HOLT Electricians Practice Exam

A transformer requires overcurrent protection in the secondary device if the primary device is set at not more than what percent of the rated primary current?

1. 100 percent

2. 110 percent

3. 125 percent

4. 250 percent

The correct answer is: 250 percent

The correct answer is based on the National Electrical Code (NEC) requirements for transformer protection. A transformer requires overcurrent protection on the secondary side if the primary device is set at not more than 125 percent of the rated primary current. However, for the context of the question, it’s crucial to understand that when the primary device is set at 250 percent of the rated primary current, it can protect the transformer in a way that does not necessitate further overcurrent protection on the secondary side. This higher setting typically applies to certain types of industrial or large transformers, where the design anticipates that the primary protection is inefficient under normal operating conditions, thus allowing for a less strict requirement for the secondary side. With a primary protection device set to 250 percent, the expectation is that it provides adequate overcurrent protection for the transformer, allowing it not to require the additional layer of protection which would imply ensuring lower percentages like 100, 110, or even 125 percent, where secondary protection would be more critical to avoid potential damage to the transformer.