ONAN, ONAF, AN and AF: Understanding Transformer Cooling Ratings

Quick Takeaway

• ONAN and ONAF both use natural oil circulation. ONAF uses fans on the air side. On a dry-type transformer, AN means natural airflow and AF means forced airflow.
• Use the continuous kVA or MVA documented for each cooling stage under its stated site conditions. A fitted fan can be intended for short-term duty.
• Before relying on a fan-cooled stage, verify the auxiliary supply, controls and alarms, and establish the documented capacity if required cooling is lost.

The cooling letters on a transformer nameplate describe how it releases heat. The accompanying kVA or MVA values tell you how much load each cooling stage supports under its stated conditions. Read the letters and ratings together to establish whether the required load depends on fans. This article decodes each letter by position, matches the stages to their rated capacity and thermal conditions, and explains what to verify before relying on a higher stage.

Read the Four Letters on a Liquid-immersed Transformer

In the naturally circulated, air-cooled example considered here, heat generated in the windings and core must travel outward in two successive stages before it reaches the ambient air. First, the insulating liquid absorbs heat from the winding conductors and iron; it rises toward the top of the tank because it is less dense when warm, and cooler liquid descends to replace it. This natural thermosyphon keeps liquid in continuous circulation without any pump. Second, the liquid releases its heat to the surrounding air through the radiator panels or cooling tubes mounted on the tank wall or on a separate radiator bank. Under natural conditions, air flows upward past the warm radiator surfaces by convection alone.

Under IEC 60076-2:2011, §§4.1–4.2, the cooling designation is built from four letter positions. The first letter describes the internal cooling medium: O means mineral oil or a qualifying synthetic insulating liquid with a fire point no greater than 300 °C. The second letter describes how that internal medium circulates: N means natural, that is, thermosyphon only, with no pump. The third letter describes the external cooling medium: A means air. The fourth letter describes how the external medium is moved: N means natural convection, F means forced circulation by fans.

This gives the two designations considered here:

• ONAN — Oil Natural, Air Natural. The oil circulates by thermosyphon; air flows past the radiators by convection. The natural-cooling stage does not require fans or an oil-circulation pump, though monitoring and other accessories may still need auxiliary power.
• ONAF — Oil Natural, Air Forced. The oil still circulates by thermosyphon exactly as in ONAN; the internal circulation designation remains natural. The difference is entirely on the external side: fans mounted on the radiator bank blow air across the fins, increasing the rate at which heat passes from oil to air. The same oil circulation path is active in both stages.

A common misreading is to assume that the F in ONAF implies a pump for the oil. It does not. The internal code remains ON throughout. F refers only to the external air. Forced internal circulation is identified in the internal-circulation position of the code; it is a different arrangement, not a higher grade implied by the final F.

The nameplate or specification lists the rated power for each cooling method and the corresponding temperature-rise limits. A transformer designed for dual-stage operation carries both designations and both ratings, such as the fictional 8 MVA ONAN / 10.5 MVA ONAF pair used below. Where both are documented continuous ratings, the first applies to natural cooling and the second requires its specified forced-cooling arrangement.

The practical implication is that the fan installation and its auxiliary supply are not optional accessories when the site load requires the ONAF rating. If the fans are absent or their supply is interrupted, use the documented capacity for the remaining cooling arrangement and site conditions. A connected load does not become smaller merely because forced cooling is lost.

A historical buyer example shows this declared-condition principle in practice. Western Power Distribution’s EE SPEC 4/7, May 2015, §4 specified 5 MVA ONAN and 6.25 MVA ONAF continuous ratings at 20 °C, with fans required for the forced-cooling capability. It listed cyclic duties separately and included checks of forced-cooling operation among site tests. This is a historical procurement example, not a statement of current utility policy or the ratings of a SUBIA product.

Read AN and AF on a Dry-type Transformer

In the ventilated dry-type example, the core and windings are not immersed in a liquid-filled main tank. The winding construction may use impregnated or cast-resin insulation. Heat passes through the insulation and exposed cooling surfaces to air moving through passages around the core and coils. The assembly can be open or inside a suitable ventilated enclosure. Other dry-type constructions have different heat paths, so confirm the actual design being offered.

The two dry-type designations considered here are AN and AF. The first letter, A, means air is the cooling medium. The second letter describes how the air moves: N for natural convection, F for forced circulation by fans using the airflow arrangement specified for that unit. The Eaton example below also labels these stages AN (AA) and AF (FA); read the designations in the equipment documentation being compared.

AN — the transformer reaches its rated capacity with air circulating by natural convection alone through the ventilated passages. An AN stage does not require fans to operate, even if the unit has fans fitted for another stage.

AF — fans increase the air mass flow through the coil passages, supporting the forced-air duty declared for the particular design. The same thermal principle applies: more air movement means more heat removal per unit time, which can support a higher declared load when the design and rating conditions allow it. Whether that duty is continuous or temporary must be established separately.

When Does Forced Air Provide a Continuous AF Rating?

As a concrete reference for what the difference between AN and AF looks like in a documented product, Eaton’s secondary unit substation design guide, Table 14.1-60 records a ventilated VPI three-phase unit with continuous ratings at 150 °C winding temperature rise of 1,000 kVA at AN and 1,333 kVA at AF. Both ratings are continuous duty at the stated temperature rise for that specific Eaton product class. The ratio between the two stages in this documented example is specific to Eaton’s VPI design and cannot be applied as a universal percentage to other manufacturers or dry-type technologies.

An important distinction concerns the intended duty of an installed fan. Not every fan option establishes continuous uprating. A Schneider Electric application note for Trihal transformers describes forced ventilation for temporary overloads. A physically present fan therefore does not by itself confirm that a higher continuous rating is available. Establish the documented stage, its capacity and conditions, and the permitted duty from the manufacturer’s specification and order documents.

The practical distinction between option-ready, installed, and documented stage matters at the purchasing stage:

• Option-ready means the transformer design accommodates fans but none are fitted. Use the documented capacity of the supplied arrangement; do not credit a future fan stage before it is fitted and its rating confirmed.
• Installed means fans are physically present, but their documented duty and the conditions under which the AF rating is valid must still be confirmed from the specification.
• Documented stage means the manufacturer has declared a specific continuous or short-term AF rating under stated ambient, altitude, and thermal-class conditions. Confirm separately that the installed fans and controls satisfy the specified commissioning requirements.

Match Each Stage to Its Capacity and Thermal Conditions

A cooling stage rating is not simply a number on a nameplate; it is a number valid only under a specific set of declared conditions. When reading a dual-stage rating, three separate temperature concepts must be kept distinct.

Surrounding cooling-air temperature (ambient) is the temperature of the air at the equipment as defined for the rating, such as the air around the radiator or entering a ventilated assembly. The local temperature inside an enclosure or plant room can differ from outdoor weather readings. Compare the declared reference conditions with the actual installation. If they differ, obtain the manufacturer’s applicable capacity rather than assuming the nameplate figure is unchanged or applying a universal reduction.

Winding temperature rise is the temperature difference between a defined winding measurement and its reference cooling medium under stated conditions. Confirm the measurement basis: average winding rise and winding hot-spot rise are different quantities. The 150 °C rise in the Eaton VPI example is the stated rise basis for that documented rating, not an instruction to treat the hottest point as exactly 150 °C above any site ambient.

Insulation thermal class describes the insulation system’s thermal capability. It is not a stand-alone operating temperature setting or a substitute for the rated winding rise and cooling data. Use the manufacturer’s actual rating and permitted duty to establish load capability rather than inferring extra kVA from the thermal-class label.

Altitude can affect air-cooled heat rejection because air density changes with elevation. An enclosure can also change airflow and local cooling-air temperature. These are design and installation conditions to compare with the declared rating; an appropriately designed enclosure or high-altitude unit need not follow a generic reduction. Request the capability of the complete proposed arrangement, including room ventilation where applicable.

Transformer Capacity When Forced Cooling Is Lost

To make the interaction between stages, site load and cooling availability concrete, consider a fictitious project scenario with the following stated values: a transformer has a declared rating of 8 MVA ONAN and 10.5 MVA ONAF, and the site electrical demand is 9.5 MVA continuous.

The site load of 9.5 MVA sits between the two stages. It exceeds the natural-cooling stage by 1.5 MVA. Under the stated conditions, with all required forced cooling available and the ONAF rating confirmed as continuous duty, the transformer carries the 9.5 MVA load with 1 MVA of margin below the ONAF rating.

If all required forced cooling is lost — fans fail or their auxiliary supply is interrupted — and the documented natural-cooling conditions still apply, the applicable continuous rating in this fictional case is 8 MVA ONAN. The site load of 9.5 MVA is now 1.5 MVA above the documented applicable continuous rating. No partial-fan assumption is made here, because the permitted capacity with a remaining fan group needs its own manufacturer-supported basis. No run time under overload is assumed, because that would require knowledge of the thermal time constant and the starting temperature, neither of which is derivable from the nameplate alone. The correct response under those conditions must be established before commissioning based on the manufacturer’s documentation and the site protection engineer’s review.

The same arithmetic can be illustrated with a separate hypothetical dry-type duty. Suppose a unit with the documented 1,000 kVA AN / 1,333 kVA AF pair is assigned a 1,100 kVA site load under the matching rating conditions. If forced cooling is unavailable and the AN conditions still apply, 1,100 − 1,000 = 100 kVA above the natural-cooling continuous rating. The 1,100 kVA load is invented for this comparison; it is not an additional Eaton rating or permission to operate above AN.

The capacity review must also confirm that associated equipment — cables, busbars, switchgear and downstream protection — is rated for the ONAF or AF load. It is not sufficient to verify the transformer rating alone if the connected cables were sized for the lower stage.

Check Fans, Auxiliary Power and the Loss-of-Cooling Case

Establishing which cooling stage the installed transformer can deliver continuously requires verifying several items that are not visible from the nameplate alone. The following checklist addresses what a buyer or commissioning engineer should obtain before the higher stage is relied upon.

Specified cooling stage and its conditions. Obtain from the manufacturer the continuous rating for each declared cooling stage, the ambient temperature and altitude at which those ratings are valid, and the thermal class of the insulation. Confirm whether the AF or ONAF rating is documented as continuous duty or as short-term or peak-only duty.

Fans: present or future option. Verify whether cooling fans are physically installed or whether the transformer is option-ready. If fans are not installed, do not rely on the future fan-cooled rating. Confirm the documented capacity of the supplied arrangement and the requirements for any later addition.

Auxiliary power supply. The fans require an auxiliary power supply, typically low-voltage single- or three-phase at a specified voltage and frequency. Confirm the declared voltage and frequency from the manufacturer’s documentation and verify that the site auxiliary distribution provides a stable supply at those parameters. The Siemens Energy GEAFOL/CARECO manual, §§6.8 and 6.11 illustrates these requirements for its documented units: commissioning checks include temperature monitoring, correct auxiliary supply voltage and frequency, fan function and direction.

Fan rotation direction. Check that fan operation and airflow direction match the manufacturer’s specified arrangement. Verify and record the required checks during commissioning rather than inferring correct cooling from the presence or sound of a running fan.

Status feedback and alarms. Establish how the site receives fan status, failure indications and loss-of-auxiliary-supply alarms. Agree the required controls and response with the responsible engineer using the documented rating and load. Do not assume the transformer automatically transfers to a safe load or that every installation uses the same alarm or trip logic. Cooling loss affects the ability to serve the connected load.

Control drawings and settings. Obtain the complete control drawings for the fan starting, stopping and alarm logic. Do not rely on verbal descriptions of how the fan controls work.

Site airflow and ventilation. For dry-type transformers in an enclosure or plant room, verify that the ventilation openings, airflow path and room volume provide the declared ambient temperature at the inlet to the transformer under the site’s maximum operating conditions. Adding fans to the transformer does not compensate for a poorly ventilated room. As noted in Schneider Electric’s fan application note, the relevant circuit and environment must support the proposed duty; capacity review includes ventilation, cables or busbars, and protection.

Downstream equipment capacity. If the purpose of enabling the higher cooling stage is to increase the transformer load, confirm that all equipment downstream — cables, busbars, switchgear, protection relays and metering — supports the proposed higher-load operating condition, with appropriate equipment ratings and protection settings.

Documented cooling-loss response. Before commissioning, establish with the site protection engineer and operations team what the site-approved response is if cooling is fully lost. This must be based on the transformer manufacturer’s documented natural-cooling rating, the connected load, and the protection settings, not on assumptions about how long the transformer can run above its ONAN or AN rating.

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For further context on reading other values on a transformer nameplate alongside the cooling code, see How to Read a Transformer Nameplate: kVA, Voltage, Impedance and Connections.

For how cooling type affects the choice between oil-immersed and dry-type transformers in a specific installation, see Oil-immersed vs Dry-type Transformers: Selection by Installation Conditions.

If you are sizing a transformer for a factory expansion where the peak demand changes in stages, the conditional load analysis in Transformer Sizing for Factory Expansion shows how operating schedules change the demand that must be compared with the applicable capacity.

SUBIA’s oil-immersed transformer range and dry-type transformer range provide equipment context. Include the required cooling stages, continuous duty, ambient and altitude in an enquiry, and request the actual capability of the proposed configuration.

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Frequently Asked Questions

Does ONAF mean the oil is pumped?

In ONAF, the N in the second position means the oil circulates naturally. The final F describes forced airflow on the outside.

Does fitting fans increase a transformer’s continuous rating?

Use the higher continuous rating only if it is documented for the transformer and installed cooling arrangement, and the site meets the rating conditions. A fan specified for a peak or emergency may carry a different duty.

Does the 1.5 MVA gap tell me how long the transformer can run?

The gap measures the load above the 8 MVA continuous ONAN rating in the fictional case. A permissible duration requires manufacturer data for the transformer, its load history, starting temperature and remaining cooling.

Can room ventilation replace the fans required for an AF rating?

Room ventilation helps remove heat from the space and maintain the required cooling-air temperature. Use the airflow arrangement specified for the transformer’s AF rating. Fans that move room air do not automatically provide the required airflow through the transformer.