An air dehydrating breather assembly and method provide dehumidifying air suitable to be supplied into oil expansion vessels used in electrical devices. The air dehydrating breather assembly includes first and second air demoisturizer units respectively associated with two corresponding oil expansion vessels. The first air demoisturizer unit includes a first tank housing moisture absorber and a second tank housing moisture absorber alternatively providing dehumidified air. The second air demoisturizer unit includes a third tank housing moisture absorber providing dehumidified air. Sensors are associated with the first, second and third tanks to detect the level of saturation of their moisture absorbers. An electronic processing unit is operatively associated with the sensors for routing dehumidified air to the associated oil-expansion vessels from the tanks whose moisture absorbers are not saturated.