The building sewer requires testing under not less than a _____ -foot head of water.

The correct choice is 10 feet, which reflects the standard testing requirement for building sewers to ensure their integrity and functionality. When conducting a water test for building sewers, using a head of 10 feet of water is typically mandated to confirm that there are no leaks, that the system can withstand the anticipated pressures, and that it will perform adequately under normal operational conditions.

This pressure allows for a thorough examination of the sewer line, making it a conservative and effective method of ensuring that the plumbing systems are sound before they are put to use. Utilizing a lower head might not provide enough information about the system's resilience and may overlook potential weaknesses, while a higher head could be unnecessarily rigorous or could risk causing damage to the plumbing.

The choice of 10 feet balances the need for effectiveness in testing while remaining practical and manageable for installation and inspection processes in urban and residential plumbing systems in North Carolina.