Transformers must not be connected to circuits exceeding what percentage above the nameplate rating?

Transformers are designed to operate within specific electrical limits, and exceeding these limits can lead to excessive heating, insulation breakdown, and ultimately failure of the transformer. The industry standard for the maximum allowable voltage that can be applied above the nameplate rating is typically recognized as 5%. This is due to the fact that a 5% overrating allows for some variations and fluctuations in the system voltage without significantly impacting the reliability and safety of the transformer.

Exceeding this 5% threshold can increase the risk of damaging the insulation and causing premature aging of the transformer components. While some other percentages might be applicable in different contexts or equipment, the generally accepted maximum for transformers is 5%. Therefore, operating a transformer connected to circuits exceeding this percentage can lead to unsafe situations and negatively affect performance.