An 11-section interactive reference guide covering the core plumbing design codes, calculation methods, and engineering rules used daily in MEP and plumbing engineering practice. Includes water supply sizing (Hunter's method), sanitary drainage DFU tables, vent system rules, water heater sizing, backflow prevention assembly selection, roof drainage, medical gas verification requirements (NFPA 99), and fuel gas pipe sizing (NFPA 54 / IFGC).
Each section targets a core plumbing design discipline: plumbing code overview (IPC vs. UPC adoption, key differences in trap distances and venting), water supply sizing using Hunter's fixture unit method (WSFU to GPM via Hunter's curve, pipe material selection, pressure drop), sanitary drainage design (DFU tables, slope requirements, grease interceptors, cleanout locations), vent system types and trap-to-vent distance limits, water heater sizing and Legionella prevention per ASHRAE 188, backflow prevention device selection by hazard level (air gap, RPZ, DCVA, PVB, AVB), storm and roof drainage sizing formulas, medical gas systems per NFPA 99 with verification test requirements, fuel gas systems per NFPA 54/IFGC including CSST bonding, plumbing fixture counts and ADA compliance, and a master quick-reference table.
Use the Prev / Next buttons at the bottom, or press the arrow keys on your keyboard. Click the ☰ menu button in the top-right to open the table of contents and jump to any section. The gold progress bar at the top tracks your position through all 11 sections.
The International Plumbing Code (IPC) is adopted by most eastern US states and many others nationwide. The Uniform Plumbing Code (UPC, often adopted as the California Plumbing Code — CPC) is the primary code in California, Oregon, Washington, Nevada, Hawaii, and Arizona. Key differences include trap-to-vent distances (UPC is more restrictive), wet venting rules, and stack venting (permitted in IPC, not in UPC). Always confirm the adopted code edition with the local Authority Having Jurisdiction (AHJ) — some jurisdictions are still enforcing the 2015 or 2018 edition.
Drainage Fixture Units (DFU) from IPC Table 709.1 and Water Supply Fixture Units (WSFU) from IPC Table 604.1 are the basis for all pipe sizing. DFU values drive sanitary drain and stack sizing from IPC Tables 710.1(1) and 710.1(2). WSFU totals are converted to design GPM demand using the Hunter's curve probability method, which accounts for diversity of simultaneous fixture use rather than assuming all fixtures flow at once. Always verify both pipe size and slope satisfy the DFU capacity from Table 710.1(1) simultaneously.
Use Hunter's Method (fixture unit method per IPC Section 604). First, assign fixture units (FU) from IPC Table 604.1 to each fixture (water closet=2.5 FU, lavatory=1, shower=2). Sum all FUs for the building. Convert total FU to GPM demand using IPC Table 604.1 or Figure 604.1 (Hunter's probability curve). Then size piping from the water service inward using IPC Table 604.3 — check that velocity stays below 8 ft/s. Verify that residual pressure at the most remote, highest fixture is at least 15 psi after subtracting elevation loss (0.434 psi/ft), meter loss, and pipe friction losses.
Per IPC Section 704.1: 3-inch and smaller drain pipes require a minimum slope of 1/4 inch per foot (approximately 2%). Pipes 4 inches and larger require a minimum slope of 1/8 inch per foot (approximately 1%). These slopes maintain a self-cleaning velocity of approximately 2 ft/s when the pipe flows at half-full. Maximum slope is 3 inches per foot (45°) — steeper slopes allow liquid to outrun solids, causing deposition. Ensure as-built slopes are surveyed, especially for long horizontal runs where framing tolerances can affect slope.
An RPZ assembly (ASSE 1013) is required for high-hazard cross-connections where contaminated water could cause health harm if it entered the potable supply — examples include boiler systems (chemicals), irrigation with fertilizer injection, fire suppression with antifreeze, and any connection to a non-potable water source. A Double Check Valve Assembly (DCVA, ASSE 1015) is acceptable for low-hazard connections where contamination would cause aesthetic issues (taste/odor) but not health harm — clean irrigation water, residential fire suppression without additives. RPZ assemblies are required by most water utilities for any commercial or industrial connection regardless of hazard level; check with your local water authority.
NFPA 99 Chapter 5 requires a series of verification tests before a medical gas system can be placed in service: (1) Cross-connection test — each outlet must be tested with only the correct gas flowing; all other outlets must be confirmed dry (no gas) at the same time. (2) Pressure test — gas piping must hold 150% of operating pressure for a minimum of 24 hours with no pressure drop (using nitrogen or oil-free dry air). (3) Flow test — verify adequate flow and pressure at outlets. (4) Purity test — sample each gas at the outlet and verify purity meets NFPA 99 Table 5.1.3.3. (5) Alarm test — verify all zone and master alarms function correctly. Tests must be witnessed by a qualified verifier and documented.