In a water supply system, the sizes of water mains from largest to smallest are which sequence?

• A. Primary, Secondary, Distributor
• B. Distributor, Secondary, Primary
• C. Secondary, Primary, Distributor
• D. Primary, Distributor, Secondary

Answer: (A)

Mains are sized in a hydraulic hierarchy from the source outward: the largest are primary mains, they feed into secondary mains, which then feed distributor mains that are closest to customers. So the correct sequence from largest to smallest is primary, then secondary, then distributor. Primary mains carry bulk flow from treatment or storage to broad areas, secondary mains distribute to neighborhoods, and distributor mains bring water to the local streets and hydrants. The other sequences would place closer-to-customers lines ahead of the larger network mains, which doesn’t reflect how flow and pressure are managed across the system.