Transformer Cooling Methods: ONAN, ONAF and OFAF
Reviewed by the Fengguang engineering team · Last updated 8 Nov 2026
Cooling method sets the usable capacity, noise, maintenance and footprint. Compare ONAN, ONAF and OFAF against the real ambient and load profile.
ONAN: natural oil, natural air
ONAN relies on natural oil circulation and natural air movement over the radiators. It has no fans or pumps, so it is quiet and low-maintenance. It is the default for many distribution transformers, but the usable capacity is limited by the ambient temperature and radiator surface.
ONAF and OFAF: more capacity, more systems
ONAF adds fans to increase cooling and short-time capacity. OFAF adds forced oil circulation and is used on larger transformers or where the cooling margin is tight. Fans and pumps add auxiliary power, noise and maintenance, and they need a control and protection scheme.
Match cooling to the site
At 50 °C ambient, a unit that is comfortable at 40 °C may need a larger cooling surface or forced cooling. At high altitude, air density falls and cooling performance changes. The datasheet should state the cooling stage and the ambient basis for each rating.
| Cooling | Advantages | Watch-outs |
|---|---|---|
| ONAN | Quiet, simple, low maintenance | Larger radiators / footprint |
| ONAF | Higher short-time capacity | Fan power, noise, maintenance |
| OFAF | High capacity, compact | Pumps, controls, auxiliary supply |
| AN / AF (dry) | No oil, fire-safe | Room airflow and filters |
FAQ
Can a transformer run without fans at rated load?
Only if the nameplate rating is based on natural cooling. If the rating depends on fans, loss of fans reduces the permissible load.
Does forced cooling increase noise?
Fans and pumps add noise. If the site has a noise limit, specify the sound level with the cooling stage running.
Which cooling is best for a hot site?
It depends on the ambient, load profile, footprint and maintenance capability. Sometimes a larger ONAN unit is more robust than a smaller ONAF unit with fans.
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