When a transformer is taken off-line, certain amount of
residual flux remains in the core which is typically in
between 50-90% of the rated flux.. When the transformer is
re-energized, the initial magnitude of the magnetizing current
is determined by the amount of the residual flux. In some
extreme cases,the peak flux is greater than twice of the
design limit and due to the saturation, inrush currents up to 3
to 40 times of the load current are possible.