Allowances in water-supply design — distribution losses: For preliminary municipal water-supply sizing, what allowance is commonly assumed for system losses (leakage, theft, meter inaccuracies), expressed as a percentage of demand?

Introduction / Context:
Designing a water-supply system requires adding realistic allowances for distribution losses. While modern utilities strive for low non-revenue water (NRW), preliminary estimates in many planning exercises still include a percentage margin to account for leakage, illegal connections, meter error, and operational losses.

Given Data / Assumptions:

• Conventional municipal system without advanced district metered areas.
• Preliminary planning stage (not a detailed NRW audit).
• Typical ranges cited in many textbooks and codes (older benchmarks).

Concept / Approach:
A 15% allowance has long been used as a conventional planning value in many civil engineering syllabi and questions. Actual NRW varies widely by city and infrastructure condition, but for exam purposes, 15% is the recognizable benchmark unless a specific code dictates otherwise.

Step-by-Step Solution:

Verification / Alternative check:
Some references use 10–20%; selecting the mid/value of 15% aligns with many standard problems and avoids optimistic underestimation.

Why Other Options Are Wrong:
5–10% are optimistic for ageing systems; 22.5% reflects high losses but is not the conventional exam benchmark.

Common Pitfalls:
Confusing storage “fire demand” allowances with distribution losses; double-counting losses in both demand and storage sizing.

Final Answer:
15%