Residential Plumbing Code Essentials: IPC vs UPC

Two Codes, One Country

Unlike the NEC, which is a single national electrical code adopted (with amendments) across virtually all US jurisdictions, plumbing in the United States is governed by two competing model codes:

• IPC (International Plumbing Code) — published by the International Code Council (ICC)
• UPC (Uniform Plumbing Code) — published by the International Association of Plumbing and Mechanical Officials (IAPMO)

Roughly speaking, the eastern half of the US tends to adopt the IPC, while the western half (particularly California and the Pacific Northwest) tends to adopt the UPC. However, this is a generalization — each state and sometimes each municipality makes its own adoption decision.

Key Differences

Drain-Waste-Vent (DWV) Sizing

One of the most significant differences between the two codes is how they size drainage and vent piping:

• The IPC uses drainage fixture units (DFU) to calculate pipe sizing. Fixture unit values are assigned to each fixture, and tables are used to determine the minimum pipe size based on the total DFU load.
• The UPC uses a similar fixture unit system but with different fixture unit values for some fixtures and different pipe sizing tables. The UPC tends to require slightly larger pipe sizes in some configurations.
• Vent sizing also differs. The UPC generally requires larger vent pipes than the IPC for the same number of fixture units.

Venting Methods

Both codes allow traditional individual venting, but they differ on alternative venting methods:

• Air admittance valves (AAVs) are permitted by the IPC with fewer restrictions. The UPC has historically been more restrictive with AAVs, though recent editions have expanded their permitted use.
• The IPC permits combination waste and vent systems more broadly than the UPC.
• Wet venting provisions differ significantly between the two codes. The IPC allows more liberal use of wet venting for bathroom groups.

Water Supply Sizing

Both codes use water supply fixture units (WSFU) to size water distribution piping, but the values and methods differ:

• The IPC references Table E103.3 (in the appendix) for converting fixture units to demand in gallons per minute
• The UPC uses its own demand calculation method based on fixture units
• Pipe sizing calculations account for available pressure, pipe length, friction loss, and elevation changes in both codes, but the specific methods and tables are different

Materials

Both codes permit common plumbing materials like copper, PVC, ABS, CPVC, and PEX, but there are differences:

• The UPC was slower to accept PEX for potable water systems but now permits it broadly
• The IPC permits ABS plastic for DWV in all applications where PVC is permitted. Some UPC jurisdictions have historically restricted ABS in certain applications
• Both codes require piping materials to meet referenced standards (ASTM, NSF, etc.)

Which Code Does Your State Use?

Most states adopt one code statewide, but some allow local jurisdictions to choose. Key examples:

• California — UPC (California Plumbing Code is based on UPC with state amendments)
• Texas — IPC in most jurisdictions
• New York — IPC (statewide adoption)
• Oregon, Washington — UPC
• Florida — IPC (Florida Building Code references IPC)
• Illinois, Ohio, Pennsylvania — IPC

Always verify the adopted code and edition with your local building department before starting work. State amendments can modify the base code significantly.

Exam Implications

If you are preparing for a journeyman or master plumber exam, you must know which code your exam is based on. Study materials and practice questions are code-specific. Using IPC study materials for a UPC exam (or vice versa) will lead to incorrect answers on sizing and method questions.