Transformer Inrush Current

Why energizing a completely unloaded transformer can momentarily pull 8-12× its full-load rated current — and trip a breaker sized to handle full load without complaint.

Close the breaker on a transformer with nothing connected to its secondary and, for an instant, it can look like it's trying to draw more current than it ever would under a real fault. No load. No fault. Just a switch closing. That spike is called inrush current, and it's one of the most counterintuitive — and most normal — things a transformer does.