformers where natural oil circulation is not enough.

OFWF. Oil Forced, Water Forced. Cooling water carries the heat away, used where space is tight or where water cooling is available.

Dry type transformers use a shorter code because there is no liquid: AN for air natural and AF for air forced, with the same trade-off between quiet simplicity and higher capacity.

The same transformer can carry two ratings

Because cooling can be staged, a single transformer is often specified with two ratings: for example 1,000 kVA ONAN and 1,250 kVA ONAF. The physical unit is identical. What changes is whether the fans are running.

This has real commercial consequences that buyers frequently miss. If your load never exceeds the ONAN rating, you are paying for an ONAF capability you will never use, and you are carrying fans whose failure would be a maintenance item for the life of the asset. If your load regularly exceeds the ONAN figure, you are relying on fans, which means relying on auxiliary power, fan control relays and someone noticing when a fan stops.

The honest question to ask is not which rating is higher but which rating your load actually needs, and whether the margin above it is worth the added complexity.

Ambient and altitude derating

Ratings are quoted for a stated maximum ambient, conventionally 40 degrees Celsius, and for altitudes up to 1,000 metres. Both assumptions are frequently wrong for real export projects.

High ambient. A common rule of thumb for oil-immersed transformers is to derate by roughly one percent of rating for each degree Celsius that the maximum ambient exceeds the design value. A 1,000 kVA transformer rated at 40 degrees Celsius, installed where summer ambient reaches 50 degrees Celsius, therefore has an effective capacity of about 1,000 x (1 – 0.10), or 900 kVA. Consult the manufacturer’s loading guide, since the exact figure depends on the design and the load cycle.

High altitude. Air is thinner at altitude, so it removes heat less effectively and the unit must be derated. Altitude also reduces the dielectric withstand of air gaps, which affects clearances and bushing selection. Sites above 1,000 metres should be stated explicitly at enquiry.

The cost of getting this wrong is asymmetric. Discovering the derating requirement after delivery means either accepting an overloaded transformer or buying another one.

Cooling choice affects more than capacity

Sizing is only one of five consequences of choosing forced cooling.

Noise. Fans are usually the loudest component of a transformer installation. For a hospital, a hotel or a residential-adjacent site, a unit that meets its rating naturally is often worth more than a cheaper one that needs fans.

Auxiliary power. Fans and pumps need a supply, and that supply must remain available when the transformer is at its hottest, which is exactly when the site is drawing its most power.

Maintenance. Fans, pumps, radiators and their control circuits all become items on the maintenance schedule. Natural cooling has almost nothing to maintain.

Footprint. A more efficient cooling system usually means a larger radiator bank and more plan area, which may conflict with a compact substation layout.

Cost. Forced cooling adds capital cost, control gear and a lifetime of maintenance, in exchange for capacity that may only be needed on a handful of days per year.

How to specify temperature rise and cooling

State all of the following in the enquiry, not just the kVA rating:

Maximum and average ambient temperature at the site, in degrees Celsius

The altitude of the site above sea level

Required rating in kVA, and the cooling stage at which that rating is required

Acceptable cooling classes, whether forced cooling is permitted at all

Sound level limit in decibels at the nearest sensitive boundary

Top-oil and winding temperature rise limits, and the alarm and trip settings

Whether a temperature monitoring device is required, and whether it should include fan control

Frequently Asked Questions

What does ONAN mean on a transformer nameplate?

ONAN stands for Oil Natural, Air Natural. The oil circulates by convection and the tank and radiators release heat to the air without fans or pumps. It is the quietest and lowest-maintenance cooling class and the basis for most distribution transformer ratings.

Can a transformer be rated at two different capacities?

Yes. Where cooling is staged, the same physical unit can be given an ONAN rating and a higher ONAF rating, for example 1,000 kVA and 1,250 kVA. The higher figure applies only while the fans are running.

How much does high ambient temperature reduce capacity?

A widely used working figure is about one percent of rating for each degree Celsius above the design ambient. A 1,000 kVA unit rated at 40 degrees Celsius may therefore deliver roughly 900 kVA at a 50 degree site. Confirm the exact derating with the manufacturer.

Which cooling class should I choose for a noise-sensitive site?

Prefer a class that reaches the required rating without forced cooling, such as ONAN or a larger naturally cooled unit. Cooling fans are typically the dominant noise source on an installed transformer and will be audible at the site boundary when they run.

Related Products, Applications and Support

Need help choosing a cooling class or confirming derating for a hot or high-altitude site? Send us your site conditions and load profile, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.