4.2 Water Supply Fixture Units & the Sizing Tables (UPC 610.3, 610.4, 610.8, 610.10)

Why Fixture Units, Not Gallons per Minute

Fixtures are used intermittently, so the peak demand of a building is far less than the sum of every fixture running at once. The UPC captures this probability through water supply fixture units (WSFU). Section 610.3 says the Table 610.3 values include both hot and cold demand. Section 610.4 lets systems within the range of Table 610.4 be sized directly from that table. Larger or unusual systems use Appendix A (610.5). Washington's 610.4 amendment keeps this choice: "may be sized from that table or by the method set forth in Section 610.5."

Tables provided on the exam: PSI provides Tables 610.3, 610.4, and 610.10 with the journey level exam (along with 702.1). You need to know how to use them quickly and correctly.

Table 610.3: WSFU and Minimum Branch Sizes

Key notes to Table 610.3:

• Note 1: The minimum branch size applies to the cold branch, or to both hot and cold branches.
• Note 3: The listed values represent load on the cold water building supply. For fixtures with both hot and cold connections, the separate hot and cold values may each be taken as three-quarters of the listed total. A private lavatory (1.0) counts as 0.75 on the hot branch and 0.75 on the cold branch.
• Note 5: Continuous-flow demands are added separately in gpm.
• Note 8: The reduced value for additional hose bibbs applies when sizing the building demand or a segment serving more than one. The branch to each individual hose bibb is sized for 2.5.

Table 610.10: Flushometer Valves

Flushometer valves draw a large, short burst, so they get their own decreasing, cumulative values (610.10). The number of valves served by each section of pipe sets that section's load:

Other fixtures in the same pipe section are added at their Table 610.3 values. The UPC's Example 610.10 counts 2 WC (70) + 2 UR (35) + 2 LAV (2) = 107 FU. Piping to a flushometer valve may never be smaller than the valve inlet.

Table 610.4 and the 610.8 Procedure

Section 610.7 lists the data you need: total fixture units, the developed length from the meter to the most remote outlet, the elevation difference to the highest outlet, and the minimum street pressure. Then 610.8 says:

1. Determine the available pressure at the meter or other source.
2. Add or subtract 1/2 psi for each foot of elevation between the source and the highest outlet.
3. Choose the Table 610.4 pressure range: 30–45 psi, 46–60 psi, or over 60 psi.
4. Choose the length column equal to or longer than the developed length: 40, 60, 80, 100, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, or 1,000 feet.
5. Follow down that column to the first value equal to or greater than the total fixture units.
6. Read the meter and street service size and the building supply and branch size from the two left-hand columns.

Also remember:

• Minimum building supply: 3/4 inch (610.8 and Table 610.4 note 2).
• Pressure losses through filters, softeners, backflow preventers, or tankless heaters must be included (610.2).
• Where a pressure regulator is installed, size from 80% of the reduced pressure (608.2).
• Branches (610.9) are sized by the fixture units they serve, using the same length and meter column. No branch needs to be larger than the building supply.

Worked Example: Office Restroom Core

A building has 4 flushometer water closets, 2 flushometer urinals, 4 public lavatories, and 1 public service sink. The minimum pressure at the meter is 68 psi. The highest outlet is 24 feet above the meter, and the developed length is 110 feet.

• Fixture units: 4 WC = 105; 2 UR = 35; 4 lavatories × 1.0 = 4; service sink (public) = 3.0. Total = 147 FU.
• Pressure: 24 ft × 1/2 psi = 12 psi. 68 − 12 = 56 psi, so use the 46–60 psi range.
• Length: 110 ft rounds up to the 150-foot column.
• Table lookup: Going down the 150-ft column: 4, 14, 28, 30, 39, 52, 66, 85, 128, then 150 at the 2-inch meter / 1-1/2-inch building supply row. That is the first value of at least 147.
• If the existing meter is only 1-1/2 inches: the 1-1/2 inch meter rows allow 66 (1-1/4 supply), 128 (1-1/2 supply), and 272 (2-inch supply). The building supply must be 2 inches.

Worked Example: Two-Bath House

A house has 2 tank water closets (2.5 each), 2 lavatories (1.0 each), 1 tub/shower (4.0), 1 shower (2.0), a kitchen sink (1.5), a dishwasher (1.5), a clothes washer (4.0), and 2 hose bibbs (2.5 + 1.0). Total = 23.5 FU. The street pressure is 55 psi, the highest outlet is 15 feet up (−7.5 psi, leaving 47.5 psi, so 46–60 psi), and the developed length is 95 feet, so use the 100-foot column. The column reads 5, 17, then 33 at 3/4-inch meter / 1-inch building supply. A 3/4-inch meter with a 1-inch supply is the answer.