What is the acceptable water content limit in transformer oil?

Acceptable moisture depends on voltage class, oil type, and whether the unit is sealed or has a conservator. In practice, utilities usually target very low moisture for HV assets and treat oil if ppm rises or BDV drops. Because moisture partitions between oil and paper, a ppm number alone is not enough. The best approach is moisture ppm plus temperature, water saturation, BDV trend, and paper condition indicators.