For a grounding autotransformer, the overcurrent device must be set to what percent of the transformer’s continuous current per phase or neutral rating?

• A. 125%
• B. 100%
• C. 110%
• D. 150%

Answer: (A)

The protective device for a grounding autotransformer is sized to 125% of the transformer’s continuous current per phase or neutral rating. The continuous current rating is the current the transformer can carry indefinitely without overheating. You need a cushion to handle normal inrush and transient overloads that can occur when the grounding path is established or when the system experiences brief disturbances. Setting the overcurrent device at 125% allows normal operation to proceed without nuisance trips, while still providing protection against sustained faults. If it were set at 100%, 110%, or 150%, you’d either risk nuisance tripping or insufficient protection; 125% is the standard balance.