Water Supply

A fire protection water supply is the source, piping and equipment that deliver water to a building's sprinklers, standpipes and hydrants at the flow, pressure and duration the design needs. Every sprinkler system in the region is designed to a demand point, a number of gallons per minute (gpm) at a pressure in pounds per square inch (psi) that must be sustained for a set time, and the supply has to meet that point with a margin. When the public main cannot, the project adds a fire pump, a storage tank or a larger service, and each of those carries its own standard, its own acceptance test and its own place in the permit sequence.

This guide is written for owners, developers, architects and construction managers who need to understand how a water supply is measured, how engineers decide whether it is enough, and what happens when it is not. It covers New York State, New York City, New Jersey, Pennsylvania, Maryland and Delaware. It stays at the level of the whole topic and points to the article that owns each individual question.

What a Water Supply Has to Deliver: Flow, Pressure and Duration

A fire protection water supply is judged on three numbers at once. Flow is the volume per minute the system discharges, in gpm. Pressure is what pushes that flow through the pipe network and out of the most remote sprinkler, in psi. Duration is how long the supply must hold that flow, in minutes, and NFPA 13 (2022) Chapter 19 sets the duration and the hose stream allowance by hazard class.

The sprinkler demand is a single point: for example, 720 gpm at 58 psi at the base of the riser for 90 minutes, including 250 gpm of hose stream for the fire department. NFPA 13 (2022) Chapter 5 lists the acceptable sources (a public waterworks, a tank, a pump drawing from a reliable source, or a combination) and Chapter 28 sets what the hydraulic calculation has to show about them. A supply is adequate when the available pressure at the demand flow is above the required pressure, and NFPA 13 (2022) Chapter 28 requires the calculation to show the supply curve, the demand point and the margin between them.

Standpipes add a second demand that is usually larger than the sprinkler demand. Under NFPA 14 (2019) a Class I or Class III standpipe must deliver 100 psi residual at the most remote hose outlet, with 500 gpm for the first standpipe and 250 gpm for each additional standpipe, capped at 1,000 gpm where the building is sprinklered throughout and 1,250 gpm otherwise (Chapter 7). In a high-rise the pressure to reach the roof outlet, plus friction, is the number that decides whether a fire pump is required and how large it is.

The fire flow in the last row is a different question from the sprinkler demand. Fire flow is the water the fire department needs from hydrants to fight a fire in the building from outside, and the fire code sets it by floor area and construction type in IFC Appendix B, reduced where the building is sprinklered. New York City does not use Appendix B; its water supply and hydrant rules sit in the NYC Fire Code and in the Department of Environmental Protection's standards. This guide only places that arithmetic in the sequence.

The Water Supply Train: Main to Hose Outlet

How the Supply Is Measured and Plotted

The supply side of the comparison comes from a hydrant flow test. A gauge on one hydrant records the static pressure with nothing flowing and the residual pressure while a second hydrant on the same main flows water through a pitot gauge. NFPA 291 (2022) Chapter 4 sets the procedure, and the 2022 edition recommends enough hydrants be flowed to drop the residual pressure by at least 10 percent from static; the 2019 edition asked for 25 percent. The same chapter sets 20 psi as the residual pressure that the available flow is projected to.

Two readings define a curve. Water flow through a pipe network follows a 1.85-power relationship between flow and pressure loss, so when static and residual pressures are plotted on paper whose horizontal axis is scaled to the 1.85 power, the supply becomes a straight line. The engineer then plots the sprinkler demand point on the same graph and reads the distance between the two. The Process article on how a hydrant flow test is run and read covers the field procedure.

A supply that barely clears the demand is not a safe design. Public mains lose pressure as the system ages and as new customers connect, and a test taken on a cool morning reads higher than the same main on a summer afternoon. Most engineers in the region design to a margin of about 10 percent or 10 psi at the demand flow, and some AHJs (authorities having jurisdiction) write a minimum margin into their review standards.

Supply Curve Against the Sprinkler Demand Point

Three Formulas Behind Every Water Supply Analysis

Pitot flow (NFPA 291, 2022, Chapter 4): Q = 29.84 × c × d² × √p, where Q is flow in gpm, c is the outlet coefficient (0.90 for a smooth rounded hydrant outlet, 0.80 for a square outlet, 0.70 for an outlet projecting into the barrel), d is the outlet diameter in inches and p is the pitot pressure in psi.

Projection to 20 psi (NFPA 291, 2022, Chapter 4): Q_R = Q_F × ((P_S − P_R) / (P_S − P_F))^0.54, where Q_F is the flow measured, P_S is static pressure, P_F is the residual pressure at the test flow and P_R is the residual pressure of interest, normally 20 psi.

Fire pump curve (NFPA 20, 2022, Chapter 4): the pump must deliver at least 150 percent of rated flow at not less than 65 percent of rated pressure, and its no-flow (churn) pressure may not exceed 140 percent of rated pressure (Chapter 4, Fire Pump Unit Performance).

When the City Main Is Not Enough: Pumps, Tanks and Mains

When the supply line falls below the demand point, the engineer works through a fixed order of fixes, cheapest first. The first move is to redesign the system to lower the demand: smaller remote area through a quick-response sprinkler reduction, larger pipe, a gridded layout or a different sprinkler. The second is a larger or second service from the main, which the water purveyor must approve and which NFPA 24 (2022) governs once it leaves the public right of way. Only when those fail does the project add a pump, and only when the main cannot supply the flow at all, or the purveyor caps the draw, does it add a tank.

A fire pump adds pressure, not water. NFPA 20 (2022) requires the suction source to deliver the pump's full 150 percent flow, and the pressure at the suction flange may not drop below 0 psi gauge, with 20 psi the common purveyor and AHJ limit for a pump drawing from a public main (Chapter 4). The live Trigger article on whether a building needs a fire pump covers the decision in detail, and the live Process article on the diesel fire pump acceptance test covers how NFPA 20 Chapter 14 proves the installed pump; both are linked in the questions table below.

A tank adds water, and sometimes pressure. NFPA 22 (2023) covers the tank itself: gravity tanks, suction tanks, pressure tanks and break tanks, their capacity, their heating and their fill. A break tank is the usual answer when the main has enough pressure for domestic use but the purveyor will not allow a direct pump draw; it holds a fraction of the duration and refills from the main while the pump runs. A suction tank holds the full demand for the full duration when the site has no reliable main, which is common in Pennsylvania and Maryland industrial parks served by small systems.

The private fire service main is the piping that ties all of this together outside the building. NFPA 24 (2022) sets its materials, depth of cover, thrust restraint, flushing rate and the 2-hour hydrostatic test at 200 psi or 50 psi above working pressure (Chapter 10). The Definition article on the private fire service main, linked in the table below, defines it and separates it from the public main and the domestic service.

Supply Short? The Fix Order Engineers Follow

Standpipes and High-Rise Pressure

A standpipe is a vertical pipe with hose outlets on each floor that lets firefighters connect hose inside the building instead of dragging it up the stairs. The IBC (2021) Section 905.3.1 requires a Class III standpipe where the floor level of the highest story is more than 30 ft above the lowest level of fire department vehicle access, and allows a Class I standpipe instead where the building is fully sprinklered; other triggers in Section 905.3 cover assembly, covered malls, stages and buildings under construction. NFPA 14 (2019) sets the design: Class I for fire department use with 2 1/2 in. outlets, Class II for occupant use with 1 1/2 in. hose, and Class III with both.

Pressure is the whole problem in a standpipe design. The 100 psi required at the roof outlet, plus about 0.433 psi for every foot of elevation, plus friction in the riser, sets the pressure the pump must produce at grade. In a 20-story building that number often lands near 250 psi, and NFPA 14 (2019) caps the pressure at any hose outlet at 175 psi and requires pressure-reducing valves where the static pressure would exceed it (Chapter 7). Above a certain height the system is split into zones with separate pumps or a transfer pump.

Most standpipe systems in the region are combined systems that also feed the floor sprinklers from the same riser. NFPA 14 (2019) Chapter 7 sets the combined demand, and NFPA 13 (2022) Chapter 28 requires the sprinkler calculation to account for it. Under NFPA 14 (2019) Chapter 7 the combined standpipe demand is capped at 1,000 gpm where the building is sprinklered throughout and 1,250 gpm otherwise, and the pump is sized to the larger of that figure and the sprinkler demand; DGA's standpipe design work and the commercial high-rise sector page below describe the projects where this governs.

What the AHJ and the Water Purveyor Each Control

Two different authorities decide a water supply, and they do not always agree. The water purveyor (a municipal water department or a private utility such as New Jersey American Water, Aqua Pennsylvania or the Philadelphia Water Department) owns the public main, runs or witnesses the hydrant flow test, approves the size of the tap and the service, and often limits how far a fire pump may draw the main down. The AHJ for the building code (the building department, fire marshal or fire department) reviews the hydraulic calculation, the pump room and the standpipe design, and witnesses the acceptance tests under NFPA 20 Chapter 14 and NFPA 24.

The purveyor's rules frequently govern. Many utilities in the region will not release a flow test older than 6 or 12 months, require their own crew to run it, and require a reduced-pressure backflow preventer on the fire service that costs 10 to 15 psi of the available pressure before the water reaches the riser.

The AHJ controls the sequence after the design is set. In New York City, sprinkler and standpipe work is filed with the Department of Buildings (DOB) and the Fire Department (FDNY) witnesses the standpipe and fire pump tests; outside the city, the local code enforcement official or fire marshal does both. The approvals hub linked at the end covers each jurisdiction's filing path; the table below gives the editions and the names.

From Flow Test to Acceptance: The Water Supply Sequence

Code Editions by State

Water supply requirements come from three places in every jurisdiction: the building code that adopts NFPA 13, 14 and 20 by reference, the fire code that sets fire flow and hydrant rules, and the water purveyor's own standards. The table lists the adopted editions as of October 2026, the AHJ that reviews the design and the name of the filing. Local amendments are not shown.

Water Supply Codes, Reviewers and Filings by Jurisdiction

Jurisdiction	Adopted code and edition (as of October 2026)	Fire flow and hydrants	AHJ	Filing
New York State (outside NYC)	2025 Uniform Code (2024 I-Codes): NFPA 13 (2022), NFPA 14 (2024), NFPA 20 (2022), NFPA 24 (2022)	2024 IFC Appendix B and C where the municipality adopts them; purveyor flow test	Local code enforcement official; local fire department for hydrants	Building permit with sprinkler and standpipe drawings and hydraulic calculations
New York City	2022 NYC Building Code and 2022 NYC Fire Code: NFPA 13 (2016), NFPA 14 and NFPA 20, as amended in Appendix Q	No IFC Appendix B; hydrant flow tests through the Department of Environmental Protection; NYC Fire Code Chapter 5	Department of Buildings for design; FDNY witnesses standpipe and pump tests	DOB NOW sprinkler (SP) and standpipe (SD) work types; FDNY test witnessing
New Jersey	Uniform Construction Code (UCC) 2024 I-Codes (effective August 17, 2026): NFPA 13 (2022), NFPA 14 (2024), NFPA 20 (2022); Uniform Fire Code on a 2018 IFC basis	Fire flow per 2018 IFC Section 507 as modified by the UFC; local fire official and purveyor	Municipal fire protection subcode official; local fire official for hydrants	UCC construction permit, fire protection subcode technical section
Pennsylvania	UCC 2021 I-Codes (January 1, 2026): NFPA 13 (2019), NFPA 14 (2019), NFPA 20 (2019); no statewide fire code	IFC Appendix B and C only where a municipality adopts a fire code; Philadelphia Fire Code	Municipal or third-party code official; Philadelphia Department of Licenses and Inspections (L&I)	UCC building permit; Philadelphia fire suppression permit
Maryland	2021 IBC: NFPA 13 (2019), NFPA 14 (2019), NFPA 20 (2019); NFPA 1 (2024) for fire prevention	NFPA 1 (2024) Chapter 18 water supply and hydrant provisions; county fire marshal standards	County building official and county or State Fire Marshal	County building permit plus fire marshal plan review
Delaware	Regulation 701 (2026): NFPA 1 (2024), NFPA 13 (2025), NFPA 20 (2025), NFPA 25 (2023); IBC by county (2021 in New Castle and Sussex, 2018 in Kent and Wilmington)	NFPA 1 (2024) Chapter 18 water supply provisions; State Fire Marshal review	Office of the State Fire Marshal (Wilmington Fire Marshal inside the city)	State Fire Marshal plan review before the county building permit

Editions as listed in DGA's Fire Code Cheat Sheet for NY, NJ, PA, MD and DE.

Key Concepts

These are the terms the Water Supply articles use. Each is defined once here so the spokes can use the term without re-explaining it.

• Static pressure: the pressure in the main with no water flowing, read on the gauge hydrant at the start of a flow test.
• Residual pressure: the pressure remaining in the main while the test flow is discharging, read on the same gauge hydrant.
• Pitot pressure: the velocity pressure measured in the stream leaving a flowing hydrant outlet, converted to gpm with the pitot equation.
• Demand point: the flow and pressure the sprinkler or standpipe system requires at the point where the supply is compared, usually the base of the riser.
• Supply curve: the line through the static and residual readings on 1.85-power graph paper, showing the pressure the main can hold at any flow.
• Safety margin: the pressure difference between the supply curve and the demand point at the demand flow, held in reserve against main deterioration and seasonal change.
• Hose stream allowance: the extra flow, typically 100 to 500 gpm, added to the sprinkler demand for fire department hose use under NFPA 13.
• Rated point: the flow and pressure stamped on a fire pump nameplate, the 100 percent point on its curve.
• Churn: the pump running with no water flowing, whose pressure NFPA 20 limits to 140 percent of rated pressure.
• Break tank: a tank that receives water from the public main and holds part of the demand so a pump can draw from it without pulling the main below the purveyor's limit.
• Private fire service main: the owner-owned underground piping from the public main or private source to the risers, private hydrants and fire department connections, built under NFPA 24.
• Fire flow: the water the fire code requires to be available from hydrants for exterior firefighting, set by building area and construction type in IFC Appendix B or NFPA 1 Chapter 18.

Working With DGA on Water Supply

DGA's engineers request and witness hydrant flow tests, plot the supply against sprinkler and standpipe demands before a design is committed, size fire pumps and tanks, prepare the sealed hydraulic calculations and pump room drawings for permit, and witness NFPA 20 Chapter 14 acceptance tests for owners. The scope of work and the fee drivers are described on the fire pump design service page and the standpipe design service page, and the kinds of buildings where standpipe pressure governs are described on the commercial high-rise sector page.

DGA designs fire pumps, standpipes and water supply improvements and witnesses their acceptance tests across New York, New Jersey, Pennsylvania, Maryland and Delaware. Project briefs get confirmation and preliminary code feedback within one business day. See where DGA works →

Four tools support the first conversation. The Hydrant Flow Test Calculator turns pitot readings into gpm and projects the available flow to 20 psi. The Sprinkler Water Supply Analysis tool plots that test against a sprinkler demand and reports the margin.

The Fire Pump Test Curve Plotter checks an acceptance or annual test against the NFPA 20 churn, rated and 150 percent points. The IFC Appendix B Fire Flow Calculator estimates the site fire flow from building area and construction type where a jurisdiction adopts the appendix.

Frequently Asked Questions

This guide is general information, not a substitute for a project-specific water supply analysis. Code editions, local amendments, purveyor standards and reviewer practice change; confirm the adopted code and the authority having jurisdiction for each project.