8.2 Water Distribution Design: Flow and Pressure Criteria, Fixture Supplies & Pipe Sizing

Design criteria (IPC 604.1 to 604.4)

• 604.1 and 604.2: the design must follow accepted engineering practice, and sizing methods must be approved. Appendix E gives one method. The IPC appendices are informative unless a jurisdiction adopts them, so the building official decides which method is approved.
• 604.3: pipe must be sized so that, under peak demand, each fixture supply outlet receives at least the flow and pressure in Table 604.3. Fixtures not listed use the manufacturer's values.

Table 604.3: required capacity at fixture supply outlets

Table 604.3 sets the minimum supply at the outlet. Table 604.4 sets the maximum consumption of the fixture (for example, 1.6 gallons per flush or 2.5 gpm at 80 psi). Do not confuse the two.

Fixture supplies (IPC 604.5) and manifolds (IPC 604.10)

Table 604.5 minimum fixture supply pipe sizes:

• The fixture supply pipe must end no more than 30 inches from the point of connection to the fixture. A reduced-size flexible connector between the supply and the fixture must be an approved type, and the supply pipe must extend to the floor or wall next to the fixture.
• Footnote: in a parallel (manifold) system, where the distribution line is 50 feet or less and the pressure at the meter is at least 35 psi, each line may be one size smaller than the table.
• 604.10.1 manifold sizing (Table 604.10.1): a manifold is sized by total gpm. A 1/2-inch manifold handles 2 gpm at 4 feet per second and 5 gpm at 8 feet per second; 3/4 inch handles 6 and 11 gpm; 1 inch, 10 and 20 gpm. 604.10.2: valves at the manifold are identified by fixture. 604.10.3: manifolds with valves need access.

The fixture unit method (IPC Appendix E)

Hunter's probability method assigns each fixture water supply fixture units (wsfu), then converts the total wsfu to a design flow in gpm. The method accounts for the fact that not all fixtures run at once.

Table E103.3(2): selected wsfu values (cold / hot / total)

The total for a fixture with hot and cold supply is less than cold plus hot, because both are not drawn at full flow at the same time. Use the cold column to size cold-only piping, the hot column for hot piping, and the total for the service and main up to the water heater branch.

Table E103.3(3): estimating demand

A flushometer valve opens fully and draws about 25 gpm for a few seconds (Table 604.3), so even a few flushometers need a large design flow. A flush tank refills slowly. As a system grows, the chance of many simultaneous draws evens out, and the two curves meet at about 1,000 wsfu.

The pressure budget

Appendix E sizes pipe by finding the pressure available for friction:

1. Start with the minimum available pressure at the source (604.6: design for the minimum).
2. Subtract elevation: 0.433 psi per foot of rise. One psi lifts water 2.31 feet.
3. Subtract fixed losses: the water meter, backflow preventer, PRV, softener and filters, using manufacturers' data at the design flow.
4. Subtract the pressure required at the most remote, highest outlet (Table 604.3, or the manufacturer's value).
5. Divide what remains by the developed length plus an allowance for fittings to get the allowable friction loss per 100 feet.
6. Choose pipe sizes from friction-loss charts for the design gpm of each section, and check velocity.

Example: the street has a minimum of 60 psi. The highest flushometer valve water closet (siphonic) is 25 feet above the main, and the meter and backflow assembly lose a combined 12 psi at design flow. Available for friction: 60 − (25 × 0.433 = 10.8) − 12 − 35 = 2.2 psi. That is very little, so the designer must either use larger pipe or add a booster system (604.7).

Velocity

Apart from the manifold table and the general water hammer rule (604.9), the IPC does not set a maximum velocity. The Copper Development Association recommends about 8 feet per second maximum for cold water, about 5 feet per second for hot water up to about 140°F, and 2 to 3 feet per second where water routinely exceeds 140°F, to limit erosion-corrosion and noise. A handy formula is:

$$V = \frac{0.408 \times Q}{d^2}$$

where V is velocity in feet per second, Q is flow in gpm, and d is the inside diameter in inches. Example: 10 gpm in tube with a 0.785-inch inside diameter: V = 0.408 × 10 ÷ 0.616 ≈ 6.6 feet per second.