UPC Section 603 requires the drinking-water supply to be protected wherever it could connect to a source of contamination. It bans unprotected cross-connections and calls for a backflow device, such as an air gap or a vacuum breaker, matched to the level of hazard at each point.
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.
The drinking-water supply in a building is only safe if nothing can flow backward into it. A cross-connection is any point where the clean supply could meet a source of contamination, like a hose in a bucket, an irrigation line, or a boiler. UPC Section 603 is the rule that requires those points to be protected, so a drop in pressure can never pull pollutants into the water people drink.
What this section says
Section 603 sets a blanket requirement: the potable water system has to be protected against backflow at every cross-connection, and unprotected cross-connections are prohibited. The code states the principle plainly:
No person shall install a water-operated equipment or mechanism, or use a water-treating chemical or substance, if it is found that the equipment or mechanism could cause pollution or contamination of the domestic water supply.
The protection has to match the hazard. Where a backflow could carry something merely objectionable, a simpler device like a vacuum breaker may be enough. Where it could carry something dangerous to health, the code requires stronger protection, up to an air gap or a reduced-pressure backflow assembly. The greater the risk, the more robust the required device. The section works together with the rules on air gaps, vacuum breakers, and backflow assemblies elsewhere in the code, which spell out each device.
When this comes into play
This rule reaches far more of a building than people expect. An irrigation system tapped off the house supply is a cross-connection, because sprinkler lines sit in soil and fertilizer. So is a hose bibb, a mop sink, a commercial dishwasher, a fire-sprinkler line, and a carbonated-beverage machine. Picture a home with a lawn irrigation system: Section 603 requires a backflow device on that connection, because without one a pressure drop could siphon lawn water back into the drinking supply. Each of these points needs the device that fits its hazard level.
What this means for you
If you add anything that connects the water supply to a possible contaminant, an irrigation zone, a pool auto-fill, a water treatment loop, plan on backflow protection as part of the job. It is a code requirement, not an upgrade. To understand the underlying hazard, see what is a cross-connection and what backflow is, and for the common irrigation case, see backflow preventer for sprinklers in Arizona. Phoenix layers its own rules on top under Phoenix City Code Chapter 37. Because the right device depends on the hazard level, this is a good place to confirm the call with a licensed plumber.
Full text and source
UPC Section 603 is part of the Uniform Plumbing Code. IAPMO publishes and holds the copyright on it. The excerpt above reflects the rule as adopted; Phoenix enforces the 2024 UPC with local amendments, so verify the current text and section numbers. Read the cross-connection rules on UpCodes, or confirm local amendments through the City of Phoenix: phoenix.gov/pdd.
Keep Reading
- UPC 1003: Which Fixture Traps Are Allowed (and Banned)
- UPC Section 1101: Storm Drainage and Roof Runoff
- UPC 705: Joints and Connections in Drainage Piping
- UPC 706: Changes of Direction in Drainage Piping
- What is a cross-connection and why is it dangerous?
- Do I need a backflow preventer and annual test for my sprinklers in Arizona?
- What is backflow in plumbing?