The ISO recommends that non-looped water mains connected to fire hydrants have a diameter of at least how many inches?

• A. 6 inches
• B. 8 inches
• C. 10 inches
• D. 12 inches

Answer: (B)

This is about the minimum pipe size for non-looped mains feeding a fire hydrant to ensure adequate fire flow. In a dead-end (non-looped) main, there isn’t an alternate path to supply extra water if demand spikes, so the pipe size must be large enough to carry the required flow without dropping pressure too much. An eight-inch diameter helps keep friction losses manageable and preserves sufficient residual pressure at the hydrant when water is drawn, supporting reliable firefighting supply. Smaller sizes, like six inches, can restrict flow and reduce available water for a fire, while larger sizes such as ten or twelve inches are more than the minimum needed in many situations and add unnecessary cost.