Private fire service mains are not permitted to extend more than ______ under a building, except when they are run in a covered trench.

The correct answer is based on NFPA 24, which specifies limitations on the extension of private fire service mains under buildings to prevent potential hazards such as structural damage, maintenance issues, and access concerns. Specifically, the standard allows for these mains to extend a maximum of 10 feet under a building when they are not in a covered trench. This requirement emphasizes safety and compliance, ensuring that any necessary inspections or maintenance can occur without obstruction.

If the mains are installed within a covered trench, this allowance may be adjusted, reflecting the intention of the NFPA to provide flexibility where safety measures are in place. Understanding these regulations is essential for ensuring that private fire service mains are installed in a manner that mitigates risks while maintaining functionality.