Transformer AN vs AF (2026)

Hi Buddy, need your guidance on this:-

For each transformer, there is 2 rating involved , AN and AF. Lets say AN/AF = 2.0/2.5MVA. Shall there be any difference with the transformer with 2.0/3.0MVA?
What is the application difference ?
Is there any rule of thumb indicating the AF rating should be?

Thanks.

Forced air (ONAF) rating typically is about 50% greater than the naturally cooled (ONAN) rating. It depends on variables such as radiator design, oil circuit design internally, and fan power. The increase is therefore very dependent on the individual design. For example, the limiting factor could be an internal hot spot on the winding or core where cooling by the oil is marginal because flow is limited. Adding fans does little to help this situation and may lead to a performance improvement rather less than the typical 50% value.

----------------------------------

Sometimes I only open my mouth to swap feet...

Thanks ScottyUK !
Can you share with me more on the AN/AF things?

What is the usage of having Forsed air cooling rating to be 150% of natural cool rating?
Shall there be price difference between a transformer with 50%AF rating compare to a transformer with 20%AF rating only?
When i specify the transformer for purchase, shall the AF rating be stated?If without the AF rating stated,will caused any issue?

Thanks.

The forced cooled rating allows the transformer to handle more kVA at a very small increase in cost.

If you want the transformer to have fans and a forced-cooled rating, you must specify this and pay for it.

Another option is to specify the the transformer be self-cooled but have provisions for addition of fans in the future.

The percentage increase in kVA rating for forced air cooling is largely a function of the transformer size and construction. There is a limit to the increase possible, as mentioned.

For ANSI transformers, this is a standard value - I'm not sure about IEC transformers.