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Views: 0 Author: Site Editor Publish Time: 2026-10-23 Origin: Site
A transformer quotation gives two capacity values beside ONAN/ONAF. Which value should you use when checking the available capacity?
The answer depends on the cooling state your project can rely on. The larger number is not available simply because it appears in the quotation.
Understanding ONAN and ONAF transformer ratings helps buyers compare capacity, auxiliary supplies, and operating limits. This guide explains the dual rating, rather than providing a general transformer cooling survey.
ONAN uses natural oil circulation and natural air cooling.
ONAF retains natural oil circulation but uses fans for external air cooling.
Two stated capacities belong to different cooling states; they are not added together.
The higher rating depends on the approved cooling arrangement and service conditions, not a universal uplift percentage.
In an oil-immersed transformer, the insulating liquid also carries heat away from the windings and core. The tank and cooling surfaces then transfer heat to the surroundings.
For these cooling codes, the second letter identifies liquid circulation. The last letter identifies movement of the external cooling medium.
Both ONAN and ONAF contain N in the second position. Oil circulation remains natural in both modes.
The change is at the air side:
ONAN uses natural air convection. ONAF uses forced air, normally supplied by cooling fans.
When comparing oil-immersed transformers, keep the cooling designation beside its corresponding capacity. “Fan cooled” without a stated rating leaves the available duty unclear.
The F at the end of ONAF refers to external air movement. It does not describe an oil pump.
OFAF is a different designation. Its second letter identifies forced liquid circulation, while its final letter identifies forced external cooling.
Do not treat the two systems as interchangeable when comparing their equipment or auxiliary requirements.
Consider this hypothetical example, not a particular manufacturer’s product:
Cooling state | Assigned capacity in this example | Cooling basis |
|---|---|---|
ONAN | 1,000 kVA | Natural oil circulation and natural air cooling |
ONAF | 1,250 kVA | Natural oil circulation and the specified fan-assisted air cooling |
This is one transformer with two operating ratings. It is not a 2,250 kVA transformer, and the values do not represent two independent transformers.
In this example, the higher rating is 25% above the ONAN rating. That percentage comes only from these assumed values. It is not a rule for other transformers.
Also distinguish the higher assigned rating from an emergency overload allowance. Operation beyond the applicable rating requires a separate assessment.
If the required continuous duty depends on ONAF, the project also depends on the specified fan system.
That dependency includes more than the fan motors. Controls, auxiliary supply, radiator condition, airflow, and the approved service conditions must support the duty.
An ONAN requirement may be appropriate where the load must be carried without relying on cooling fans. Define that requirement explicitly instead of expecting the supplier to infer it.
Forced airflow can improve heat transfer from the external cooling surfaces. This can support a higher assigned capacity when the transformer is designed for that duty.
However, the windings, leads, bushings, terminals, and other relevant components must also support the resulting current.
That is why adding fans does not automatically authorize a capacity increase.
A transformer prepared for future forced-air operation is different from a standard ONAN unit with an improvised fan installation. The future rating and required equipment need engineering confirmation.
The same distinction applies to an existing unit. A cooling upgrade may be possible, but its achievable capacity is a design question.
Changing from ONAN to ONAF does not change the winding turns ratio.
Cooling stages and tap-changing arrangements are separate features. Fans do not make an off-circuit tap changer suitable for operation while energized.
Keep capacity control, voltage adjustment, and protection requirements separate in the technical schedule.
An ONAF rating depends on the defined forced-air cooling being available. Loss of that cooling requires the approved operating response.
Where an ONAN rating is assigned, it provides a reference for natural-cooling capability under its stated conditions. It is not a blanket guarantee for every abnormal condition.
For example, damaged radiators or restricted airflow can affect cooling even when the fans are not being used.
Do not assume that the existing load can continue unchanged after fan failure. Equally, do not invent a universal number of minutes before action is required.
The response depends on loading, temperatures, ambient conditions, equipment condition, and the approved protection strategy.
Losing half the fans does not automatically mean half the additional ONAF capacity remains.
Fan grouping, radiator layout, airflow distribution, and control arrangements affect the result. Obtain the permitted response for the actual configuration.
If reduced-duty operation after a cooling failure is important, include that condition in the specification before ordering.
A specified ONAF capacity is not useful if the necessary auxiliary supply is missing from the installation design.
Identify the required supply voltage and frequency, its source, and responsibility for providing it. Include any restrictions during generator operation or supply transfer.
The control description should explain how cooling stages are started and stopped. It should also identify the relevant temperature signals, alarms, and failure indications.
A command to start a fan is not the same as evidence that the required cooling is operating. Agree on the monitoring and feedback needed by the project.
Do not copy temperature settings from another transformer. Settings must match the supplied design and approved control scheme.
A natural-cooled capacity from one offer should not be compared directly with a fan-assisted capacity from another.
Start with the load that must be supplied and the cooling state permitted for that duty. Then compare both offers against the same ambient conditions, temperature-rise requirements, and installation assumptions.
Include fan power and the relevant noise requirements. A comparison of operating costs should not ignore cooling auxiliaries.
Ask what thermal evidence supports each assigned rating. Clarify the applicable standard, assessment basis, and required test documentation.
When evaluating an oil-immersed transformer manufacturer, connect the proposed cooling equipment with the capacity guarantee. A general statement that fans increase output is not enough.
Record the required ONAN and ONAF capacities separately. Identify the associated cooling equipment and any future provision.
Also state whether the higher capacity depends on all specified fan groups, and what response is required if cooling becomes unavailable.
Do not leave those questions until commissioning. They can affect the transformer selection, auxiliary design, and site operating plan.
ONAN and ONAF transformer ratings describe capacity under different cooling conditions. Read each kVA value with its matching cooling state.
The higher rating depends on an approved design, functioning auxiliaries, and the stated service conditions. It is neither an automatic fan upgrade nor an unrestricted overload allowance. SNTOOM can discuss oil-immersed transformer configurations against the required natural-cooled and fan-assisted duties.
Comparing ONAN and ONAF capacity for a project?
No universal efficiency ranking follows from the cooling code. Compare the proposed units at the same duty and include auxiliary consumption. The higher available capacity does not, by itself, establish lower energy losses.
No. Two cooling ratings do not create two independent transformers. Any cooling redundancy or reduced-duty capability must be defined separately for the actual arrangement.
No. ONAF retains natural liquid circulation. OFAF uses forced liquid circulation as well as forced external air cooling. Their cooling equipment and auxiliary requirements differ.
They are different scales of the same apparent-power unit. One MVA equals 1,000 kVA. Convert both proposals to the same unit before comparing their cooling ratings.
Not simply because a future rating has been stated. The required equipment must be installed, commissioned, and accepted under the approved design before relying on that cooling state.
