Sizing Potable Water Lines

In order to provide a consistent supply of water to every fixture in a plumbing system the water lines must be sized according to a number of factors. Gone are the days of piping in a home only with 1/2" copper, people today expect their water pressure to be consistent, regardless of the number of fixtures being used.

Sizing factors

In order to properly size the potable water system, a number of factors must be taken into account:

  1. Number and type of plumbing fixtures in the building. This is calculated in FU (fixture units), which is a standardized way to calculated the expected demand, each fixture having a numerical value given to it.
  2. Supply pressure. The supply pressure available from the water main will dictate the required size of the water service as well as the requirements of a pressure regulating valve (PRV).
  3. Longest run. Is the distance from the beginning of the water distribution system, to the most distant fixture. This will take into account friction loss in the piping and valves.
  4. Head loss. The elevation change from the service to the highest fixture must be calculated to find the pressure drop. Each meter of height drops the pressure 10Kpa, or 1 foot = 0.433 PSI.
  5. Plumbing codes for the area (for this article, the Canadian Plumbing Code will be used)

Sizing walkthrough

Practice water sizing diagramThe best way to learn is to "see and do", so print off this image (right click- Print target/save image), and get a 2005 code book. (I've requested permission to use the required tables in this article but have yet to be informed)

  1. First thing is to size all of the fixture supplies and fill in their fixture units. To do this use Table 2.6.31. (since this is a commercial building, the "public use" fixture units are used)
    1. Example: Lavatories - 3/8" supplies, 2 FU (fixture units)
    2. Water closets with flush tank - 3/8" , 5 FU
    3. SS(service sink)- 1/2", 4 FU
    4. Kitchen sink- 1/2", 4 FU
    5. Showers(each)- 1/2", 4 FU
    6. Because the Hose bibb is 3/4" it must be sized with Table 2.6.3.2. (6 FU)
    7. LT(Laundry Tray) - Not listed, use 1/2" supply and 4 FU
  2. Using the given elevation of the highest fixture (8M) find the pressure loss in the system. For each Meter of elevation you lose 10 Kpa. 442Kpa - (8M x 10Kpa) =  442Kpa - 80 Kpa = 382Kpa. Using the available pressure of 382Kpa you can now size the supply main and water distribution system using                    Table A-2.6.1.1.(1) on page A-64 of the appendix.
  3. To use the table, the longest run in Meters must be used to make up for friction loss. On this example the longest run is given as 75M, so the column 76M is used.
  4. To find the size of the distribution pipe, all the fixtures downstream of it must be totaled.
  5. The supply for the hot water tank is determined by the hot water fixture units. Add up all the hot water fixture units to find the size of the supplies in and out of the tank. In this example there are 31 HWFU so the size is 1 1/4". *The hot water fixture units are not added to the cold water fixture units for sizing the supply main, only to size the cold water supply to the tank.

Water sizing answersNow you should be able to size all potable water pipes in the system or any other system using the few tables covered here and a little math. Here is a completed drawing to check your answers against.

*For sizing "home run" manifold systems the total fixture units are used to size the manifold.