Basis for Sizing of ONAF Rating.

do you know what is kVA ,ONAN and ONAF??....

These terms essentially refer to the cooling functions used for a given transformer.

ONAN mean "Oil Natural Air Natural", while ONAF means "Oil Natural Air Forced"

A given transformer core will have a lower kVA rating with a ONAN configuration then the same core with a ONAF configuration as it is able to dissipate less heat.

Go with the ONAN version, why? Less gadgets to fail and if additional load is required in the future, the addition of fans will increase the kVA value of the core.

As to why you would want to specify ONAF, if there is some reason why natural air flow is impeded, say in an equipment vault or in some other area where air flow would be restricted, then ONAF is a wise choice.

See this thread: http://cr4.globalspec.com/thread/43826 (especially the comment # 2).

I am sure you will get what you need.

- MS

Thanks MS I also have the same opinion of non continuous load, future expansion etc with your referred thread. I wanted to verif from my learned friends.

Thank you again.

You have mentioned continuous load is 1200KVA.This I presume will be the ultimate load on MCC.A prsent you have 100KVA and 700KVA stand by.Is this correct.If so we can go to next step.ONAN is the basic rating of the transformer without extra cooling as mentioned by forum members.ONAF is selected whre there is a possibility of increase in the connected load . One basic fact has to be kept mind .Transformers have thier highest eff near 50%