Jeffrey M. Hugo, CBO 

December 30, 2025 

Combination (or “combo”) underground water lines are common in small commercial and multifamily residential construction, yet they remain one of the most misunderstood systems in the field. When a single underground water service supplies both domestic plumbing and a fire protection system, a familiar set of questions quickly arises: 

 

• Which standard applies—the International Plumbing Code (IPC) or NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances? 

• Are fire protection flushing and hydrostatic testing required? 

• Does the pipe need to be listed for fire protection use? 

• Who installs the system? 

• And who inspects it? 

In practice, the answer depends less on theory and more on how the Authority Having Jurisdiction (AHJ) enforces fire code requirements. That enforcement approach can vary by jurisdiction, project type, or even department involvement. 

Regardless of the enforcement path, one principle is constant: the IPC applies to the domestic water service, and NFPA 24 applies to the fire sprinkler supply when required. When both functions are present, both codes are in play. 

This blog references the 2024 model codes including the International Building Code (IBC), International Fire Code (IFC), and NFPA 1, along with the 2022 editions of NFPA 13, NFPA 14, and NFPA 24. 

How This Is Enforced in the Field 

Across the U.S., AHJs generally enforce underground fire service mains in one of two ways: 

1. Through NFPA 1, Fire Code, or 

1. Through the International Fire Code (IFC) 

Both approaches lead to the same technical requirements. 

• When NFPA 1 is adopted, NFPA 24 is enforced by reference through NFPA 1 Section 13.5.  

• When the IFC is adopted, NFPA 24 is enforced by reference through IFC Section 507. 

In both cases, the IPC remains in force for the domestic water service portion of the piping. The difference is not which standards apply, but how NFPA 24 enters the picture. 

Why Combination Lines Are Different 

A combination line is not simply a domestic water service with a fire tap added downstream. Once a water service supplies a fire sprinkler system, it becomes part of the fire protection infrastructure. From an enforcement standpoint: 

• The domestic water portion is always regulated by the IPC. 

• The fire protection supply portion is regulated by NFPA 13, NFPA 14 or another NFPA standard, with underground piping governed by NFPA 24, enforced either through NFPA 1 or through the IFC. 

Because the same underground pipe serves both purposes, neither code can be ignored. 

When NFPA 24 Applies — and When It Does Not 

NFPA 24 is the default standard for underground piping supplying fire sprinkler systems. However, its own scope language creates specific, limited exceptions. NFPA 24 does not apply to: 

• Underground mains serving sprinkler systems designed under NFPA 13R, where the underground main is less than 4 inches in nominal diameter 

• Underground mains serving sprinkler systems designed under NFPA 13D 

These same exclusions are reflected whether NFPA 24 is enforced through NFPA 1 or through the IFC. In practical terms: 

• NFPA 13 systems → NFPA 24 applies 

• NFPA 13R systems 

• 4 inches and larger → NFPA 24 applies 

• Less than 4 inches → NFPA 24 does not apply, plumbing code applies 

• NFPA 13D systems → NFPA 24 does not apply, plumbing code applies 

Outside of these narrow exceptions, an underground pipe supplying a fire sprinkler system is treated as a private fire service main. 

Pipe Materials: Where IPC and NFPA 24 Overlap 

Where NFPA 24 applies, the underground pipe must satisfy both plumbing and fire protection material requirements. IPC Table 605.3 and NFPA 24 Table 10.1.1.1 identify common acceptable underground materials, including: 

• Ductile iron pipe 

• PVC pressure pipe 

• Polyethylene (PE) pressure pipe 

• Copper tube or pipe 

• Austenitic stainless steel pipe 

Selecting one of these materials allows the underground pipe to comply with IPC potable water requirements and NFPA 24 fire service requirements at the same time. 

Fire protection–specific product listing is not required for underground piping when the material complies with one of the standards recognized by NFPA 24. 

Pressure Requirements: IPC Is the Floor, Not the Ceiling 

The International Plumbing Code (IPC) establishes a baseline pressure requirement for underground water service piping. The IPC requires underground water service pipe installed outside a building to have a minimum working pressure rating of 160 psi at 73.4°F, or to be rated for the highest available pressure where it exceeds that value. 

This requirement is adequate for domestic plumbing systems, but it represents a minimum threshold, not a design target, for combination fire and domestic undergrounds. 

Where NFPA 24 applies, higher pressures must be anticipated. Underground piping supplying NFPA 13 systems, and qualifying NFPA 13R systems, is subject to NFPA 24 hydrostatic testing, typically 200 psi or 50 psi above the maximum system pressure, whichever is greater.  

As a result, combination fire and domestic undergrounds governed by NFPA 24 almost always require pipe pressure classes that exceed IPC minimums. 

Where NFPA 13R is permitted and the underground service is less than 4 inches, lower-pressure pipe may be used, but only if the system is arranged so elevated pressures are not introduced when the fire department connection (FDC) is boosted. 

This distinction explains why plumbing pipe that may be acceptable for NFPA 13D systems is no longer acceptable once NFPA 24 applies. 

Installation and Acceptance Testing: Driven by Fire Code Enforcement 

A domestic-only water service would not normally be flushed, hydrostatically tested, or forward-flow tested. A fire sprinkler supply governed by NFPA 24 must be. 

Where NFPA 24 applies, whether enforced through NFPA 1 or the IFC, the underground piping is required to undergo: 

• Underground flushing 

• Hydrostatic pressure testing 

• Leakage allowances 

• Valve operation testing 

• Forward-flow testing of the backflow prevention devices 

Even though part of the system serves domestic plumbing, fire protection requirements govern acceptance. In real-world enforcement, the more restrictive fire standard controls. 

After Acceptance: NFPA 25 Applies Everywhere 

Once the system is placed in service, the installation enforcement path no longer matters. Because the combination line supplies a fire sprinkler system, that system is subject to NFPA 25, the Standard for Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems for the life of the building. This includes: 

• Periodic inspection of the water supply 

• Required testing of sprinkler system components 

• Ongoing owner responsibility for system reliability 

Domestic plumbing components remain subject to IPC maintenance provisions, but fire sprinkler ITM is governed by NFPA 25 nationwide through the IFC and NFPA 1.  

Who Installs and Inspects the New Installation  

Installer and inspector qualifications are determined by state and local law, but enforcement typically follows this division: 

• Plumbing officials inspect IPC compliance for the domestic water service. 

• Fire officials inspect NFPA 24 compliance for the fire sprinkler supply. 

It is common and appropriate for two inspectors to review the same installation under different authorities. 

The Bottom Line 

In practice, combination fire and domestic water lines are regulated through two parallel code systems: 

• IPC always applies to the domestic water service 

• NFPA 24 applies to the fire sprinkler supply, enforced either: 

• through NFPA 1, or 

• through the International Fire Code 

• NFPA 24 does not apply only where its scope explicitly says it does not, for example, less than 4 inch mains for NFPA 13R systems.  

• NFPA 25 governs post-construction inspection, testing, and maintenance 

The enforcement path may vary, but the technical outcome is the same. Understanding that reality avoids failed inspections, rework, and jurisdictional disputes and ensures the system performs when it matters most. 

About the Author:

Jeffrey M. Hugo, CBO, is Vice President of Codes & Standards, Public Fire Protection, and Training & Education at the National Fire Sprinkler Association (NFSA). A code official since 1995 and with NFSA since 2005, he participates in ICC and NFPA code and standards development and works closely with authorities having jurisdiction, engineers, and contractors on the practical application of water-based fire protection requirements. Jeff’s background includes service as a chief building official and municipal code consulting, and he is known for clear, field-focused guidance that connects code intent to real-world performance.