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# Water Supplies - Expected vs. Theoretical Summarized Demand

## Calculating expected demand for water supply service lines.

The total theoretical demand for a water supply system can easily be calculated by adding known maximum demand for all fixtures in the system. Due to the nature of intermittent use this will unfortunate add up to unrealistic demands for the main supply service lines. A realistic demand for a supply system will always be far less than the total theoretical demand.

Expected demand in a water supply system can be estimated as

qet = qnl + 0.015 ( Σqn - qnl  ) + 0.17 ( Σqn - qnl  )1/2                                           (1)

where

qet = expected total water flow (l/s)

qnl = demand of largest consumer (l/s)

Σqn = total theoretical water flow - all fixtures summarized (l/s)

Note that minimum expected total water flow can never be less than the demand from the largest fixture. This equation is valid for ordinary systems with consumption patterns like

• homes
• offices
• nursing homes
• etc.

Be aware when using the equation for systems serving large groups of people where the use is intermittent, like in

• hotels
• hospitals
• schools
• theaters
• wardrobes in factories
• etc

For these kind of applications, like a wardrobe, it is likely that all showers are used at the same time. Using the formula blindly would result in insufficient supply lines.

### Example - Main Water Supply to a Nursing Home

If the theoretical demand from all fixtures in a nursing home adds up to 50 l/s and the larges fixture requires 0.4 l/s, the expected water supply demand can be estimated like

qet = (0.4 l/s) + 0.015 ((50 l/s) - (0.4 l/s)) + 0.17 ((50 l/s) - (0.4 l/s))1/2

= 2.3 (l/s)

### Total Theoretical Water Flow and Expected Flow

Expected demand for a supply system at different total theoretical demand can based on the formula above be expressed as

Water Supplies - Expected vs. Theoretical Summarized Demand
Total Theoretical Demand Summarized
(liter/s)
Expected Demand
(liter/s)
0.2 0.2
0.8 0.4
1.6 0.5
4.0 0.6
8.0 0.85
15 1.1
20 1.5
30 1.8
40 2.1
65 2.8
70 2.9
100 3.7

The maximum fixture load is 0.2 liter/s.

## Related Topics

• ### Water Systems

Hot and cold water service systems - design properties, capacities, sizing and more.

## Related Documents

• ### Domestic Hot Water Service Systems - Design Procedures

Design procedures for domestic hot water service systems.
• ### PE - Polyethylene High Density Water Supply Pipes - Properties

Nominal pipe size, outside diameter, wall thickness, weight and working pressure.
• ### Water - Human Activity and Consumption

Activity and average water consumption
• ### Water Distribution Pipes - Materials

Materials used in water distribution pipes.
• ### Water Service Pipe Lines

Water service pipe lines extends from the potable water source to the interior of buildings.
• ### Water Supply - Cross-Contamination Control

It is fundamental to keep the potable water in the water supply systems uncontaminated.
• ### Water Supply due to Fixtures Water Requirements

Supply requirements for common fixtures.
• ### Water Supply Fixture Units - WSFU vs. GPM and Liters/sec

Converting WSFU - Water Supply Fixture Units - to GPM.
• ### Water Supply Fixture Units WSFU

WSFU is used to calculate water supply service systems.
• ### Water Supply Lines - Sizing

Sizing water supply service and distribution lines based on Water Supply Fixture Units (WSFU).
• ### Water Supply Pipe Lines - Sizing

Sizing of water supply pipe lines.
• ### Water Supply Public Buildings

Required water supply to public buildings.
• ### Water Supply Systems - Impurity Limits

Typiclal limits for impurities in drinking water.
• ### Water Supply Systems - Online Design Application

Free online tool for designing water supply systems in buildings.

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## Citation

• The Engineering ToolBox (2008). Water Supplies - Expected vs. Theoretical Summarized Demand. [online] Available at: https://www.engineeringtoolbox.com/water-supply-expected-demand-formula-d_1076.html [Accessed Day Month Year].

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2.19.10