There is no single permissible moisture value that fits all transformers, because it depends on oil type, temperature, voltage class, and insulation condition. For mineral oil at operating temperature, many utilities consider 20-30 ppm acceptable for new or well-maintained units, while above 40-50 ppm usually triggers closer monitoring. More important than ppm is relative saturation or water activity, because risk of free water and paper wetting depends on temperature. In practice, keeping paper moisture below about 2% is a common long-term target.