Oklahoma Electrical Practice Exam

For a 30-volt lighting system, what is the maximum size of a branch circuit allowed to feed the line side of a transformer?

1. 15 amps

2. 20 amps

3. 25 amps

4. 30 amps

The correct answer is: 25 amps

In residential and commercial electrical systems, branch circuits are designed to safely carry a certain amount of load current without overheating or becoming a safety hazard. For a 30-volt lighting system, the National Electrical Code (NEC) provides specific guidelines regarding the maximum size of branch circuits feeding a transformer. The maximum size of a branch circuit for any system takes into account the transformer’s primary rating and the sizing requirements for conductors based on their ampacity. In this context, a 30-volt lighting system typically operates at a reduced voltage and requires careful consideration in sizing the branch circuit to avoid excessive loading. A branch circuit feeding a transformer must not exceed 25 amps when it comes to a 30-volt system. This is because the feed needs to allow for safe operation of the transformer and connected lighting loads. Therefore, a 25-amp maximum branch circuit is aligned with the design and safety parameters necessary for low-voltage systems. Choosing a branch circuit larger than this could lead to wire overheating or potential failure of the transformer due to excess current, particularly in low-voltage applications, where the tolerances and limitations are tighter compared to standard voltage systems. This makes 25 amps the most appropriate and safe choice for the maximum size of