Trade Exam · Module 3
Water Supply Rough-In
Rough in the potable water system the way the 2025 California Plumbing Code reads: the 15 psi minimum and 80 psi maximum, when a pressure regulator and an expansion tank are required, copper tube weights and color codes, shutoff and control valve locations, hot on the left, hose bibb backflow protection, which backflow assembly fits which hazard, support spacing, burial depth, slab rules, and the pressure test before cover.
Key Takeaways
- Residual pressure at least 15 psi (608.1). Static pressure over 80 psi needs an approved pressure regulator with a strainer, set to 80 psi or less (608.2).
- A check valve, backflow preventer or other normally closed device creates a closed system that needs a listed expansion tank or similar device (608.3).
- Copper water tube is at least Type L; Type M is allowed above ground in or on a building, or underground outside structures (604.3). Color codes: K green, L blue, M red (604.4).
- Fullway valve after the meter and on the cold supply to each water heater (606.2); control valve ahead of each water supplied appliance and each slip joint or appliance supply (606.5).
- Hot water is on the left when facing the fixture (417.5).
- Hose outlets need a nonremovable hose bibb backflow preventer or vacuum breaker, or an AVB at least 6 in above the highest use point (603.5.7).
- Test the water system at working pressure with water, or 50 psi with air (never air on plastic), for at least 15 minutes (609.4).
Why This Matters
Water supply rough-in is part of Underground and Rough Systems, which the CSLB C-36 study guide weights at 22 percent of the trade exam. Potable water piping is under constant pressure and it carries the water people drink, so the code rules focus on three things: keeping the pressure in a safe range, keeping contamination out, and making sure the pipe is the right material, properly supported and tested before it is covered.
Every number below comes from Chapter 6 of the 2025 California Plumbing Code (CPC) or from its support table, Table 313.3.
Pressure: 15 psi Minimum, 80 psi Maximum
Too little (608.1): where the main will not provide a residual pressure of at least 15 psi after friction and other losses, a tank and pump or other means must provide it. If a fixture or fitting needs more than 15 psi to work, that higher residual pressure must be provided.
Too much (608.2): where static pressure in the water supply piping exceeds 80 psi, install an approved pressure regulator preceded by an adequate strainer, and reduce the static pressure to 80 psi or less. Other details from the same section:
- Regulators comply with ASSE 1003 or AWWA C530.
- Regulators 1-1/2 inches and larger do not need a strainer.
- The regulator controls the pressure to the outlets in the building unless the AHJ approves otherwise.
- The regulator and strainer are accessibly located above ground, or in a vault with a properly sized and sloped drain to daylight, protected from freezing, with the strainer cleanable without removing the regulator.
The code sets the ceiling at 80 psi. It does not require any particular downstream setting below that.
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