Upon completing this chapter, you will be able to:
4.Locate and apply the key sections of the 2018 International Plumbing Code (IPC) governing water supply system design, sizing, and installation.
5.Calculate minimum fixture supply pipe sizes and estimate demand using the IPC fixture unit method.
6.Identify the requirements for water distribution piping materials, protection, and installation.
7.Apply the Maryland State Plumbing Code modifications that affect water supply work.
8.Recognize OSHA 29 CFR 1926 requirements that apply to water supply work on construction sites.
9.Avoid common exam traps involving table misuse, fixture unit confusion, and code cross-references.
1.1 Scope and Code Hierarchy
The water supply system chapter in the IPC is Chapter 6 (Water Supply and Distribution), but water supply work is also governed by Chapter 3 (General Regulations), Chapter 4 (Fixtures), and Chapter 10 (Traps and Interceptors) where cross-connections are concerned. The Maryland State Plumbing Code adopts the IPC with state-specific amendments; you must know which sections are modified. For the exam, the primary reference is the IPC, with Maryland amendments applied where noted.
Key principle: The code regulates the potable water system from the point of service (utility connection or private well) to the last fixture. The master plumber is responsible for the design, sizing, and safe installation of this entire path.
1.2 Definitions You Must Know Cold
Potable water: Water free from impurities in amounts sufficient to cause disease or harmful physiological effects.
Service pipe: The pipe from the water main or other source to the water-distributing system of the building.
Water-distributing pipe: Pipe that carries water from the service pipe to the fixtures.
Fixture supply pipe: The pipe connecting a fixture to the branch pipe.
Branch: Any water supply pipe that supplies two or more fixtures.
Riser: A vertical water supply pipe.
Cross-connection: Any physical connection between a potable water supply and any non-potable source.
Backflow: The flow of water or other liquids into the potable water system from any source other than the intended source.
Back-siphonage: Backflow caused by negative pressure in the supply system.
Air gap: The unobstructed vertical distance through free atmosphere between the lowest opening from a pipe or faucet and the flood level rim of a receptacle.
Exam trap: The air gap is measured from the flood level rim of the receiving vessel, not from the bottom of the sink or tank. The minimum air gap for a water distribution system is two times the effective opening diameter of the supply pipe, but never less than 1 inch (IPC Table 608.15.1 logic).
1.3 Required Water Supply Capacity
IPC Section 607 establishes the minimum water supply capacity. The code requires that the water supply system be designed to provide the required flow rates at the required pressures. The key concept is the fixture unit method.
Water supply fixture units (WSFU) are assigned to each fixture type in IPC Table 607.6.2 (or the combined table in Chapter 4). This table is different from the drainage fixture unit (DFU) table — do not confuse them.
The total WSFU load is converted to an estimated demand flow (gallons per minute) using IPC Table 607.6.2 (the demand curve table) or the corresponding Hunter’s curve logic.
Critical distinction: The WSFU values for water supply are not the same as DFU values. For example, a public lavatory is 2 WSFU but 1 DFU. A private toilet (1.6 gpf) is 2.5 WSFU but 3 DFU. Always use the correct table.
Demand estimation:
34.Sum the WSFU for all fixtures served.
35.Use the table to find the corresponding flow rate in gpm.
36.For systems with predominantly flush valves, use the flush valve column; for flush tanks, use the flush tank column. The code requires the more conservative (higher) demand when both types are present.
Exam trap: The table has a footnote about continuous flow fixtures (e.g., hose bibbs, irrigation). Add the continuous flow rate (in gpm) to the demand flow after converting WSFU to gpm. Do not convert continuous flow to WSFU.
1.4 Sizing the Water Supply System
IPC Section 607.6 requires that pipe sizing be based on the demand flow and the available pressure. The master plumber must:
42.Determine the available pressure at the water meter or service entrance (typically 40–60 psi from a municipal system).
43.Subtract the pressure losses through the meter, backflow preventer, and other devices.
44.Subtract the elevation loss (0.433 psi per foot of rise).
45.Subtract the friction loss through the piping (based on pipe length, diameter, and flow).
46.The remaining pressure must be at least the minimum required fixture pressure (typically 8 psi for most fixtures, but 15 psi for flush valves per IPC Table 607.6.2 notes).
Sizing method: The code allows sizing by the velocity method or the pressure-drop method. The velocity method is simpler: limit velocity to 5 fps for cold water and 4 fps for hot water (to prevent noise and erosion). The pressure-drop method uses friction loss charts (Hazen-Williams or Darcy-Weisbach) and is required for large or complex systems.
Minimum pipe sizes (IPC Table 607.6.2):
Individual fixture supply pipes: ⅜ inch minimum for lavatories and sinks, ½ inch for most others, ¾ inch for flush valves.
Branch and riser sizes are determined by WSFU load and the table.
Exam trap: The ¾-inch minimum for flush valve supplies is a minimum, not a recommendation. A 1-inch supply may be required for a flush valve if the developed length is long or pressure is low.
1.5 Pressure Requirements and Pressure-Reducing Valves
IPC Section 607.3 requires that the water pressure at any fixture not exceed 80 psi. If the static pressure exceeds 80 psi, a pressure-reducing valve (PRV) must be installed.
The PRV must be installed at the point where the pressure exceeds 80 psi, typically at the building service entrance.
A thermal expansion tank is required on the water heater when a PRV or backflow preventer creates a closed system (IPC Section 607.3.1). This is a common Maryland inspection issue.
Minimum pressure: The code requires a minimum of 40 psi at the service entrance for most systems. If the utility cannot provide this, a booster pump is required.
Exam trap: The 80 psi limit is static pressure. A system may have higher transient (surge) pressure, but the code addresses this through the requirement for a water hammer arrestor (IPC Section 607.9) where quick-closing valves are used.
1.6 Protection of the Potable Water Supply
IPC Chapter 6, Part 6 (Sections 607 through 608) covers protection. The key requirements:
Backflow prevention: Every cross-connection must be protected. The type of protection depends on the degree of hazard:
Air gap: For direct connections to a non-potable source (highest protection).
Reduced pressure principle backflow preventer (RPZ): For high-hazard connections (e.g., irrigation systems, boilers with chemical additives).
Double check valve assembly: For low-hazard connections (e.g., fire sprinkler systems).
Atmospheric vacuum breaker: For back-siphonage only, on hose bibbs and similar outlets.
IPC Table 608.1 lists the required protection for specific connections. You must know the degree of hazard classification: health hazard (could cause illness or death) vs. non-health hazard (objectionable but not dangerous).
Hose bibb protection: All hose bibbs must be protected by a non-removable hose bibb vacuum breaker (IPC Section 608.3.4).
Boiler and water heater connections: The water supply to a boiler must be protected by a backflow preventer if chemicals are added (IPC Section 608.5).
Maryland note: The state has adopted additional requirements for irrigation systems — an RPZ is required for all commercial irrigation, and a testable double check is required for residential irrigation where a backflow preventer is installed.
1.7 Water Distribution Piping Materials
IPC Section 602 lists approved materials. For water supply, the common materials are:
Copper (Types K, L, M) — the most common for residential and light commercial. Type M is not allowed for underground service in some jurisdictions (Maryland requires Type K or L for underground).
CPVC (chlorinated polyvinyl chloride) — for hot and cold water, but must be rated for the water temperature and pressure.
PEX (cross-linked polyethylene) — allowed by the IPC, but must be installed with proper support and not exposed to UV light.
Galvanized steel — allowed but rarely used for new work due to corrosion issues.
Stainless steel — for special applications.
Prohibited materials: Do not use galvanized pipe for hot water if the water has a high mineral content (accelerated corrosion). Do not use PVC (non-chlorinated) for hot water — it is not rated for temperatures above 140°F.
Exam trap: The IPC requires that all water piping materials be lead-free (defined as not more than 0.2% lead content for solder and flux, and not more than 0.25% lead for pipe and fittings). This is a federal requirement incorporated into the code.
1.8 Installation Requirements
IPC Section 603 covers installation:
Support: Horizontal pipes must be supported at intervals per IPC Table 603.3.1 (e.g., ½-inch copper every 6 feet, ¾-inch copper every 8 feet, PEX every 32 inches). Vertical pipes must be supported at each floor or at intervals not exceeding the table values.
Grading: Pipes must be graded to drain at fixtures and low points. The code requires that the entire system be designed to drain (for winterization or maintenance).
Protection from damage: Pipes passing through concrete or masonry must be sleeved. Pipes in or under slabs must be protected from corrosion.
Access: Shutoff valves must be accessible. The main shutoff valve must be at the point of entry to the building (IPC Section 603.4.1).
Water hammer arrestors: Required where quick-closing valves are used (IPC Section 607.9). The sizing is based on the fixture unit load and the length of pipe — use IPC Table 607.9.2 for sizing.
Field point: As the master plumber, you are responsible for ensuring that all shutoff valves are accessible after the building is finished. A common mistake is installing a valve behind a finished wall or ceiling — this is a code violation and a practical problem for maintenance.
1.9 Hot Water Systems
IPC Section 607 also covers hot water:
Hot water temperature: The code requires that hot water be supplied at a temperature of 110°F to 130°F at the point of use for most fixtures (IPC Section 607.1). For public buildings, the maximum temperature at public lavatories is 110°F (to prevent scalding).
Hot water supply: The system must be sized to provide the required flow rate. For buildings with high demand, a circulating system (recirculation line) is required to maintain temperature at the point of use.
Water heaters: Must be sized per the manufacturer’s rating and the building demand. The code requires that water heaters be installed with a temperature and pressure relief valve (T&P valve) — this is a critical safety device.
Maryland note: The state requires that all new water heaters be installed with a thermal expansion tank if a backflow preventer or PRV is present. This is a common inspection failure.
1.10 OSHA Requirements for Water Supply Work
29 CFR 1926 applies to construction work, including plumbing. Key requirements:
Excavation safety (1926 Subpart P): When installing water service lines, the trench must be protected from collapse. For trenches 5 feet or deeper, a protective system (shoring, sloping, or shielding) is required.
Personal protective equipment (1926 Subpart E): Hard hats, safety glasses, and gloves are required.
Ladder access (1926.1053): A ladder is required in any trench over 4 feet deep.
Lockout/tagout (1926.417): When working on existing water systems, the water must be shut off and locked out to prevent accidental pressurization.
Exam trap: OSHA questions are typically straightforward — know the trench depth thresholds (5 feet for protection, 4 feet for ladder) and the requirement for a competent person to inspect excavations.
1.11 Code Navigation: Where to Find It
Concept
Location in 2018 IPC
Definitions
Chapter 2 (definitions of potable water, backflow, etc.)
Approved materials
Section 602
Installation requirements
Section 603
Water supply capacity
Section 607.6
WSFU values
Table 607.6.2 (also Table 4.1.2 for fixture counts)
Demand flow conversion
Table 607.6.2 (demand curve)
Minimum fixture pressures
Table 607.6.2 notes
Pressure limit (80 psi)
Section 607.3
Thermal expansion
Section 607.3.1
Backflow prevention
Section 608
Air gap requirements
Section 608.15 and Table 608.15.1
Hose bibb protection
Section 608.3.4
Pipe support intervals
Table 603.3.1
Water hammer arrestors
Section 607.9 and Table 607.9.2
Hot water temperature
Section 607.1
T&P relief valve
Section 607.4 (and Chapter 5 for water heaters)
Maryland amendments
Maryland State Plumbing Code (adopted regulations)
OSHA trenching
29 CFR 1926 Subpart P
1.12 Common Exam Traps
112.Fixture unit confusion: Always verify whether you are using WSFU (water supply) or DFU (drainage) values. The tables are in different chapters.
113.Continuous flow addition: Add continuous flow (gpm) after the WSFU-to-gpm conversion, not before.
114.Air gap measurement: Measured from the flood level rim, not the bottom of the vessel.
115.80 psi limit: This is static pressure, not flow pressure. A PRV is required if static exceeds 80 psi.
116.Thermal expansion: Required only in closed systems (with PRV or backflow preventer).
117.Pipe support intervals: Memorize the common values — ½-inch copper at 6 feet, ¾-inch at 8 feet, PEX at 32 inches.
118.Flush valve supply: Minimum ¾-inch, but sizing may require 1-inch for long runs.
119.OSHA thresholds: 5 feet for trench protection, 4 feet for ladder access.
1.13 Practical Field Points for the Master Plumber
Pressure testing: The IPC requires that the water supply system be tested at 1.5 times the working pressure or 100 psi (whichever is greater) for at least 2 hours before being concealed. As the responsible licensed person, you must document this test.
Flushing: Before connection to fixtures, the system must be thoroughly flushed to remove debris.
Cross-connection survey: On existing buildings, you are responsible for identifying and correcting cross-connections during any renovation work.
Permit and inspection: In Maryland, the master plumber must pull the permit and be present for inspections. The final inspection includes verification of backflow protection and pressure settings.
1.14 Summary
Water supply systems are governed by IPC Chapter 6, with critical input from Chapter 2 (definitions) and Chapter 3 (general regulations). The master plumber must master the fixture unit method, understand the relationship between pressure, flow, and pipe sizing, and be able to navigate the code quickly. Maryland amendments add requirements for thermal expansion tanks and irrigation backflow protection. OSHA rules for trenching and excavation are separate but essential for field safety. Focus your study on the tables — they are the heart of the open-book exam.
