The expected lifespan of a transformer depends on a variety of factors, including its size, design, operating conditions, and maintenance history. In general, the lifespan of a transformer can range from 15 to 40 years, with an average lifespan of around 25 years. However, some transformers may last longer or shorter depending on the specific circumstances. For example, a transformer that is well-maintained and operated within its design limits may last longer than a transformer that is poorly maintained and subjected to frequent overloads or surges. It is also worth noting that the lifespan of a transformer may be extended by replacing certain components, such as the oil or the bushings, before they reach the end of their useful life.