There is no standard size of oil conservator tank for the oil filled transformer. As a thumb of rules, it is by general common sense. Conservator oil tank for the transformer is usually design to have the minimum of oil filled, so that it create a litter bit of atmosphere pressure to maintain the transformer winding of the transformer is fully filled with oil.
The oil conservator tank also to help to filled up the oil inside the transformer, if there is a small oil leak from the transformer otherwise, your transformer will goes up in smoke.
Generally, the oil conservator tank size will depend on the transformer manufacture. Some manufacture design the oil conservator tank to have 5% to 8% of the total oil inside the transformer and some have more than 10%.
Of course, you don't design big conservator tank for small transformer and small conservator tank for the big transformer. It will look funny. Most of the oil conservator tank design are having the same length with the width of the transformer.
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Do the Conservator positioning (either In Top-middle or side) is relevant??? Why does other transformers have their conservator tank placed in side while others are in middle top?