Do I need a larger circuit breaker or fuse when reverse feeding a transformer?
When a transformer is reverse fed or back-fed, what is traditionally the secondary or inner winding is energized first. This can create much larger inrush currents which can cause nuisance tripping of fuses and circuit breakers. This is often considered when an application needs to step up a voltage.
HPS does not recommend this practice. Step up transformers are readily available in stock. Increasing the sizing of short circuit devices can cause dangerous situations. Some modifications needed to do this may violate transformer warranties and agency approvals. Nothing clearly defines on the nameplate that the transformer is a step up transformer which can lead to improper de-energization. Recent changes to the NEC electrical code have added further restrictions to this practice. Transformers should always be selected and designed for the proper voltage change.