Definitions, General Requirements, and Plumbing System Design
Definitions, General Requirements, and Plumbing System Design
This domain tests precise vocabulary and the design principles that apply before pipe sizing begins. Definitions matter because code requirements are triggered by terms such as plumbing, fixture, fixture unit, developed length, flood level rim, trap seal, backflow, and cross-connection. General requirements cover materials, support, protection, access, and coordination with other trades.
Core concepts
- Plumbing includes the water supply, fixtures, drainage, venting, and appurtenances inside or adjacent to a building, up to the connection to the public system or private well/septic.
- A fixture unit is a measure of probable load, not a count of fixtures. Drainage fixture units and water-supply fixture units use different scales.
- Developed length is the measured length along the pipe run including fittings and changes of direction, not the straight-line distance on plan.
- The flood level rim is the top edge of the receptacle from which water can overflow. Backflow protection and air gaps are measured relative to that rim.
- An air gap is a physical separation; a cross-connection is any actual or potential link between potable and nonpotable water.
Worked example
A plan shows a 2-inch drain terminating 1 inch above a floor sink rim. The effective opening is the pipe inside diameter. An air gap must be at least twice the effective opening, so a 2-inch pipe needs at least 4 inches of separation. The proposed 1 inch is a cross-connection; raise the pipe or use an approved air gap fitting.
Traps
Candidates lose marks by treating developed length as horizontal distance only, by confusing supply and drainage fixture units, and by measuring an air gap from the floor instead of the flood level rim.