Plumbing Systems and Fixtures
This domain tests fixture selection, maximum flow rates, clearances, and installation details - the parts of the trade an inspector can see and measure. Expect questions on fixture units, trap sizes, accessibility dimensions, and what must be installed under a shower or around a water closet.
Flow rates and fixture units
Federal standards cap fixture flow: water closets at 1.6 gallons per flush, urinals at 1.0 gpf, showerheads at 2.5 gpm at 80 psi, and lavatory and kitchen faucets at 2.2 gpm. Replacement fixtures must meet the same limits. Drainage fixture units are assigned per fixture, so a bathroom group is added before any pipe is sized.
Setting and clearances
A water closet needs at least 15 in from its centerline to a side wall and 30 in of clear width. Accessible fixtures need a 60 in turning circle, knee clearance under a lavatory, and grab bars at the tub or shower. Shower pans require a pre-slope, a listed liner, and weep holes kept open at the drain.
Worked example
A bathroom group has a tank water closet (3 DFU), lavatory (1 DFU), and tub/shower (2 DFU) = 6 DFU. A 2 in horizontal fixture branch is limited to 6 DFU, so this branch is exactly at its limit and cannot accept an added laundry tray. The fix is a 3 in branch or a separate branch. Reasoning: total the fixture units first, then compare with the table value for that pipe size and branch type - branch limits are not stack limits.
Traps
Mixing gpf with gpm, using a 1-1/4 in trap on a kitchen sink, forgetting the shower pan pre-slope, sealing the weep holes, and ignoring accessibility clearances are the marks that go most often.