Master DFU: toilet = 4, tub = 2, sink = 1. Add all fixtures on a drain to get total load. Use CPC Table 7-3 and sizing tables to match load to pipe size. 2-inch pipe handles 20 DFU, 4-inch handles 160 DFU. Practice calculations—they're on every exam.
Key Takeaways
Why This Matters on the Exam
Fixture drain sizing is the most practical skill you need on the job and on the exam. Every bathroom, every kitchen, every commercial project starts with one question: What size drain pipe? The California Plumbing Code uses drainage fixture units (DFU) and tables to size all drainage piping. Master this and you're estimating jobs accurately and installing code-compliant systems. Get this wrong and you're undersizing drains, causing backups, and losing credibility. Every contractor I respect knows DFU cold.
What You Must Know
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