General Plumbing Knowledge
This domain tests the physical and math fundamentals behind every plumbing installation: gravity drainage, trap seals, pressure, fixture units, approved materials, and safe work practices. Exam items often ask you to apply a rule to a field situation rather than simply recall a definition.
Core rules and concepts
- Drainage relies on gravity. Under IPC-style codes, 2-inch and smaller horizontal drains generally need 1/4 inch fall per foot; 3-inch and larger commonly need 1/8 inch per foot. Local amendments can change details, so verify the enforced edition.
- A trap seal is normally 2 to 4 inches deep. The vent protects that seal from siphonage and back pressure; a trap without a proper vent is not a substitute for a vented trap arm.
- Static pressure changes about 0.433 psi per foot of elevation. Friction losses in pipe and fittings reduce the pressure actually available at the fixture.
- Drainage fixture units (DFU) express drainage load; they are not gallons per minute. Water-supply fixture units are a separate sizing basis.
- Pipe, fittings, solder, and joining methods must be listed and approved for the application. Manufacturer instructions are part of the installation requirements.
Worked example
A horizontal branch is 25 feet long and must slope at 1/4 inch per foot. Total fall = 25 × 0.25 = 6.25 inches. If the same run used 3-inch pipe at 1/8 inch per foot, fall = 25 × 0.125 = 3.125 inches. The reasoning is that slope is vertical drop per horizontal foot, not a percentage of the pipe’s inside diameter.
Traps that cost marks
Using 1/8 inch per foot on 2-inch pipe, mixing DFU with gallons per minute, ignoring trap-arm length limits, and assuming a larger drain automatically solves a venting problem are common errors. Another is treating static pressure at the meter as the pressure at an upper-floor fixture.
Key ideas: apply the slope to the actual run length, separate drainage load from water-supply load, and remember that venting protects the trap seal.