The problem is not directly with the wires, its the addition of the quasi-DC induced currents in an AC transformer. Even the addition of a small DC current can

The problem is not directly with the wires, its the addition of the quasi-DC induced currents in an AC transformer. Even the addition of a small DC current can lead to unwanted harmonics or in extreme cases spot heating (this is what you think of when people say 'knocking out a transformer'). If you want a very in-depth review: https://doi.org/10.1002/2016SW001501 https://doi.org/10.1002/2016SW001501