Applies to transformers in the range 16kVA to 1000kVA for continuous service at 50Hz, for highest voltage for equipment 7.2kV or 12kV. Applies also to single and three phase two winding oil immersed naturally cooled transformers suitable in all respects for indoor or outdoor service (except where specially defined as an indoor type) connected by air oil bushings, oil oil bushings, or bushing into cable boxes.