Journeyman Plumber ExamFixtures, Faucets, and Other AppliancesHard
A plumber is installing a dishwasher in a residential kitchen. The dishwasher manufacturer specifies a minimum water supply pressure of 20 PSI for proper operation. The building's static water pressure is 60 PSI. What is the maximum allowed length of the 1/2-inch copper supply tubing to the dishwasher before excessive pressure drop becomes a concern for the appliance?
- A30 feet
- B20 feet
- C50 feet
- D40 feet
Show answer & explanationAnswer & explanation
Correct answer: A. 30 feet
While specific code limits for supply line length are rare, a common rule of thumb for 1/2-inch copper tubing supplying a single fixture like a dishwasher with a 20 PSI minimum requirement from a 60 PSI static pressure is to limit the length to around 30 feet to maintain adequate dynamic pressure. Longer runs can lead to excessive friction loss and insufficient operating pressure.
Why the other options are wrong
- B. 20 feet might be overly cautious, as 1/2-inch copper can typically supply a dishwasher effectively over longer distances.
- C. 50 feet is likely too long for 1/2-inch copper to maintain sufficient dynamic pressure for the dishwasher's minimum requirement.
- D. 40 feet could lead to significant pressure drop, potentially falling below the 20 PSI minimum required by the dishwasher.
Fixture Supply Line Length
The practical maximum length of a fixture's supply line to maintain adequate dynamic pressure, considering pipe size and fixture requirements.
- Longer pipes increase friction loss.
- Smaller pipe diameters increase friction loss.
- Dynamic pressure at the fixture is crucial for proper operation.
- Rule of thumb often used for common fixture types.
Memory trick: Pressure drops with pipe's length and turns, so keep it short and smooth.