A water supply fixture unit (WSFU) is a load number that reflects how likely a fixture is to run at the same time as others. A designer totals WSFUs for each pipe, converts the total to a peak flow in gallons per minute, then picks a pipe size that delivers the required pressure at the farthest fixture.
What is a water supply fixture unit?
It is a measure of probability. The Plumbing and Drainage Institute's sizing standard, as reproduced in Plumbing & Mechanical Engineer, defines it this way: "A water supply fixture unit is defined as a non-dimensional unit of the probability of simultaneous use of water in a plumbing water distribution system." In other words, a toilet with a flush valve carries a higher number than a lavatory, and a building with hundreds of fixtures will never see all of them at full flow in the same instant. For the general idea, see what a plumbing fixture unit is.
Values also change with the type of fixture control and occupancy. The fixture unit table printed with PDI's material lists a public flush valve water closet at 10 units, a public flush tank water closet at 5, and a public stall or wall urinal with a flush valve at 5 (a pedestal urinal with a flush valve is 10). In the same table, private-use fixtures carry lower values; the private bathroom group with a flush valve closet is 8.
Why not just add up each fixture's flow?
Because the total would be absurdly large. The IPC's Table 604.3 sets the capacity each fixture supply outlet must deliver at peak, and the numbers for flush valves are big: "Water closet, blow out, flushometer valve" is 25 gpm at 45 psi, and "Urinal, valve" is 12 gpm at 25 psi, while a public lavatory is 0.4 gpm at 8 psi. Add the table values for ten blow-out flushometer toilets and the pipe would be sized for 250 gpm, which no real restroom needs all at once. WSFUs and the conversion curve exist to avoid that overbuilding.
What are the steps to size a commercial water line?
IPC Section 604.1 gives the legal frame: "The design of the water distribution system shall conform to accepted engineering practice. Methods utilized to determine pipe sizes shall be approved." The working steps are:
- 1Count WSFUs by fixture and pipe. Separate hot and cold, public and private, flush tank and flush valve. Each branch carries only the fixtures downstream of it.
- 2Add up the load on each pipe, working from the farthest fixtures back to the service.
- 3Convert to peak demand. A conversion curve or table turns the WSFU total into gpm. The conversion depends on whether the system is mainly flush tanks or flush valves, as the two columns in Phoenix's meter table below show.
- 4Check pressure at the critical fixture. Table 604.3 sets the flow pressure each outlet needs. Pipe friction, elevation and devices such as backflow preventers use up the pressure the service starts with.
- 5Pick the pipe size that keeps flow and pressure inside limits at every branch.
Phoenix adopted both sizing options. The city's adoption of IPC Appendix E says the appendix "provides two methods of water pipe sizing not provided in the body of the code." For the Uniform Plumbing Code, the justification for adopting Appendix A says it "provides an alternative engineered method of water pipe sizing." The designer chooses the code, and the drawings show which method was used.
How do the utility's limits show the conversion?
Phoenix's Water Services Design Standards Manual gives each meter size a maximum flow and fixture unit limits. For flush valve fixtures under the IPC, the manual lists 50 units on a 1 inch meter at 50 gpm, 245 units on a 1 1/2 inch meter at 100 gpm, and 625 units on a 2 inch meter at 160 gpm. For flush tank fixtures the 2 inch meter allows 696 units. The manual adds: "Project designs which exceed the listed gpm unit values shall be upsized to the next larger meter."
Look at the ratio. A 1 inch meter allows 50 flush valve units and 50 gpm, about 1 gpm per unit. A 2 inch meter allows 625 units and 160 gpm, about a quarter gallon per unit. That is the diversity effect: the bigger the building, the less each added fixture adds to peak demand. Our page on what sets a commercial meter size and capacity fee covers the utility side in more detail.
What goes wrong when a line is undersized?
The code gives the test: Table 604.3 sets the flow and pressure each fixture outlet must have at peak demand, so a fixture that starves when others run is not meeting it. Sizing problems often appear after a tenant buildout adds fixtures to an existing line without a new count. See why water pressure drops when several fixtures run and, for the drain side of the same idea, how commercial drain lines are sized with drainage fixture units.
The numbers above come from the model code and Phoenix's published tables; your own drawings and plan reviewer decide the actual pipe sizes. HQ is licensed and insured and gives free estimates, so a plumber can look at a low-pressure commercial building and tell you whether the line, the pressure or a fixture is the cause.