6.2 Drainage Fixture Units, Trap Sizes, & Flow Conversions (UPC Table 702.1, 702.2, 702.3)

What a Drainage Fixture Unit Represents

A drainage fixture unit (DFU) measures a fixture's probable load on the drainage system, based on its discharge rate, volume, and frequency of use. Section 702.1 says the unit values in Table 702.1 are based on the size of the trap required. Fixtures not listed are rated by their trap or trap arm size. You add DFUs along the direction of flow, then size each pipe from Table 703.2 (Section 6.3).

Occupancy columns: Private covers dwellings and private rooms, public covers general commercial use, and assembly covers public-use assembly occupancies. Public and assembly values are higher because those fixtures are used more often. Part 2 drawings state which column to use. When the drawing does not say, a single-family house is private.

Selected Rows of Table 702.1

Table notes worth memorizing

1. Indirect waste receptors are sized by the total drainage capacity of the fixtures that drain into them, using Table 702.2(2).
2. Some fixtures (kitchen and laundry sinks, dishwashers, bar and special-purpose sinks, urinals) require a 2-inch minimum drain.
3. A battery of three or more clothes washers is rated at 6 DFU each when sizing common horizontal and vertical drainage piping.
4. Water closets count as 6 DFU when sizing septic tanks under Appendix H.
5. Trap sizes may not be increased to the point that the fixture discharge can no longer scour the trap.

Maximum Load on a Trap (Table 702.2(1))

The table does not apply to self-service laundries. It lets you rate an unlisted fixture from its trap size and check that a trap is not overloaded.

Converting Flow to DFU

Intermittent flow, Table 702.2(2), applies to devices that discharge occasionally at a known rate:

Rates over 50 gpm are set by the AHJ.

Continuous flow (702.3): A pump, sump ejector, air-conditioning unit, or similar device discharging continuously counts 2 DFU per gpm. A 12 gpm continuous discharge therefore adds 24 DFU. Section 710.5 applies the same 2 DFU per gpm to building drains that receive pump or ejector discharge.

Worked Example: Totaling a Bathroom Group

A private bathroom has a 1.6 gpf water closet (3.0), a lavatory (1.0), and a bathtub/shower (2.0). Total: 6.0 DFU.

• The same group in a public restroom (water closet, lavatory, and a public shower instead of a tub): 4.0 + 1.0 + 2.0 = 7.0 DFU.
• A single-family house with two such private bathrooms, a kitchen sink (2.0), a dishwasher (2.0), a clothes washer (3.0), and a laundry sink (2.0): 6.0 + 6.0 + 2.0 + 2.0 + 3.0 + 2.0 = 21 DFU on the building drain.

Common Exam Traps

• Lavatory trap 1-1/4 in, but shower trap 2 in. A shower is not a 1-1/2-inch fixture in the UPC.
• Water closet DFU depends on the column. Private 3, public 4, assembly 6.
• Kitchen sink with disposer and dishwasher on the same trap is still 2.0 DFU private. The row reads "with or without food waste disposer, dishwasher, or both." A dishwasher with its own independent drain adds 2.0.
• Clothes washer standpipe trap is 2 inches. Standpipe height rules are in Chapter 8 (Section 7.3).