Which pipe size is the minimum for a direct cold water system in a dwelling?

• A. 10 mm
• B. 12 mm
• C. 15 mm
• D. 22 mm

Answer: (C)

In a direct cold water system, the pipe size must be able to deliver enough water to multiple outlets without a large pressure drop. If you use a very small pipe, like 10 mm or 12 mm, the flow available to taps when several fixtures are open at once would be limited, leading to weak pressure and poor performance. A 15 mm pipe provides the needed flow capacity for typical domestic use (kitchen, bathroom, basin) while keeping friction losses at a reasonable level. Larger sizes, such as 22 mm, are used for main distribution trunks or long runs to reduce pressure loss, but they aren’t the minimum required. So the minimum standard for a dwelling’s direct cold water system is 15 mm.