In the long term, the limit is the degradation of the insulation on the wires. Like most chemical reactions, its rate increases exponentially with temperature.

In the long term, the limit is the degradation of the insulation on the wires. Like most chemical reactions, its rate increases exponentially with temperature. [0] gives a relative damage rate of 9.8e-18 * exp(15e3/T) with T in Kelvin. Around the usual operating temperature of 100C, every 10C increase in temperature reduces the transformer life by 3x. So transformers that should last 40 years last 13 years. [0] http://erlphase.com/downloads/papers/Fundamental_Principles_Of_Transformer_Thermal_Loading_And_Protection_ERLPhase_TexasAM2010.pdf http://erlphase.com/downloads/papers/Fundamental_Principles_...