Plumbing (C-36) · Lesson 1 of 8

Fixture Units and Pipe Sizing

"Fixture Units and Pipe Sizing" is lesson 1 of 8 in the Plumbing (C-36) Learning Hub. It opens with "A Fixture Unit Is Not a Fixture Count" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. A Fixture Unit Is Not a Fixture Count
  2. Common Private (Residential) DFU Values - Table 702.1
  3. Trap Size Sets a Hard DFU Ceiling
  4. Building Drain and Branch Sizing - Table 703.2
  5. Add DFUs Fixture by Fixture, Then Look Up the Pipe

The worked example in the course applies this lesson to a realistic job in 4 steps, from "List the fixtures" through "Add them up".

What you'll be able to do

  • Explain the main idea of "Fixture Units and Pipe Sizing" in plain language you can use on the exam.
  • Apply this step with confidence: "Trap Size Sets a Hard DFU Ceiling".
  • Apply this step with confidence: "Building Drain and Branch Sizing - Table 703.2".
  • Apply this step with confidence: "Add DFUs Fixture by Fixture, Then Look Up the Pipe".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
FIXTURE UNITS AND PIPE SIZING CPC Table 702.1 (DFU values) and Table 703.2 (pipe sizing) TABLE 702.1 - PRIVATE DFU Lavatory1.0 Bathtub / bath-shower combo2.0 Single-head shower2.0 Kitchen sink2.0 Clothes washer standpipe3.0 Water closet, 1.6 GPF tank3.0 values in DFU (drainage fixture units) MAX DFU PER TRAP SIZE (Table 702.2(1)) 1 1/4" trap1 DFU 1 1/2" trap3 DFU 2" trap4 DFU 3" trap6 DFU 4" trap8 DFU hard ceiling - never exceed it THE 3-STEP METHOD 1 List every fixture on the branch or stack 2 Add each DFU value from Table 702.1 3 Match the total to Table 703.2 for the minimum pipe size 2" drainup to 8 DFU 3" drainup to 35 DFU 4" drainup to 216 DFU CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: A Fixture Unit Is Not a Fixture Count

A drainage fixture unit (DFU) is a standardized load number the code assigns to each fixture, based on how much waste it discharges and how fast. It is NOT the same as counting fixtures. A kitchen sink and a lavatory are both "one fixture," but the sink is rated heavier: a lavatory is 1.0 DFU, a kitchen sink is 2.0 DFU. You size pipe by adding up DFUs, not by counting fixtures.

Step 2: Common Private (Residential) DFU Values - Table 702.1

From California Plumbing Code Table 702.1, private/residential fixtures: lavatory = 1.0 DFU, bathtub or bath/shower combo = 2.0 DFU, single-head shower = 2.0 DFU, kitchen sink (with or without disposer/dishwasher) = 2.0 DFU, domestic clothes washer standpipe = 3.0 DFU, water closet (1.6 GPF gravity tank) = 3.0 DFU. Memorize these six - they cover most residential bathrooms and kitchens.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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