Transformer overcurrent protection is typically rated at what percentage more than the transformer capacity?

Transformer overcurrent protection is typically set at 125% of the transformer’s capacity to prevent nuisance tripping while still offering adequate protection. This is rooted in the need to accommodate temporary inrush currents that occur when transformers are energized. During starting, transformers can experience currents that are several times greater than their rated capacity.

By rating the overcurrent protection at 125%, it allows for these higher transient currents without tripping the protection devices unnecessarily. This setting strikes a balance between ensuring that the transformer can handle normal operational conditions and safeguarding it against genuine overload conditions that could lead to overheating or failure.

The other options, while they might seem reasonable, either do not provide enough margin for inrush protection or are excessive and could lead to a delay in tripping for overload conditions.