What operating oil temperature ranges are acceptable in transformers?

Acceptable ranges depend on design and insulation class, but operators commonly monitor top oil and winding hot spot limits defined by manufacturer and applicable standards. Normal operation typically keeps top oil well below alarm thresholds, while short term overloads allow higher temperatures within specified limits. Sustained high temperature accelerates oil oxidation and paper aging. Because limits vary by cooling class and insulation system, the correct approach is to follow nameplate and OEM guidance and trend temperatures with load and ambient conditions.