Why do I need a bigger breaker when reverse

Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results.

In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online.