Domestic plumbing sizing: In residential bathrooms and lavatories (domestic water supply), what nominal internal diameter is commonly used for the supply pipes to individual fixtures?

Introduction / Context:
Fixture branch sizing in domestic buildings must balance adequate flow and pressure with cost and water conservation. Common nominal sizes are standardized by plumbing codes and practical experience.

Given Data / Assumptions:

• Typical single-family residence or small apartment.
• Normal pressure conditions, standard taps and valves.
• Short branch lengths from risers or manifolds.

Concept / Approach:
Half-inch nominal pipes (≈ 12 mm internal diameter) are conventionally used for bathroom and lavatory connections to fixtures such as wash basins, bib-taps, and flush valves. Larger diameters are reserved for risers, mains, or multiple fixtures; smaller lines can cause excessive headloss and poor delivery.

Step-by-Step Solution:

Verification / Alternative check:
Check local code tables (fixture units vs. pipe size). Many allow 12–15 mm for single fixtures under typical pressures.

Why Other Options Are Wrong:
6 mm is too small; 18–24 mm are oversized for single fixtures and uneconomical; 32 mm is used for mains or groups, not individual taps.

Common Pitfalls:
Undersizing long branches causing poor flow; ignoring simultaneous use leading to pressure drops.

Final Answer:
12 mm