The minimum diameter of a private service main to a hydraulically calculated sprinkler system that also supplies a fire hydrant is

According to NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, the size of a private fire service main is determined by hydraulic analysis. However, the standard establishes a mandatory minimum nominal size. When the main is intended to supply one or more fire hydrants in addition to other systems like sprinklers, the nominal size shall not be less than 6 in. (150 mm). This ensures sufficient water flow capacity for both automatic sprinkler operation and manual firefighting efforts via hydrants.

A. 3 in. (75 mm): This size is below the minimum requirement for any private fire service main supplying a fire hydrant.

B. 4 in. (100 mm): While a 4-inch pipe might be a minimum for certain sprinkler system risers, it is not permitted as the minimum size for an underground main that also supplies a fire hydrant.

D. 8 in. (200 mm): While hydraulic calculations for a large system might necessitate an 8-inch or larger main, it is not the prescribed minimum size.

1. NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, 2022 Edition.

Section 10.4.1.1: "The size of the private fire service main shall be determined by a hydraulic analysis, but in no case shall the nominal size be less than 6 in. (150 mm) where the main is intended to supply one or more fire hydrants."

2. NFPA 13, Standard for the Installation of Sprinkler Systems, 2022 Edition.

Section 27.2.4.7.1: "The nominal size of the underground pipe shall be determined by hydraulic calculations as covered in Section 27.2.3, but shall not be less than 6 in. (150 mm) where the pipe supplies both sprinklers and hydrants."

3. Oklahoma State University, Fire Protection & Safety Technology Program Courseware (FST 3333 - Fire Protection Systems).

Module on Water Supplies for Fire Protection: Course materials emphasize that NFPA 24 dictates the minimum pipe sizing for private mains. It is explicitly noted that if a main serves hydrants, the absolute minimum diameter is 6 inches, irrespective of the sprinkler system's calculated demand, to ensure adequate flow for manual firefighting.