HOLT Electricians Practice Exam

A transformer not over 600 volts requires overcurrent protection on the primary side set at not more than what percent of rated primary current?

1. 75 percent

2. 100 percent

3. 125 percent

4. 150 percent

The correct answer is: 125 percent

The requirement for overcurrent protection on the primary side of a transformer not exceeding 600 volts is established by the National Electrical Code (NEC). Specifically, the NEC stipulates that the overcurrent protection must be set at not more than 125 percent of the rated primary current for the transformer. This 125 percent rule is designed to provide adequate protection while allowing for temporary overload conditions that can occur during the normal operation of the transformer, such as inrush currents when the transformer is first energized. By setting the protection at this level, it reduces the likelihood of nuisance tripping while still offering a reasonable margin to protect the transformer from sustained overloads that could lead to overheating or damage. Options suggesting lower percentages, such as 75 percent or 100 percent, would not provide sufficient headroom for normal operation, particularly under conditions like inrush current. On the other hand, setting the protection at a higher percentage, such as 150 percent, would increase the risk of overcurrent situations leading to damage, as it would allow for higher sustained currents than the transformer's capacity to handle safely. Thus, the choice of 125 percent balances safety with the operational realities of transformer performance.