Oklahoma Electrical Practice Exam

A 600v, 20a transformer will have an individual overcurrent device on the primary side rated at largest of how many amps?

1. 15 amps

2. 20 amps

3. 25 amps

4. 30 amps

The correct answer is: 25 amps

When determining the appropriate overcurrent protection for a transformer, you generally refer to the National Electrical Code (NEC) requirements, specifically NEC 440.22 regarding the protection of transformers. According to these guidelines, the overcurrent device should be rated at a certain percentage above the full-load current of the transformer, while also considering the transformer's size and characteristics. For a transformer operating at 600 volts and having a primary side current of 20 amps, the NEC allows for the overcurrent device to be sized based on a percentage of the transformer's capacity. Specifically, the primary overcurrent protection can be sized up to 125% of the full-load current of the transformer. Calculating 125% of 20 amps gives us: 125% of 20 A = 0.125 × 20 A + 20 A = 25 A. However, the next standard size for overcurrent devices according to the available options presents a rating of 30 amps, which is a standard size that can be used. This means that using a 30 amp overcurrent device is compliant, as it is greater than the calculated maximum of 25 amps. Thus, the correct answer reflects a comprehensive approach to ensuring the protection of the transformer while also