Trade Exam · Module 3
Fixture Drain Sizing
Size drainage piping the way the 2025 California Plumbing Code does it: look up each fixture's drainage fixture units and minimum trap size in Table 702.1 (private, public or assembly column), add continuous and intermittent flows, total the load on each section of pipe, and pick the smallest pipe that Table 703.2 allows, watching the notes on water closets, slope and vertical versus horizontal pipe.
Key Takeaways
- DFU values and minimum trap and trap arm sizes come from Table 702.1, which has separate private, public and assembly columns.
- Private values: lavatory 1, bathtub 2, shower 2, domestic kitchen sink 2, clothes washer 3, 1.6 gpf water closet 3. A 1.6 gpf water closet is 4 in public use and 6 in assembly use.
- Continuous flow counts 2 DFU per gpm (702.3). Intermittent flow uses Table 702.2: up to 7-1/2 gpm = 1, over 7-1/2 to 15 = 2, over 15 to 30 = 4, over 30 to 50 = 6.
- Table 703.2 horizontal maximums: 1-1/2 in 1, 2 in 8, 3 in 35, 4 in 216, 6 in 720. Vertical: 1-1/2 in 2, 2 in 16, 3 in 48, 4 in 256.
- No water closet on 2 in pipe; no more than six water closets on 3 in pipe (Table 703.2 notes 3 and 4).
- At 1/8 in per ft slope, multiply the horizontal DFU limit by 0.8 (Table 703.2 note 5).
- A trap may be at most one pipe size larger than Table 702.1 and matches its trap arm (1003.3); do not upsize traps so far that they stop self-scouring (Table 702.1 note 7).
Why This Matters
The CSLB C-36 study guide lists "pipe sizing and venting requirements" under Planning and Estimating and "piping layout and installation" under Underground and Rough Systems, and it tells you some questions require math. Drain sizing is that math. The method in the 2025 California Plumbing Code (CPC) is simple and repeatable: look up, add up, look up again. The only way to get it wrong is to use the wrong numbers, so this lesson uses only the values printed in the code.
Step 1: Look Up Each Fixture in Table 702.1
Table 702.1 gives each fixture two things: the minimum trap and trap arm size, and a drainage fixture unit (DFU) value. The DFU value depends on the occupancy column you are in: private (dwellings), public, or assembly.
Private column, common fixtures:
- Lavatory: 1-1/4 inch trap, 1 DFU
- Bathtub or combination bath and shower: 1-1/2 inch, 2 DFU
- Shower, single head: 2 inch, 2 DFU
- Domestic kitchen sink, with or without disposer, dishwasher or both: 1-1/2 inch trap, 2 DFU (with a 2 inch minimum drain per note 2)
- Domestic dishwasher on its own drain: 1-1/2 inch, 2 DFU
- Laundry sink: 1-1/2 inch, 2 DFU (2 inch minimum drain per note 2)
- Clothes washer standpipe: 2 inch, 3 DFU
- Floor drain: 2 inch, 2 DFU
- Bar sink: 1-1/2 inch, 1 DFU
- Water closet, 1.6 gpf or less (gravity tank, flushometer tank or flushometer valve): 3 inch, 3 DFU
Public column examples:
- Water closet 1.6 gpf or less: 4 DFU (assembly 6)
- Water closet over 1.6 gpf: 6 DFU (private 4, assembly 8)
- Urinal with integral trap, 1.0 gpf: 2 inch, 2 DFU (assembly 5)
- Lavatory: 1 DFU
Other rules around the table:
- Fixtures not listed (702.1): base the unit value on the size of the trap or trap arm, using Table 702.1(1): 1-1/4 inch 1 unit, 1-1/2 inch 3 units, 2 inch 4 units, 3 inch 6 units, 4 inch 8 units.
- Laundries (note 5): buildings with a battery of three or more clothes washers rate them at 6 DFU each for sizing the common drain.
- Septic sizing (note 6): water closets count as 6 DFU when sizing septic tanks under Appendix H.
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