6.2 Water Use, Demand Patterns & Distribution System Design
Key Takeaways
- Average day demand in North Carolina residential systems commonly runs 50 to 100 gallons per capita per day, and design flows are built by multiplying that average by peaking factors: roughly 1.5 to 2.0 for maximum day and 2.5 to 4.0 or more for peak hour.
- Fire flow requirements are usually the largest single demand a small system must meet, and 15A NCAC 18C .0901 prohibits hydrants on mains smaller than six inches or on systems not designed to carry fire protection flows.
- Under 15A NCAC 18C .0902, no more than 20 residences may connect to a two-inch main, a looped two-inch main may serve no more than 40, and a two-inch main may not exceed 1,000 feet in length.
- Unaccounted-for water is the difference between water produced and water billed or otherwise accounted for; a water audit separates real losses such as leakage from apparent losses such as meter under-registration.
- Pressure zones are created where elevation change exceeds roughly 100 to 150 feet, because a single zone spanning that range cannot hold every customer within an acceptable pressure band.
6.2 Water Use, Demand Patterns & Distribution System Design
The Distribution needs-to-know outline opens with Water Use and System Design for a reason: every hydraulic decision an operator makes — which tank to fill, which pump to run, where to flush — rests on understanding the demand pattern.
1. Categories of water use
| Category | Typical share | Notes |
|---|---|---|
| Domestic / residential | 40–60% | Interior use is fairly steady year round; irrigation drives summer peaks |
| Commercial / institutional | 10–30% | Schools, hospitals, laundries, restaurants |
| Industrial | 0–40% | Highly site-specific; a single process user can dominate a small system |
| Public / unbilled authorized | ~5% | Flushing, firefighting, street cleaning, municipal buildings |
| Losses | Variable | Leakage, unauthorized use, meter error |
Per capita demand. A common North Carolina planning figure is 50 to 100 gallons per capita per day (gpcd) for residential systems; systems with substantial commercial or industrial load run higher. The operational value of the number is that it lets you sanity-check production: a 4,000-person system producing 900,000 gpd is producing 225 gpcd and almost certainly has a major leak or an unmetered industrial user.
2. Variations and peaking factors
Peak hour demand ~2.5 - 4.0 x average day (higher in small systems)
Maximum day demand ~1.5 - 2.0 x average day
Average day demand 1.0
Minimum hour ~0.25 - 0.5 x average day
- Time of day: morning and early evening peaks, minimum in the early morning hours.
- Day of week and season: summer irrigation and tourism can double a coastal system's demand.
- Smaller systems peak harder. With few customers there is no statistical smoothing, so a 200-connection system can see a peak-hour factor well above 4.
Design rule of thumb: the distribution system must deliver the larger of (a) maximum day demand plus fire flow, or (b) peak hour demand — while still holding at least 20 psi everywhere under 18C .0901.
3. Fire flow
Fire flow requirements come from the Insurance Services Office rating schedule and local fire code, and depend on building construction, occupancy, exposure, and area. Typical needed fire flows range from 500–1,000 gpm for two hours in single-family residential areas to 3,500 gpm or more for four hours in commercial or industrial areas.
North Carolina's rules constrain what may be built:
- Fire hydrants shall not be installed on water mains of less than six inches diameter, or on mains or systems not designed to carry fire protection flows (18C .0901).
- Systems not designed for fire flows must hold at least 30 psi throughout the system during peak flow — a higher routine standard than the 20 psi fire-flow floor, precisely because there is no fire-flow event to design around.
- Adding a fire protection system to a distribution system changes its classification under 18D .0205(b), and therefore the grade of ORC it requires.
4. Main sizing constraints in 15A NCAC 18C
| Requirement | Rule |
|---|---|
| Minimum main size: 2-inch nominal | .0901 |
| Minimum pressure: 20 psi at all points at peak demand (fire flow); 30 psi at peak flow for non-fire-flow systems | .0901 |
| No more than 20 residences (or equivalent) on a 2-inch line unless looped or supplied from two connections | .0902(a) |
| A looped 2-inch main serves no more than 40 residences; a 2-inch main may not exceed 1,000 feet | .0902(b) |
| Dead ends require a hydrant or adequately sized flush valve with above-ground discharge | .0903 |
| Cover: below the frost line or 30 inches, whichever is greater; 12 inches clearance from other utilities | .0904 |
| New mains tested for leakage per AWWA standards | .0905 |
| 10 feet lateral separation from sewers (or 18 inches vertical with the water main above, in a separate trench) | .0906 |
5. Pressure zones
Pressure at a point equals the elevation difference to the hydraulic grade line times 0.433 psi per foot. A 100-foot elevation span therefore produces a 43 psi spread across a single zone — which is why systems split into pressure zones roughly every 100 to 150 feet of elevation, joined by booster pump stations (to raise pressure going uphill) and pressure reducing valve stations (to drop it going downhill). Each zone normally has its own storage, so its tank sets the zone's hydraulic grade line.
6. Water loss accounting
Unaccounted-for water = water produced − (water billed + authorized unbilled use). A modern water audit separates:
- Real losses: leakage on mains, services, and storage overflows. Reduced by leak detection surveys, pressure management, and main replacement.
- Apparent losses: meter under-registration, data handling errors, unauthorized consumption. Reduced by meter testing and replacement, and by billing audits.
Night flow analysis is the operator's most powerful tool: isolate a district, read the master meter during minimum-hour demand, and compare with expected legitimate night use. Persistent high night flow is leakage until proven otherwise.
[!NOTE] Tie demand to water quality. Oversized mains and tanks built for a growth that never came are the leading cause of excessive water age, disinfectant residual loss, nitrification in chloraminated systems, and DBP formation in the far reaches of the system. Design and operation are the same problem viewed at different time scales.
A system serves 3,200 people and produces an average of 288,000 gallons per day. What is the per-capita demand, and what does it suggest?
Under 15A NCAC 18C .0902, how many residences may be served by a single unlooped two-inch water main, and what is the maximum length of a two-inch main?
A utility isolates a district and finds 38 gpm flowing at 3 a.m., when legitimate night use is estimated at 6 gpm. What does this indicate?