On a horizontal branch, two public lavatories drain together before any other fixture joins. What is the minimum size of that horizontal segment under Table 703.2?

2 inches. Two lavatories total 2 DFU. In the horizontal column of Table 703.2, both 1-1/4-inch and 1-1/2-inch pipe are limited to 1 DFU, while 2-inch pipe allows 8. The segment must therefore be 2 inches. Each lavatory's own trap arm is 1-1/4 inches per Table 702.1.

A horizontal building drain carries 31.5 DFU from four 1.6 gpf flushometer water closets and other fixtures in an office building. What is its minimum size?

4 inches. A 3-inch horizontal drain would carry 35 DFU, but Table 703.2 note 4 allows no more than three water closets on a 3-inch horizontal branch or drain. Four water closets require 4 inches. Washington's exception allowing four closets on 3 inches is limited to single-family dwelling additions or alterations with AHJ approval.

A vertical 3-inch stack receives two floors, each with two water closets, totaling 28 DFU. Does the stack comply with Table 703.2?

Yes; 3-inch vertical pipe allows 48 DFU and up to four water closets. Table 703.2 allows 48 DFU on 3-inch vertical drainage pipe, and note 4 limits a 3-inch vertical pipe or stack to four water closets or six-unit traps. With 28 DFU and four water closets, the stack complies. A fifth water closet would require 4 inches.

Part 2 Worked Example: Accumulating DFU and | Free Guide 2026

Key Takeaways

In UPC Table 703.2, 1-1/2-inch horizontal drainage pipe carries only 1 DFU, so two lavatories on a horizontal branch require 2-inch pipe.
A drain segment receiving a water closet must be at least 3 inches, because Table 703.2 note 3 bars water closets from 2-inch pipe.
A 3-inch vertical stack may carry up to 48 DFU and four water closets, but a 3-inch horizontal drain only 35 DFU and three water closets (Table 703.2 note 4).
Public Table 702.1 values used on commercial drawings include 4 DFU for a 1.6 gpf water closet, 1 for a lavatory, 2 for a 1.0 gpf integral-trap urinal, and 3 for a mop basin.
A drain may not get smaller in the direction of flow; once a segment is 3 inches, everything downstream stays 3 inches or larger.

Step 1: DFU and Trap Size for Every Fixture (Table 702.1, Public)

Fixture	Minimum trap and trap arm	Public DFU
Water closet, 1.6 gpf flushometer valve	3 in	4.0
Lavatory	1-1/4 in	1.0
Urinal, integral trap, 1.0 gpf	2 in	2.0
Floor drain	2 in	2.0
Service or mop basin (2-inch trap)	2 in	3.0
Drinking fountain	1-1/4 in	0.5

Restroom group total: 2 × 4.0 + 2 × 1.0 + 2.0 + 2.0 = 14.0 DFU, including 2 water closets.

Step 2: Build the Ledger (Second-Floor Branch, Horizontal)

The limits below come from Table 703.2, horizontal column: 1-1/4 in = 1; 1-1/2 in = 1; 2 in = 8 (no water closets); 3 in = 35 (maximum 3 water closets); 4 in = 216.

Segment	What joins	Running DFU	Water closets	H/V	Minimum size	Why
A	LAV-1 trap arm	1.0	0	H	1-1/4 in	Trap arm at the Table 702.1 minimum; 1-1/4-inch arm carries up to 1 DFU (Table 702.2(1))
B	+ LAV-2	2.0	0	H	2 in	1-1/2-inch horizontal is limited to 1 DFU; 2 inches carries 8
C	+ urinal	4.0	0	H	2 in	4 is 8 or less; urinal trap is 2 inches
D	+ floor drain	6.0	0	H	2 in	6 is 8 or less
E	+ WC-1	10.0	1	H	3 in	Water closets not allowed on 2-inch pipe (note 3)
F	+ WC-2	14.0	2	H	3 in	14 is 35 or less, and 2 water closets is 3 or fewer

Checkpoint: Segment B is the classic trap. Many candidates write 1-1/2 inches for two lavatories. In the horizontal column, 1-1/2-inch pipe allows only 1 DFU, so the answer is 2 inches.

Step 3: The Stack (Vertical)

The limits below come from Table 703.2, vertical column: 1-1/2 in = 2; 2 in = 16 (no water closets); 3 in = 48 (maximum 4 water closets); 4 in = 256. Vertical length limit for 3 inches: 212 feet.

Segment	Load	Water closets	Minimum size	Why
G (stack, 2nd floor to 1st floor)	14.0	2	3 in	14 is 48 or less; 2 water closets is 4 or fewer; stack well under 212 feet

Step 4: First-Floor Branch (Horizontal)

Segment	Load	Water closets	Minimum size	Why
H (restroom group + mop sink + drinking fountain)	14.0 + 3.0 + 0.5 = 17.5	2	3 in	17.5 is 35 or less; 2 water closets is 3 or fewer

Inside the branch, the mop sink's 2-inch trap arm and the drinking fountain's 1-1/4-inch trap arm are sized from Table 702.1. Any horizontal section carrying only those two fixtures (3.5 DFU) could be 2 inches. Downstream of the first water closet connection, the branch is 3 inches.

Step 5: Building Drain (Horizontal)

Segment	Load	Water closets	Minimum size	Why
J (stack + first-floor branch)	14.0 + 17.5 = 31.5	4	4 in	31.5 fits within 3-inch horizontal (35 DFU), but four water closets exceed the three allowed on a 3-inch horizontal (note 4).

Two more checks:

Slope: at 1/8 inch per foot (4-inch and larger, AHJ approval), the 4-inch capacity becomes 216 × 0.8 = 172 DFU, still far above 31.5.
Washington exception to note 4 (four 1.6 gpf water closets on a 3-inch horizontal) applies only to single-family dwelling additions or alterations with AHJ approval. It does not apply to this office building.

Step 6: Answer Common Question Forms

Question	Answer from the ledger
"How many fixture units at segment D?"	6
"Minimum size of segment B?"	2 inches
"Minimum size of the stack?"	3 inches
"Fixture units in the building drain?"	31.5
"Minimum size of the building drain?"	4 inches

Variations the Exam Can Throw at You

Private occupancy: Change the water closet value to 3.0 and recompute. A private restroom group of 2 water closets, 2 lavatories, and 1 tub/shower totals 3 + 3 + 1 + 1 + 2 = 10 DFU.
A 2-inch trap arm feeding 5 DFU: Table 702.2(1) limits a 2-inch trap to 4 DFU. An unlisted fixture rated by trap size cannot exceed that.
Continuous flow: A 10 gpm continuous discharge adds 20 DFU (702.3).
Battery of three or more clothes washers: use 6 DFU each for common piping (Table 702.1 note 5).
A 1-1/2-inch vertical waste serving two lavatories: allowed (vertical 2 DFU). Two kitchen sinks at 2 DFU each on 1-1/2-inch vertical pipe are not allowed (note 2: sinks may not exceed 1 fixture unit on this size).
Never reduce downstream. If segment E is 3 inches, segments F, G, and J cannot be smaller than 3 inches, even if their DFU alone would allow it.

Washington Journeyman Plumber

11.2 Part 2 Worked Example: Accumulating DFU and Sizing Every Drain Segment