The water service pipe may be placed in the same trench as the sewer if the bottom of the water service pipe at all points is at least ________ inches above the top of the sewer at its highest point.

The correct standard requires that the water service pipe must be installed at least 18 inches above the top of the sewer line at its highest point. This regulation is primarily in place to prevent contamination of the potable water supply from the sewage. If the water service line is placed in the same trench as the sewer line, maintaining this vertical separation is crucial for safeguarding public health.

The specific distance of 18 inches serves as a safety buffer to ensure that any potential leaks or breaches in the sewer line do not lead to direct contamination of the water supply. It is a recognized standard that balances the practicalities of installation with crucial health guidelines, demonstrating the careful consideration given to plumbing systems in preventing cross-contamination.

While other distances like 12, 24, or 30 inches may seem like viable options, they do not match the established regulatory requirement that specifically stipulates 18 inches as the minimum separation for safety purposes. Thus, adherence to this standard helps maintain the integrity and safety of the water system, providing assurance that potable water remains uncontaminated.