Two transformers on the same 1,000 kVA duty can carry completely different thermal margins, occupy different footprints and cost different amounts to run — and the only place that difference appears on paper is the four-letter cooling code. Buyers who skip it usually discover the problem at commissioning, when the supplied unit delivers its 1,000 kVA only with fans running, and there is no auxiliary supply available for them.
This guide decodes the transformer cooling class system defined in IEC 60076-2, explains what each class actually buys you in capacity, noise and maintenance, and sets out how to specify the class so the quotation matches the duty you need.
A transformer’s rated power is not a property of its core and windings alone. It is the power at which the unit stays within its permitted temperature rise, in a defined cooling environment. Change the cooling, and the same active part carries a different rating.
That is why a nameplate can read 1,600/2,000 kVA. The winding is one design; the second figure exists only because fans run. Lose the fans — a tripped auxiliary breaker, a failed controller, a fan motor bearing — and the permitted loading silently drops by a quarter. If nobody has written down that relationship, nothing trips and nothing alarms; the insulation simply ages faster until it fails years earlier than expected.
For liquid-immersed transformers, IEC 60076-2 identifies the cooling method with four letters in a fixed order: internal cooling medium, internal circulation, external cooling medium, external circulation.
| Position | Meaning | Letters you will see |
|---|---|---|
| 1st | Internal cooling medium | O mineral oil or synthetic liquid with fire point ≤300 °C · K liquid with fire point >300 °C (natural or synthetic ester) · L liquid with no measurable fire point (silicone) |
| 2nd | Internal circulation | N natural thermosiphon · F forced by pump · D forced and directed into the windings |
| 3rd | External cooling medium | A air · W water |
| 4th | External circulation | N natural convection · F forced (fans or water pump) |
Read left to right and the code becomes a sentence. ONAN is oil natural, air natural: no pump, no fan, nothing to power. ODAF is oil directed, air forced: a pump pushes oil through ducts straight onto the winding hotspots while fans blow air across the radiators.
| Class | Auxiliaries | Where it is used | Practical notes |
|---|---|---|---|
| ONAN | None | Distribution transformers up to a few MVA; the default for pole-mounted and pad-mounted units | Silent, nothing to fail, lowest maintenance. The base rating for every other class |
| ONAF | Radiator fans | Dual-rated distribution and medium power units, typically 1,000–30,000 kVA | Adds 15–25% on oil units depending on size; fans are temperature-controlled and add noise |
| OFAF | Oil pump + fans | Large power transformers where natural oil flow can no longer reach the winding | Pump flow substantially raises the oil-side heat transfer coefficient |
| ODAF | Directed pump + fans | Large and extra-high-voltage power transformers | Oil is guided into winding cooling ducts; the most efficient air-cooled arrangement per kilogram of active part |
| OFWF / ODWF | Pump + water heat exchanger | Hydro, large industrial and where a cooling water circuit already exists | Compact and quiet, but oil pressure must always be kept above water pressure to prevent ingress |
| KNAN / KNAF | None / fans | Ester-filled units indoors, in tunnels, near water or in fire-sensitive buildings | Ester fire point is typically above 300 °C, so the fire load is materially lower than mineral oil |
For most distribution projects the real choice is between ONAN and dual-rated ONAN/ONAF. OFAF and above belong to power transformers where natural circulation physically cannot move enough heat.
Dry-type transformers use a two-letter code because there is no internal liquid: AN (air natural) and AF (air forced). Some manufacturers also declare the enclosure class alongside — for example AN/AF in an IP20 ventilated casing, or an encapsulated unit rated for outdoor service.
Two things differ importantly from the oil side:
How the enclosure and protection level are declared, and what the options mean in practice, is covered in the enclosure and IP rating guide.
A dual-rated unit is written as two figures, for example 1,600/2,000 kVA ONAN/ONAF. The first is the rating with no auxiliaries running; the second applies while the fans are energised.
The economics are attractive: the core and windings for 2,000 kVA already exist in a 1,600 kVA ONAN design, so the incremental cost is fans, a controller and a fan-ready tank. You are buying future headroom at a fraction of the cost of a larger unit — and in many designs the fans can be retrofitted later if the tank was prepared for them.
The catch is that the second rating depends on equipment that can fail. Three specification points prevent that from becoming an operating risk:
A cooling class is meaningless without the temperature rise it is declared against. Two units with the same kVA and the same cooling code can carry different ratings if one is built to a 55 K rise and the other to 60 K.
| Type | Parameter | Typical limit |
|---|---|---|
| Oil-immersed, class A insulation | Top-oil rise over ambient | 60 K (55 K is a common cooler-running option) |
| Oil-immersed, class A insulation | Average winding rise | 65 K |
| Dry-type, class 105 (A) | Average winding rise | 60 K |
| Dry-type, class 120 (E) | Average winding rise | 75 K |
| Dry-type, class 130 (B) | Average winding rise | 80 K |
| Dry-type, class 155 (F) | Average winding rise | 100 K |
| Dry-type, class 180 (H) | Average winding rise | 125 K |
The rise limit and the insulation class are two halves of one thermal design, which is why they are best specified together. The relationship between class, rise and expected life is set out in the insulation class and temperature rise guide; a lower rise on the same insulation class means a longer life at the cost of more material.
Ratings assume a maximum ambient of 40 °C, a daily average of 30 °C and a yearly average of 20 °C, at altitudes up to 1,000 m. Every one of those assumptions is a lever on the delivered rating.
| Condition | Effect on the rating | What to do |
|---|---|---|
| Ambient above 40 °C | Rating must be reduced; a 50 °C room costs roughly 8–12% of nameplate kVA | Derate in the specification, or ventilate the room to bring the ambient down |
| Altitude above 1,000 m | Air density falls, so air cooling and external insulation both weaken | Apply the altitude correction factor from IEC 60076-1 and state the site altitude |
| Enclosed room with no free ventilation area | Hot exhaust air recirculates and the effective ambient climbs well above the room thermostat reading | Size louvres for the total loss in kW, and keep outlet and inlet separated |
| Radiators close to a wall | Restricted airflow cuts the effective cooling and invalidates the class | Respect the manufacturer’s clearance; the dimension and clearance chart gives the figures per size |
The room is not a site detail to be sorted out later. A cooling class is a promise about heat transfer, and it only holds if the installation lets the heat leave the building.
Fans and pumps change three operating characteristics, and all three appear in the tender evaluation whether or not they are priced:
Hydro and large industrial projects justify the complexity because the alternative is a physically larger transformer. A 1,600 kVA commercial substation rarely does.
If the transformer rating itself is still open, settle that first — the cooling class is a modifier on a rating, not a substitute for the load calculation. The kVA sizing method covers the five steps that produce the number.
A cooling specification that cannot be misread contains five items:
| Item | Example wording |
|---|---|
| Cooling class and rating | 1,600 kVA ONAN / 2,000 kVA ONAF, IEC 60076-2 |
| Temperature rise | 60 K top oil, 65 K average winding (or 55 K if a cooler-running design is required) |
| Reference conditions | 40 °C maximum ambient, altitude 1,200 m, indoor ventilated substation |
| Auxiliary supply | Fans 400 V three-phase from a monitored circuit, controller with alarm contacts |
| Sound level | Declared dB(A) at 1 m in ONAN and in ONAF, measured to IEC 60076-10 |
Add the enclosure or protection level for dry-type units, and state whether the site is indoor or outdoor. For oil-immersed units, the liquid itself is worth naming: mineral oil versus ester changes the first letter of the code and the fire risk assessment, and the cost difference is real. The oil-immersed transformer overview covers what that choice changes in cost and installation.
The four letters are a contract about heat. ONAN means the unit delivers its rating with nothing running; ONAF means a quarter more capacity exists only while the fans do. Specify the class, the temperature rise, the reference ambient and the auxiliary supply together, and the quotation will match the duty. Specify a kVA rating alone and you are leaving the most consequential variable to the supplier’s default.
If you have a load profile and a room, the engineering team can confirm which cooling class actually delivers the required continuous rating in that space — including the case where dual rating is unnecessary and the simpler ONAN unit is the better buy.
ONAN stands for Oil Natural, Air Natural: the insulating oil circulates by natural convection and the radiators dissipate heat to ambient air without fans. It is the base rating of most distribution transformers, requires no auxiliary power and has no cooling equipment to maintain.
On oil-immersed distribution and medium power transformers, fans typically add 15–25% to the ONAN rating depending on size and radiator design; larger units sit toward the higher end. On dry-type transformers the equivalent AN to AF step is larger, commonly around 33%. Always use the manufacturer’s declared figures rather than a rule of thumb, because the increment depends on the specific radiator and fan arrangement.
No. The ONAF figure is valid only while the fans are running. With fans stopped, the unit reverts to its ONAN rating, and operating above it accelerates insulation ageing without necessarily tripping anything. Protection should reduce the permitted load, or alarm, when fan operation is lost.
AN is simpler, quieter and has no auxiliary supply to maintain, so it is preferred wherever the rating fits. AF is worth specifying when the load peaks for limited periods and the additional capacity defers a larger unit, or where a compact footprint matters more than fan maintenance. In both cases the declared rating must correspond to the actual enclosure, because a sealed casing reduces heat rejection.
Yes. Above 1,000 m the lower air density reduces both the cooling capability and the external insulation strength, so the unit must be derated or the design adjusted. State the site altitude in the enquiry; a manufacturer quoting to nameplate at sea level is quoting a rating the site cannot use.
