What is the acceptable moisture content in transformer oil?

The acceptable moisture content in transformer oil is typically very low, as moisture can significantly reduce the oil’s insulating properties and lead to electrical breakdowns. Generally, the moisture content should be below 20 parts per million (ppm) for new or recently purified oil. For oil in service, moisture levels should ideally be kept below 30-35 ppm to maintain the reliability and longevity of the transformer.