# Critical Level (C-L): Building Code Definition > How the IPC 2009 define critical level (c-l), where the wording differs, and which chapters depend on the term. - Canonical: https://harth.build/resources/glossary/critical-level-c-l/ - Last updated: 2026-09-07 - Data source: Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control. ## Key facts | Fact | Value | | --- | --- | | Defined in | IPC 2009 (2009 editions transcribed in this corpus) | | Primary source | IPC 2009 Definitions (model text, not a jurisdiction's adopted text) | | Chapters using the term | 1 (counted across the codes hosted here) | | Wording identical across codes | Single code (compare before citing one code's wording under another) | ## Direct answer Critical level (c-l) is a defined term in the International Plumbing Code (IPC) 2009, in Definitions. The code's own words are: “An elevation (height) reference point that determines the minimum height at which a backflow preventer or vacuum breaker is installed above the flood level rim of the fixture or receptor served by the device. The critical level is the elevation level below which there is a potential for backflow to occur. If the critical level …” No other I-Code in this corpus defines it, so the IPC 2009 definition is the one to work from. The term is used in 1 chapters of this corpus, most often in Definitions (IPC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted. *Content and cited inputs reviewed by [Scott Barrington](https://nz.linkedin.com/in/barrington), Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.* ## What does this term mean? **Critical Level (C-L)** — IPC 2009, Definitions: > An elevation (height) reference point that determines the minimum height at which a backflow preventer or vacuum breaker is installed above the flood level rim of the fixture or receptor served by the device. The critical level is the elevation level below which there is a potential for backflow to occur. If the critical level marking is not indicated on the device, the bottom of the device shall constitute the critical level. ![Labeled code-term definition diagram: Critical Level (C-L): Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/critical-level-c-l/diagram-1344.webp) *How the IPC 2009 define critical level (c-l), where the wording differs, and which chapters depend on the term.* ## What must be verified before using this guidance? **Critical Level (C-L)** is defined once in this corpus, in the [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/ipc-2009/definitions.md) chapter of the International Plumbing Code (IPC) 2009: > An elevation (height) reference point that determines the minimum height at which a backflow preventer or vacuum breaker is installed above the flood level rim of the fixture or receptor served by the device. The critical level is the elevation level below which there is a potential for backflow to occur. If the critical level marking is not indicated on the device, the bottom of the device shall constitute the critical level. ## Where critical level (c-l) is used in the code The term appears in 1 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next. | Chapter | Code | Mentions | | --- | --- | --- | | [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/ipc-2009/definitions.md) | IPC 2009 | 1 | ## The terms critical level (c-l) is built out of The definition above leans on 5 other defined terms. Each carries its own code definition, and a local amendment to any of them changes what critical level (c-l) covers: - **[Flood Level Rim](https://harth.build/resources/glossary/flood-level-rim.md)** — IPC 2009: The edge of the receptacle from which water overflows. - **[Vacuum Breaker](https://harth.build/resources/glossary/vacuum-breaker.md)** — IPC 2009: A type of backflow preventer installed on openings subject to normal atmospheric pressure that prevents backflow by admitting atmospheric pressure through ports to the discharge side of the device. - **[Backflow](https://harth.build/resources/glossary/backflow.md)** — IPC 2009: Pressure created by any means in the water distribution system, which by being in excess ofthe pressure in the water supply mains causes a potential backflow condition. - **[Receptor](https://harth.build/resources/glossary/receptor.md)** — IRC 2009: A fixture or device that receives the discharge from indirect waste pipes. - **[Device](https://harth.build/resources/glossary/device.md)** — IRC 2009: Aunit of an electrical system that carries or controls electrical energy as it principal function. ## Critical Level (C-L): what to open, and in what order Every row is a document this term depends on. The [ICC code library](https://codes.iccsafe.org/codes) and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) cover the middle rows. | Check | Where | Confirmed | | --- | --- | --- | | Read the definition in the adopted IPC | Definitions | | | Re-check the requirement that uses it | Definitions (IPC 2009) | | | Confirm the dependent definition | Flood Level Rim | | | Confirm the dependent definition | Vacuum Breaker | | | Confirm the dependent definition | Backflow | | | Local amendment to any of the above | Jurisdiction's amendment list | | **A research aid, not a code determination.** The authority having jurisdiction decides which edition and which amendments apply. ## Where can you explore related guidance? - [Explore all glossary guidance](https://harth.build/resources/glossary.md) - [Flood Level Rim: Building Code Definition](https://harth.build/resources/glossary/flood-level-rim.md) - [Vacuum Breaker: Building Code Definition](https://harth.build/resources/glossary/vacuum-breaker.md) - [Receptor: Building Code Definition](https://harth.build/resources/glossary/receptor.md) - [Backflow: Building Code Definition](https://harth.build/resources/glossary/backflow.md) ## Sources and method Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control. ### Source links - [codes.iccsafe.org](https://codes.iccsafe.org/codes) - [iccsafe.org](https://www.iccsafe.org/advocacy/code-adoption-maps/) ## Frequently asked questions ### What is the code definition of critical level (c-l)? The International Plumbing Code (IPC) 2009 defines it in Definitions as: “An elevation (height) reference point that determines the minimum height at which a backflow preventer or vacuum breaker is installed above the flood level rim of the fixture or receptor served by the device. The critical level is the elevation level below which there is a potential for backflow to occur. If the critical level marking is not indicated on the device, the bottom of the device shall constitute the critical level.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally. ### Which parts of the code depend on critical level (c-l)? In this corpus the term is used across 1 chapters, led by Definitions in the IPC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own. --- - Continue in Harth (open this topic in the Harth agent): https://app.harth.build/agent?context=%2Fresources%2Fglossary%2Fcritical-level-c-l%2F - View this page in a browser: https://harth.build/resources/glossary/critical-level-c-l/ - All resources for agents: https://harth.build/llms.txt