The Uniform Plumbing Code sizes water supply pipe so every fixture gets enough flow and pressure at once. It counts each fixture's demand in water supply fixture units, adds them up for a line, and factors in pipe length and available pressure to set the minimum pipe diameter.
The Uniform Plumbing Code is published and copyrighted by IAPMO. This page explains the section in our own words with a short excerpt only. Read the full official text at the source.
Turn on a shower while the washing machine fills and the dishwasher runs, and the water should still work at each one. That only happens if the supply pipes were sized for the whole load, not just one fixture. The Uniform Plumbing Code sets a method for sizing water supply pipe so a home or building delivers enough flow and pressure to every fixture that might run at the same time.
What the code requires
The code sizes water pipe from demand, not guesswork. Each fixture is assigned a water supply fixture unit value, a number that represents how much water it draws and how often. A code states the aim plainly:
The water supply system shall be designed and installed to supply water to each fixture at rates and pressures adequate for their proper operation.
To hit that, the designer adds up the fixture units on a given pipe run, then reads a code table that ties total fixture units to a minimum pipe size. Two more factors adjust the answer. Developed length, the actual routed distance the water travels, matters because longer runs lose more pressure to friction. And available pressure, the pressure coming from the street or well, sets how much there is to work with. A long run on low street pressure needs larger pipe than a short run on high pressure carrying the same fixtures. The result is a minimum diameter for each section, from the main down to each branch.
When this comes into play
This method governs any new water system and most remodels that add fixtures. Picture a bathroom addition on the far end of a house: adding a shower, sink, and toilet raises the fixture-unit load on the line feeding that side, and the long run to reach it loses pressure along the way. The existing pipe may be too small for the new total, so the branch, or even part of the main, has to be upsized. Get it wrong and fixtures starve when several run at once.
What this means for you
If your water pressure drops whenever a second fixture turns on, undersized supply pipe is one possible cause, alongside a failing pressure regulator or partially closed valve. It is a design issue, not something a cleaning fixes. For related pressure symptoms, see signs a pressure regulator is going bad and low water pressure through the whole house. The installed pipe also has to meet UPC 609 water piping installation. Before adding fixtures, it is worth having a plumber check whether the existing pipe can carry the new load.
Full text and source
The UPC water-sizing rules are part of the Uniform Plumbing Code, published and copyrighted by IAPMO. The excerpt above reflects the approach as adopted; Phoenix enforces the 2024 UPC with local amendments, so verify the current tables and section numbers. Read the sizing rules on UpCodes, or confirm local amendments through the City of Phoenix: phoenix.gov/pdd.
Keep Reading
- UPC Section 604: Approved Materials for Water Pipe
- UPC Section 313: How Often a Pipe Must Be Supported
- UPC 1003: Which Fixture Traps Are Allowed (and Banned)
- UPC 1007: Trap Seal Protection for Floor Drains
- What are the signs my pressure-reducing valve is going bad?
- Why does my water pressure drop when two fixtures run at once?
- Why is my water pressure low throughout the whole house?