What is the minimum size of a cold water supply pipe 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, water comes straight from the mains to the outlets without storage, so the pipe size must be large enough to carry the needed flow without a big pressure drop. Fifteen millimetres is the smallest size that reliably provides adequate flow to typical domestic fixtures (taps, basins, baths) at normal mains pressure and still allow multiple outlets to run simultaneously without a noticeable drop in performance. Smaller sizes like ten millimetres or twelve millimetres would throttled flow and cause weak performance, especially when more than one fixture is drawing water. A larger size, such as twenty-two millimetres, is typically used for the main service into the building or long runs to reduce losses, but it isn’t the minimum inside the dwelling. So fifte en millimetres is the standard minimum for a direct cold water supply.