General Plumbing Knowledge
What this domain covers
General Plumbing Knowledge is the framing domain: definitions, pipe and fitting materials, jointing methods, support and testing, plus the arithmetic that shows up in every other section - pressure versus head, fixture units, and grade.
Rules and concepts that get tested
- A plumbing system is the water service, distribution piping, fixtures and fixture fittings, soil and waste piping, vents, and the building drain/sewer up to the point of connection.
- Potable water piping must be lead-free (0.25 percent weighted average lead in wetted surfaces); copper joints use lead-free solder and approved flux.
- Dissimilar metals need a dielectric union or approved transition fitting; steel-to-copper contact causes galvanic corrosion and early failure.
- Support spacing depends on material: copper horizontal tubing 1¼ inch and smaller at 6 feet, larger sizes at 10 feet; plastic DWV is supported much more closely per the adopted table.
- DWV systems are tested before concealment: typically a 10-foot head of water held 15 minutes, or an air test where the code permits, with all openings plugged and the test witnessed by the inspector.
- Grade and elevation arithmetic: 1 psi of pressure equals 2.31 feet of head, or 0.433 psi per foot of rise.
Worked example
A meter delivers 60 psi static. A shower valve sits 30 feet above the meter. Elevation loss = 30 ÷ 2.31 = 13 psi, so static pressure at the valve is 60 − 13 = 47 psi. Subtract friction loss through the developed length (say 8 psi) and flowing pressure is about 39 psi - enough for the fixture, but the point is that elevation is charged before friction, not after.
Traps that cost marks
Adding friction loss to static pressure instead of subtracting it; forgetting to convert feet of head to psi; assuming a 10-foot water test also proves a gas line; skipping support spacing because "the pipe is short."