What is the minimum diameter allowed for piping serving fire hydrants on private fire service mains?

According to NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, the minimum nominal size for private fire service mains is 6 in. (150 mm). While the final size must be determined by hydraulic analysis, the standard sets this absolute minimum. An exception exists for a 4 in. (100 mm) main, but it is only permitted when the main supplies a single automatic sprinkler system, a single standpipe system, or a single fire pump. Since the question specifies the main is serving fire hydrants, this exception does not apply, and the 6 in. minimum is the correct requirement.

A. 4 in. (101.6 mm) is only permitted under specific exceptions that do not include serving fire hydrants.

C. 8 in. (203.2 mm) may be required based on hydraulic demand but is not the minimum size mandated by the standard.

D. 10 in. (254 mm) may be required based on hydraulic demand but is not the minimum size mandated by the standard.

---

1. NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, 2022 Edition, Section 6.2.1, "The size of private fire service mains shall be determined by a hydraulic analysis, but in no case shall the nominal size be less than 6 in. (150 mm)." and Section 6.2.1.1 (Exception).

2. NFPA, Fire Protection Handbook, 21st Edition, Volume 2, Section 16, Chapter 3, "Private Water Supply Systems," subsection "Piping," p. 16-42. The text reiterates the NFPA 24 requirement: "The minimum nominal size for private fire service mains is 6 in. (150 mm)."