The answer first: read the cooling code letter by letter, and design to a 60 K top-oil rise and 65 K average winding rise (IEC 60076-2), a hot-spot temperature of about 98°C at rated load, and a life that halves for every extra 6°C — ONAN is self-cooled, ONAF adds fans, ODAF adds pumps and directed oil flow
A transformer is never 100% efficient. A modern distribution unit loses roughly 0.5–2% of its rating as heat — the no-load (core) loss plus the load (winding) loss — and every watt of it raises the temperature of the insulation that separates the windings. Insulation, not steel or copper, is the life-limiting component: the paper and pressboard age by a chemical reaction whose rate climbs steeply with temperature. The entire job of the cooling system is to move that heat from the windings to the outside air fast enough to keep the insulation below the temperature at which its life collapses. The code stamped on the nameplate — ONAN, ONAF, OFAF, ODAF, KNAN, or AN/AF for dry-type — is the compact description of how the heat is transported. The answer in one line:
- Top-oil temperature rise is limited to 60 K for mineral-oil transformers, and average winding rise to 65 K (measured by resistance) per IEC 60076-2.
- That 65 K rise plus a hot-spot allowance puts the hottest point of the winding at roughly 98°C at rated load with a 20°C annual-average ambient.
- Insulation life halves for every extra 6°C of hot-spot temperature — the “6-degree rule” from the IEC 60076-7 and IEEE C57.91 loading guides.
- Cooling class scales with rating: ONAN up to roughly 2.5 MVA, ONAF from ~3 MVA to ~60 MVA, and OFAF/ODAF for large power transformers where fans and pumps shrink the radiator bank.
Each part of that answer has a standard behind it, and knowing them is what lets you specify a unit that will not quietly cook itself to an early retirement.
How to read the four-letter cooling code
IEC 60076-2 writes the cooling class as a four-position code — two letters for the inside of the tank, two for the outside:
- Position 1 — internal cooling medium in contact with the windings: O = mineral oil or synthetic liquid with fire point ≤ 300°C; K = insulating liquid with fire point > 300°C (natural or synthetic esters such as FR3 / MIDEL); L = insulating liquid with no measurable fire point.
- Position 2 — circulation of that medium: N = natural (thermosiphon only); F = forced (pumped), but flow through the windings still by thermosiphon; D = forced and directed into at least the main windings.
- Position 3 — external cooling medium: A = air; W = water.
- Position 4 — circulation of the external medium: N = natural; F = forced (fans or pumps).
The common combinations you will actually meet on a nameplate:
| Code | Meaning | Typical rating | How the heat gets out |
|---|---|---|---|
| ONAN | Oil Natural / Air Natural | ≤ 2.5 MVA distribution | Oil rises by thermosiphon to the radiators, cooled by natural air convection. No fans, no pumps, zero auxiliary load. |
| ONAF | Oil Natural / Air Forced | ~3–60 MVA | Fans blow air across the radiators; the oil still circulates naturally. |
| OFAF | Oil Forced / Air Forced | Large power (> 60 MVA) | Oil pumps plus radiator fans; oil is pumped, flow through windings by thermosiphon. |
| ODAF | Oil Directed / Air Forced | Largest power units | Pumps direct oil into the windings for maximum heat transfer; fans cool the radiators. |
| KNAN | Ester (K liquid) Natural / Air Natural | Fire-sensitive indoor / distribution | Identical to ONAN but with a high-fire-point natural ester instead of mineral oil. |
| AN / AF | Air Natural / Air Forced (dry-type) | Dry-type, cast-resin | Air cools the windings directly; AF adds fans. |
The code is worth more than a glance because it tells you two things at once: how much heat the unit can shed and how much auxiliary power and maintenance the cooling plant will demand over its life.
The temperature rise limits that keep insulation alive
Temperature rise is the difference between the coolant (ambient) temperature and the temperature measured in the unit — deliberately expressed in kelvin (K) so the limit is the same in every climate. IEC 60076-2 sets the design limits for a mineral-oil transformer:
| Quantity | Limit (mineral oil, IEC 60076-2) |
|---|---|
| Top-oil temperature rise | 60 K |
| Average winding rise (resistance method) | 65 K |
| Hot-spot winding rise | ≈ 78 K (65 K + hot-spot allowance) |
| Ambient reference (IEC 60076-1) | 40°C max / 30°C monthly avg / 20°C annual avg |
Add the 20°C annual-average ambient to the 78 K hot-spot rise and you arrive at the familiar 98°C hot-spot temperature that IEC loading guides use as the normal design point. North American practice (IEEE C57.12.00) phrases the same idea as a 55°C or 65°C average winding rise over a 30°C ambient, with hot-spot temperatures of 95°C and 110°C respectively. The two systems are not contradictory — they use different ambient references — but you must know which one your specification calls for, because a “65 K rise” and a “65°C rise” are not the same machine.
Dry-type units follow IEC 60076-11 and use insulation thermal classes instead: Class F (155°C) and Class H (180°C), with typical average winding rises of 100 K and 125 K respectively. For more on choosing between the two technologies, see our oil-immersed vs dry-type selection guide.
Hot-spot temperature and the 6-degree rule
The average winding rise is a convenient test measurement, but it is the hot spot — the single hottest point inside the winding — that governs insulation life. Cellulose insulation degrades by an Arrhenius-type reaction: hotter means exponentially faster aging. The loading guides (IEC 60076-7, IEEE C57.91) condense this into a rule of thumb engineers actually use:
- The 6-degree rule: every 6°C of extra hot-spot temperature roughly halves the insulation life; every 6°C below design roughly doubles it.
- Aging acceleration factor ≈ 2(Δθh / 6), where Δθh is the hot-spot temperature above the design point.
- Normal hot-spot limit: about 98°C for cyclic loading; short-time emergency limit of 120°C is tolerated only briefly and is bought with a measurable slice of life.
This is why continuous overload is a silent killer. A transformer loaded 10% above nameplate does not fail today — it just ages its insulation several times faster, and the 30-year unit quietly becomes a 15-year unit. The cooling class does not change that arithmetic; it only determines how large an overload the tank can absorb before the hot spot crosses the limit.
When does forced cooling actually pay for itself?
Fans and pumps are not free. They add capital cost, consume auxiliary power (typically 1–3% of the unit rating in fan/pump load for a large OFAF), introduce moving parts that need maintenance, and can fail — which is why a forced-cooled unit is normally specified with controls that alarm or trip if cooling is lost. The economic trade-off:
- ONAN is the default for distribution (up to ~2.5 MVA): zero auxiliary load, nothing to maintain, and the radiators are sized to shed full-load heat by convection alone.
- ONAF wins where footprint and weight matter (3–60 MVA): fans roughly double the heat a given radiator surface can shed, so the tank and radiator bank shrink for the same rating.
- OFAF/ODAF are the territory of large power transformers, where the marginal cost of fans and pumps is small next to the savings in tank size, oil volume and transport weight.
- Lower losses mean less heat to shed in the first place. A low-loss design — an S13/S20 loss grade or an amorphous-alloy core — runs cooler for the same cooling class, which is part of why the loss grade you choose is a cooling decision as much as an efficiency decision.
Ambient conditions also move the answer. A unit designed for a temperate 20°C annual average will not meet the same rise limits in a 40°C desert or at 3,000 m altitude unless it is derated or over-cooled — the topic covered in our high-altitude & high-temperature derating guide. When the environment is hotter than the standard reference, the cheapest fix is often to specify a lower rise (for example a 55 K unit where IEC would allow 65 K) or a forced-cooling stage, rather than to let the hot spot eat the insulation life.
Specifying cooling correctly, first time
Get these five things right and the cooling system will never be the reason a transformer retires early:
- State the rise in K, not °C, and name the standard — “65 K per IEC 60076-2” means something precise; “65°C rise” alone is ambiguous across IEC and IEEE.
- Fix the ambient conditions (max, monthly-average and annual-average temperature, plus altitude) so the rise limits are evaluated against the real site, not a default.
- Choose the cooling class by rating and duty, and confirm the auxiliary power and control scheme (fan/pump auto-start, cooling-loss alarm) are in the scope of supply.
- Ask for the heat-run test in the routine test report — it verifies the top-oil and winding rises against the nameplate, which is the only proof the cooling design actually works.
- Match the loss grade to the cooling class: the less heat the core and windings generate, the more margin the cooling system has for overload and hot climates.
QDTB supplies oil-immersed and dry-type transformers with cooling classes and rise limits specified to your site, including amorphous-alloy low-loss designs that run cooler for the same rating. Use the QDTB Engineering Toolbox to size the kVA and voltage class, then contact us with your ambient conditions and duty profile for a cooling-specified quotation with a full routine test report.