Master Plumber Practice Exam

The correct answer is that water closet tank ballcocks shall be installed with the vacuum breaker critical level at least 1 inch above the full opening of the overflow tube. This requirement is important for preventing backflow and ensuring that potable water does not become contaminated.

The overflow tube serves as a safety mechanism to prevent tank overflow; therefore, the vacuum breaker must be positioned sufficiently high to maintain the safety barrier against potential siphoning of contaminated water back into the potable water supply. Installing the vacuum breaker 1 inch above the full opening of the overflow tube creates a physical separation that helps to prevent the possibility of backflow, thereby adhering to plumbing codes and ensuring the system's overall hygiene and safety.

Other options suggest different vertical separations of the vacuum breaker from various components, but they do not meet this critical regulatory requirement that focuses specifically on the relationship between the vacuum breaker and the overflow tube to ensure adequate protection against contamination.