A pressure vacuum breaker shall be installed a minimum of _________ inches above the highest point of usage.

The correct answer is 12 inches because plumbing codes and standards specify that a pressure vacuum breaker must be installed at least this distance above the highest point of usage to ensure proper functioning and to prevent back siphonage. The purpose of this elevation is to maintain an air gap that protects the potable water supply from contamination due to negative pressure conditions that could draw water back into the system.

Installing the pressure vacuum breaker at the specified height creates a buffer, allowing air to enter the system in case of a drop in pressure without allowing the water from the system to flow backwards. This helps to safeguard public health by preventing potential contaminants from entering the drinking water supply.

Additionally, the specific height requirement is defined in plumbing codes like the Uniform Plumbing Code (UPC) or the International Plumbing Code (IPC), reinforcing the importance of adhering to these guidelines for the safe use of plumbing fixtures and systems.