2.1 Drainage Fixture Units (DFU) & Load Values

Why DFU values matter on the exam

Every drain-size question on the Wisconsin journeyman exam starts with the same step: add up the drainage fixture unit (dfu) load that passes through the pipe you are sizing. SPS 382.30 (3) (a) 1. says the load factor on drain piping "shall be computed in terms of drainage fixture unit values specified in Table 382.30-1." Once you have the total, you go to Table 382.30-2 (stacks and horizontal branches) or Table 382.30-3 (building drains and sewers), which are covered in the next three sections.

Wisconsin writes its own plumbing code, chs. SPS 381 to 387. Its dfu values are not the IPC or UPC values. Candidates who memorized a national table lose points on the most basic items, so open the book to Table 382.30-1 and learn where it sits in your binder.

Key values from Table 382.30-1

The trap column marked "per standard" means the trap size for water closets and urinals comes from the product standards referenced in SPS 384.20, not from this table.

Fixtures and devices that are not listed

Wisconsin gives three fallback rules:

1. Unlisted sinks (footnote i): 1 dfu for a 1-1/4 inch tailpiece, 2 dfu for a 1-1/2 inch tailpiece and 3 dfu for a 2-inch tailpiece.
2. Receptors of indirect waste are rated by outlet diameter: 1-1/4 inch = 1, 1-1/2 inch = 2, 2-inch = 3, 3-inch = 4, 4-inch = 6 and larger than 4 inches = 8 dfu.
3. Intermittent-flow devices not in the table count 1 dfu per gallon per minute of flow (SPS 382.30 (3) (a) 2.). A code note lets equipment with a timed discharge cycle of 2 minutes or less be treated as intermittent.

Continuous-flow devices

SPS 382.30 (3) (b) is a favorite calculation item. Pumps, ejectors, air-conditioning equipment "or similar devices that discharge continuously" count 2 dfu for each gallon per minute. A cooling unit that discharges 4 gpm continuously therefore adds 8 dfu, not 4. Don't mix this up with the intermittent rule above, which is 1 dfu per gpm.

$$\text{dfu}_{\text{continuous}} = 2 \times \text{flow (gpm)}$$

Future fixtures and design flexibility

• Future fixtures (SPS 382.30 (4) (d)): if the installer provides for fixtures to be added later, their dfu values must be included when sizing drains and vents. The rough-in must end in plugged fittings.
• Lower assigned values: SPS 382.30 (4) (a) allows loads below the table limits only through an approved alternate standard under SPS 381.20 (2) or an analysis by a Wisconsin registered architect, professional engineer or permitted plumbing designer. A journeyman cannot simply assume a smaller value.

Worked load totals

Public restroom. Four public water closets, two urinals, three lavatories and one 2-inch floor drain:

Eight-unit apartment building. Each unit has one bathroom group (6), a kitchen sink (2), a residential dishwasher (2) and a residential clothes washer (4), so each unit is 14 dfu. Eight units carry 112 dfu. Apartment water closets are nonpublic fixtures, but they are already inside the 6-dfu bathroom-group value, so don't count them again.

Mechanical room. A cooling-tower bleed pump discharges 3 gpm continuously, and an unlisted piece of process equipment dumps 5 gpm on a timed cycle of under 2 minutes. The pump counts 2 x 3 = 6 dfu under the continuous-flow rule. The process equipment may be treated as an intermittent device, so it counts 1 x 5 = 5 dfu. The drain downstream of both carries 11 dfu, which needs at least a 3-inch horizontal branch (a 2-inch branch is limited to 6 dfu).

Common exam traps

• Using IPC values (lavatory 1, water closet 3 or 4) or UPC values instead of Table 382.30-1.
• Forgetting that a residential clothes washer is 4 dfu in Wisconsin.
• Applying 1 dfu per gpm to continuous equipment; the correct factor is 2.
• Counting a bathroom group's water closet separately and then adding the group value too.