The United States doesn't have a single plumbing code. Roughly two-thirds of the country follows the International Plumbing Code (IPC), published by ICC. The Western states, including California, largely adopt the Uniform Plumbing Code (UPC), published by IAPMO. Both codes regulate the same systems, but their prescriptive requirements diverge in ways that create real problems for engineers working across jurisdictions.
This post covers the differences that matter most during drawing review: the ones that generate correction notices when an engineer applies the wrong code's requirements.
The jurisdictional split
The IPC is adopted by most Eastern, Southern, and Midwestern states as part of the ICC family of I-Codes (alongside the IBC, IMC, and IECC). The UPC is adopted primarily in California, Washington, Oregon, Nevada, Hawaii, and several other Western states. A handful of jurisdictions use their own plumbing codes or heavily amended versions of either model code.
Fixture unit systems
Both codes use fixture units to size drain and water supply piping, but the unit values differ. A water closet (flush valve) is 4 DFU in the IPC (Table 709.1) and 4 DFU in the UPC (Table 702.1) for drainage, so that one aligns. But a lavatory is 1 DFU in the IPC and 1 DFU in the UPC. Where they start to diverge is in how those fixture units translate to pipe sizes.
The IPC and UPC use different pipe sizing tables with different capacity assumptions. A 3-inch horizontal drain can carry 35 DFU at 1/4-inch per foot slope in the IPC (Table 710.1(2)), but the UPC allows 35 DFU on a 3-inch branch at 1/4-inch per foot (Table 703.2). The numbers look similar at first glance, but the cumulative effect on building drain sizing diverges on larger projects, especially high-rise residential where fixture counts are high.
Key differences
| Topic | IPC | UPC |
|---|---|---|
| Venting methods | Allows wet venting, circuit venting, combination waste and vent | Allows wet venting, circuit venting, combination waste and vent, plus island fixture venting per Section 909 |
| Wet vent sizing | IPC Section 912: wet vent sized based on fixture unit load of wet-vented fixtures | UPC Section 908: wet vent sized based on total connected fixture units, generally more conservative |
| Air admittance valves (AAVs) | Permitted where approved by the AHJ (Section 918) | Historically prohibited, now conditionally permitted in the 2021 UPC (Section 917) but many California jurisdictions still prohibit them |
| Trap arm length | Maximum trap arm length tied to pipe diameter, per Table 1002.2 (e.g., 6 feet for 1-1/2 inch) | Maximum trap arm distance also tied to pipe diameter but with different limits per Table 1002.2 |
| Grease interceptors | Section 1003: required for commercial food service, sized per PDI-G101 or engineered design | Section 1009 and 1014: more prescriptive sizing requirements, different flow rate assumptions |
| Rainwater piping | IPC Chapter 11: rainfall rate from local data, sizing per Table 1106.2 | UPC Appendix D: similar approach but different table values, plus stricter secondary drainage requirements in some editions |
| Medical gas | References NFPA 99 for medical gas and vacuum systems | References NFPA 99 but UPC Chapter 13 adds additional prescriptive requirements for medical gas piping |
| Testing requirements | Water test or air test per Section 312, 10 psi minimum air test | Water test preferred, air test at 5 psi per Section 609.4, different test duration requirements |
Air admittance valves: the biggest practical difference
AAVs are probably the single biggest functional difference between IPC and UPC jurisdictions in everyday commercial design. In IPC states, AAVs are a standard tool for venting fixtures that can't practically connect to a conventional vent stack. In UPC states, many engineers have never used one. The 2021 UPC now conditionally permits AAVs, but adoption of that edition varies, and several California jurisdictions continue to prohibit them outright.
This means a plumbing layout designed with AAVs in an IPC jurisdiction may need a complete vent redesign when the same building type is built in a UPC jurisdiction. That's not a minor markup. It can affect ceiling space coordination, shaft sizing, and roof penetrations.
Wet venting
Both codes allow wet venting, but the sizing rules and limitations differ. In the IPC, Section 912 permits wet venting of a bathroom group with the wet vent sized for the drainage load of the wet-vented fixtures only. The UPC's wet venting provisions in Section 908 are generally more conservative, requiring larger wet vent sizes for the same fixture configuration in some cases.
Engineers moving from IPC to UPC territory sometimes find that their standard bathroom group vent layout needs upsizing. This is especially relevant in hotel and multi-family residential projects where bathroom groups are repeated on every floor and a half-inch change in vent pipe size affects every unit.
What this means for drawing review
The most common review findings on projects that cross the IPC/UPC boundary fall into predictable categories: AAV usage where the jurisdiction prohibits them, pipe sizes calculated under the wrong code's tables, venting configurations that rely on provisions available in one code but not the other, and grease interceptor sizing using the wrong flow rate methodology.
None of these differences are obscure. They're the kind of thing that experienced plumbing engineers in each region know by habit. The risk shows up when a firm takes a project outside its usual territory, or when a national developer uses the same drawings across state lines. Automated code checking catches these cross-code discrepancies before they become correction notices.
Checking both codes simultaneously
Callout supports both IPC 2024/2021 and UPC 2021. You can select either code individually or, on projects where the jurisdiction is uncertain, select both and get findings flagged against each. The review output tags every finding with the specific code reference, so you can filter by which code generated the comment and resolve any conflicts with the AHJ's adopted edition.