6.3 Sizing Drainage Piping with Table 703.2 (Vertical, Horizontal, Water Closet Limits, & WA Note 4)

The Table You Must Memorize

L&I states that Tables 702.1, 610.3, 610.4, and 610.10 are provided on the journey level exam, and Table 703.2 is not. Part 2 asks you to provide fixture units and pipe sizes for a waste and vent drawing using minimum sizing, so you need this table in your head.

UPC Table 703.2: Maximum Unit Loading and Maximum Length of Drainage and Vent Piping

For 10-inch pipe, drainage is 5,600 vertical and 4,680 horizontal. For 12-inch, it is 8,400 vertical and 8,200 horizontal. Horizontal drainage length is unlimited.

The notes (where most exam questions come from)

1. Unit loadings exclude the trap arm; trap arms are sized from Table 702.1 and Table 1002.2.
2. 1-1/2-inch vertical (2 DFU): except sinks, urinals, and dishwashers, which may not exceed 1 fixture unit on this size.
3. 2-inch pipe (16 vertical, 8 horizontal): except six-unit traps or water closets. No water closet may drain into 2-inch pipe.
4. 3-inch pipe: only four water closets or six-unit traps on a vertical pipe or stack, and not more than three on a horizontal branch or drain.
   • Washington exception: In a single-family dwelling addition or alteration where a 4-inch horizontal waste is not readily available, four water closets not exceeding 1.6 gpf each may be allowed on a 3-inch horizontal waste when approved by the AHJ.
5. Horizontal values are based on 1/4 inch per foot slope. At 1/8 inch per foot, multiply horizontal fixture units by 0.8.
6. Vent notes (used in Chapter 8): an individual vent is at least 1-1/4 inches and at least one-half the diameter of the drain it serves. No more than one-third of a vent's permitted length may be horizontal. Increasing a vent one pipe size for its entire length removes the length limits.

How to Size a Drain

1. Total the DFU entering each section of pipe, working downstream from the most remote fixture (Table 702.1).
2. Decide whether the section is vertical or horizontal. Horizontal limits are lower because the pipe runs partly full.
3. Pick the smallest size whose limit is equal to or greater than the load.
4. Apply the notes: check water closet counts (notes 3 and 4) and slope (note 5).
5. Check the trap and trap arm minimums from Table 702.1. A pipe can never be smaller than the trap it serves.
6. Do not reduce size in the direction of flow. A smaller pipe downstream retards the flow, which 310.5 prohibits, so a drain should never be smaller than any drain upstream of it.

Worked Examples

Example 1: Single-family house. Two private bathrooms (3 + 1 + 2 each), a kitchen sink (2), a dishwasher (2), a clothes washer (3), and a laundry sink (2) total 21 DFU, with 2 water closets.

• Building drain (horizontal): 3 inches allows 35 DFU and up to 3 water closets. 3 inches works.
• If the design grows to 4 water closets, note 4 forces a 4-inch horizontal drain. In Washington, the exception permits 3 inches only in a single-family addition or alteration, with 1.6 gpf closets, when 4-inch is not readily available and the AHJ approves.

Example 2: Commercial restroom branch. Public values: 6 water closets (6 × 4 = 24), 4 lavatories (4), 3 integral urinals (6), and 1 floor drain (2) total 36 DFU.

• DFU alone: 3-inch horizontal allows only 35, so 36 already needs 4 inches.
• Water closet count: 6 closets on a horizontal branch exceeds note 4's limit of three. 4 inches either way.

Example 3: Reduced slope. A 4-inch horizontal drain installed at 1/8 inch per foot (permitted for 4-inch and larger by 708.1 when the AHJ approves) carries at most 216 × 0.8 = 172.8, so 172 DFU.

Example 4: Stack. A three-story stack carrying 3 floors of private bathrooms (3 × 6 = 18 DFU, 3 water closets): 3-inch vertical allows 48 DFU and 4 water closets, and the stack height must stay within 212 feet. 3 inches works.

Vertical Length Limits

The "maximum length" rows limit how tall a vertical drain or vent of that size may be. A 2-inch vertical drain may be no taller than 85 feet, and a 2-inch vent no longer than 120 feet of developed length. Part 2 drawings rarely approach these limits, but Part 1 may ask for the number directly.

Memory Aids

• "16-8, 48-35, 256-216" gives vertical and horizontal DFU for 2, 3, and 4-inch pipe.
• "Two-inch: no toilets. Three-inch: 4 up, 3 across."
• Vent DFU row: "1-8-24-84-256" for 1-1/4 through 4-inch. It matches the drainage vertical row from 4 inches up.