Lesson 6: Water Supply and Distribution, Part 1: Service, Pressure, and Sizing
IPC 602 to 606, Tables 604.3 and 604.5 · 14 practice questions
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Reading
Chapter 6 of the 2021 IPC answers a simple question: will water reach every fixture at a usable pressure,
and will it arrive safely? This lesson follows the water from the street main to the fixture: the service
pipe, its separation from the sewer, the flow and pressure each fixture needs, the valve that caps building
pressure, and the shutoff valves an inspector expects to find. Lesson 7 continues with hot water circulation
and protection of the potable supply. The numbers are model code numbers; Houston enforces the Uniform
Plumbing Code, so confirm the adopted code first.
Water service from the main to the fixtures: valves along the way, a PRV above 80 psi, and the pressure each fixture needs.
Required supply and the water service (602, 603.1)
Any structure with plumbing fixtures that people occupy or live in must have a potable water supply in the
amounts and pressures the chapter sets (602.1). Only potable water may serve fixtures used for drinking,
bathing, cooking, or for processing food, medical, or pharmaceutical products, and unless the code says
otherwise, potable water serves every fixture (602.2). The water service pipe is sized to deliver the
required quantities at the required pressures and may not be smaller than 3/4 inch in diameter (603.1).
Separating service and sewer (603.2)
A water service may share a trench with the building sewer only if the sewer is made of a material listed
in Table 702.2. Otherwise the two are kept at least 5 feet apart horizontally, in undisturbed or compacted
earth. The 5-foot rule does not apply at a crossing if the service is sleeved to at least 5 feet on each side of
the sewer centerline, or if the bottom of the service within 5 feet of the sewer is at least 12 inches above the
top of the sewer. A potable service may never run in, under, or above a cesspool, septic tank, drainage field,
or seepage pit (603.2.1). Every yard rehab that trenches a new service beside an old sewer lateral of unknown
material meets this rule.
Flow rate and pressure at the fixture (604.3)
The distribution system is designed so that, at peak demand, each fixture supply outlet delivers at least
the flow rate and flow pressure in Table 604.3 (604.3). The pressure column is what matters at the meter: it
is the pressure that must remain at the outlet after friction and elevation are subtracted. Design uses the
lowest street pressure the main ever shows (604.6). Where the street cannot deliver the Table 604.3 pressures,
a booster system is required (604.7).
Fixture supply outlet
Flow, gpm
Pressure, psi
Lavatory, private
0.8
8
Lavatory, public
0.4
8
Shower, ordinary valve
2.5
8
Shower or bathtub with a balanced-pressure or thermostatic valve
2.5 or 4
20
Water closet, tank type or flushometer tank
1.6 to 6
20
Urinal, valve
12
25
Water closet, flushometer valve (siphonic / blowout)
25
35 / 45
Key numbers
Water service pipe: 3/4 in diameter minimum (603.1)
Service to sewer: 5 ft horizontal separation, or a sleeve, or 12 in vertical (603.2)
Flow pressure at the fixture: 8 psi for most; 20 psi for tank closets and mixing valves; 25 psi for valve urinals; 35 to 45 psi for flushometer closets (Table 604.3)
Pressure-reducing valve: required above 80 psi static, with a strainer (604.8)
Fixture supply pipe ends within 30 in of the fixture (604.5)
Underground service pipe pressure rating: 160 psi minimum (605.3)
Booster pump low-pressure cutoff: at 10 psi or less on the suction side (606.5.5)
Pressure-reducing valves (604.8)
Where pressure in the building exceeds 80 psi static, an approved pressure-reducing valve with a strainer
must reduce it to no more than 80 psi static (604.8). Two exceptions exist: service lines to sill cocks and outside
hydrants, and main risers, where the pressure is brought to 80 psi or less at the individual fixtures. The valve
is designed to stay open if it fails, so a failed valve delivers full main pressure rather than no water
(604.8.1). The valve and strainer must be arranged so parts can be repaired or removed without cutting the line
or taking the valve out (604.8.2).
Worked example: a restaurant sits on a main that measures 95 psi static. The building distribution exceeds
80 psi, so a valve and strainer are required, set at or below 80 psi static. A flushometer closet needs 35 to
45 psi at the fixture, so with the valve set at 80 psi the design has up to 35 psi to lose to friction and
elevation before a blowout flushometer closet drops below its 45 psi. If the main showed only 30 psi at peak, a booster system
would be required for flushometer fixtures (604.7).
Booster systems (604.7, 606.5)
Where the main cannot supply the minimum pressures and quantities, the supply is supplemented by an
elevated tank, a hydropneumatic system, or a booster pump (606.5.1). Every booster pump needs a low-pressure
cutoff that protects the suction side from a vacuum when suction pressure falls to 10 psi or less (606.5.5).
Fixture supply sizes (604.5)
A fixture supply pipe ends no more than 30 inches from its connection to the fixture and extends to the
adjacent floor or wall. Table 604.5 gives the minimum sizes. Note how large the flushometer closet supply is.
Fixture
Minimum supply pipe
Lavatory, flush-tank water closet, drinking fountain
3/8 in
Kitchen sink, bathtub, single shower head, service sink, hose bibb
1/2 in
Urinal, flushometer valve
3/4 in
Water closet, flushometer valve
1 in
Materials, water hammer, and valves (605, 604.9, 606)
At summary level: pipe and fittings in the water supply may contain no more than 8 percent lead, and those
supplying drinking or cooking water must meet a weighted average of 0.25 percent or less (605.2). Underground
service pipe must be listed for potable use and rated for at least 160 psi (605.3). Piping may not be
installed in soil or groundwater contaminated with solvents or fuels, and a chemical analysis is
required where that is suspected (605.1). Where quick-closing valves are used, a water hammer arrestor is required (604.9).
Full-open valves belong on the service near the curb, on the discharge side of every meter, at the entrance
to the structure (one for each tenant in a multi-tenant building), at the base of most risers, on the water
supply to every water heater, and to tanks (606.1). Shutoff valves belong at each fixture supply, each sillcock,
and each appliance (606.2). All of these valves need access (606.3), and service and remote valves must be
identified (606.4).
In the field
In a three-tenant strip center rehab, the owner asks GCHS to swap out an old meter and tie the new service
to the existing risers. Confirm that each tenant has its own shutoff valve where the branch enters the space,
that the meter has a valve on its discharge side, and that the PRV and its strainer can be serviced
without cutting the line. Ask what the street pressure reads: a static reading over 80 psi at the meter
means a PRV is required, and a low reading may mean a flushometer closet will never flush well.
Practice questions
1. A designer for a Group B office shows a 1/2-inch water service pipe from the meter to the building. What is the finding under 2021 IPC Section 603.1?
It is too small: the water service pipe must be at least 3/4 inch
It is acceptable: 1/2 in is the minimum
It is too small: the minimum is 1 in for any commercial building water service
It is acceptable if the meter is 5/8 in
Answer: a. The water service pipe is sized for the required quantities and pressures and is never smaller than 3/4 inch in diameter.
2. A new water service is laid in the same trench as an existing building sewer of a material that is not listed in Table 702.2, at a Group R-2 site. What separation does Section 603.2 require?
At least 12 in horizontally, with the service placed on a bench in the trench wall
At least 3 ft vertically only
At least 5 ft horizontally of undisturbed or compacted earth
No separation is required
Answer: c. If the sewer is not made of a Table 702.2 material, the service and sewer are separated by at least 5 feet horizontally of undisturbed or compacted earth, unless a listed exception applies.
3. A water service crosses over a building sewer at a Group B site. The bottom of the service pipe is 14 inches above the top of the sewer within 5 ft of the crossing. Under Section 603.2, the sleeve requirement:
Applies: a sleeve is always required wherever a water service crosses a sewer line
Applies unless the pipe is copper
Does not apply only if the sewer is cast iron
Does not apply: the service bottom is 12 in or more above the sewer top
Answer: d. The 5-foot separation does not apply where the bottom of the service within 5 feet of the sewer is at least 12 inches above the highest point of the sewer. A 14-inch gap qualifies.
4. A rural GCHS rehab plans to run a new potable water service across a septic tank drainage field. What does Section 603.2.1 say?
It is allowed if the pipe is sleeved
It is prohibited: potable service pipe may not be in, under or above a drainage field
It is allowed if buried 12 in deep
It is allowed with a backflow preventer at the building and warning tape above the pipe
Answer: b. Section 603.2.1 bars potable water service pipes from being located in, under, or above these sources of pollution.
5. A Group R-2 apartment building has a static pressure of 92 psi at the entrance to the building water distribution piping. What does Section 604.8 require?
A pressure-reducing valve with a strainer, set at or below 80 psi
Nothing, because the limit is 100 psi at the entrance for buildings with risers
A pressure-reducing valve set at 92 psi
A water hammer arrestor on each riser
Answer: a. Where building pressure exceeds 80 psi static, an approved valve with a strainer reduces it to no more than 80 psi static.
6. A Group A-3 pool building has a hose bibb line that runs ahead of the pressure-reducing valve at 90 psi static. What does the exception to Section 604.8 say?
Every branch must be reduced to 60 psi, including lines to outside hydrants
Only the water heater branch may skip the valve
Service lines to sill cocks and outside hydrants are excepted
There is no exception in the section
Answer: c. The exception covers service lines to sill cocks and outside hydrants, and main supply risers where the pressure is reduced to 80 psi or less at the individual fixtures.
7. What must a pressure-reducing valve do if it fails, according to Section 604.8.1?
Close fully to protect the fixtures and stop water from reaching them
Vent to the drain
Reduce flow to 50 percent
Remain open to allow uninterrupted water flow
Answer: d. The valve is designed to remain open on failure, so occupants keep water service.
8. A design calculation for a Group A-3 community hall shows 30 psi left at a siphonic flushometer valve water closet at peak demand. What does Table 604.3 conclude?
It is enough: 30 psi is the minimum for any flushometer
It is short: the outlet needs 35 psi, so add a booster or larger piping
It is too high: the maximum for a siphonic flushometer valve is 25 psi at peak flow
It is enough: flushometer closets need only 8 psi
Answer: b. A siphonic flushometer valve closet needs 35 psi (a blowout type needs 45 psi). With 30 psi left, the design fails and Section 604.7 points to a booster system.
9. On the top floor of a Group R-2 building, a design shows 15 psi remaining at close-coupled tank water closets at peak demand. What is the finding?
It is short: tank-type closets need 20 psi at the outlet
It is enough: 15 psi is the minimum for tank closets
It is enough: tank closets need 8 psi
It is short: tank closets need 35 psi at the outlet, as flushometer closets do
Answer: a. Table 604.3 lists 20 psi flow pressure for tank-type water closets, so 15 psi is not enough.
10. A Group B high-rise design needs a water pressure booster pump. What low-pressure protection does Section 606.5.5 require on the pump?
A pressure gauge only
A check valve at the discharge
A low-pressure cutoff to prevent a vacuum when suction pressure is 10 psi or less
A relief valve set at 150 psi and a high-pressure cutoff set at 125 psi at the pump
Answer: c. A low-pressure cutoff protects the suction side from negative pressure when positive suction pressure is 10 psi or less.
11. A Group A-4 locker room uses flushometer-valve water closets. What is the smallest fixture supply pipe permitted by Table 604.5?
3/8 inch
1/2 inch
3/4 inch
1 inch
Answer: d. Table 604.5 lists 1 inch for a water closet with a flushometer valve. Flush-tank closets are 3/8 inch (a one-piece closet is 1/2 inch), and flushometer valve urinals are 3/4 inch.
12. An inspector finds a lavatory supply stub-out that ends 40 inches from the lavatory, with flexible connectors made up at the fixture. Under Section 604.5, the supply pipe:
May end up to 6 ft away
Must end not more than 30 in from the point of connection to the fixture
May end up to 60 in from the fixture where flexible connectors are used at the stop
Must end 12 in or less from the floor
Answer: b. The fixture supply pipe terminates not more than 30 inches from the point of connection to the fixture.
13. A three-tenant Group M strip center has a common water supply system. What does Section 606.1 require at each tenant space?
A pressure-reducing valve for each tenant
A backflow preventer for each tenant
A main shutoff valve for each tenant
A separate meter for each tenant
Answer: c. In multiple-tenant buildings with a common water supply, a full-open main shutoff valve is provided for each tenant.
14. A contractor in a Group R-2 building is laying new underground water service pipe outside the structure. What minimum working pressure rating does Section 605.3 require at 73.4 degrees F?
160 psi
80 psi
100 psi
250 psi
Answer: a. Water service pipe installed underground outside the structure must be rated at least 160 psi at 73.4 degrees F, and higher if the available pressure is higher.
Glossary
Static pressure
Water pressure with no flow; the pressure-reducing valve rule caps it at 80 psi in building piping.
Flow pressure
The pressure at a fixture outlet while water is flowing; Table 604.3 sets the minimum for each fixture.
Water service pipe
The pipe from the public main or other source to the building, at least 3/4 in in diameter.
Pressure-reducing valve
A valve with a strainer that holds building pressure at or below 80 psi static and fails open.
Full-open valve
A valve, such as a gate or ball valve, that leaves the pipe unobstructed when open; required at the curb, meter outlet, and entrance.
Booster system
A pump, tank, or hydropneumatic system that raises pressure where the main cannot meet the Table 604.3 values.
Fixture supply
The short pipe from the distribution piping to a fixture, ending within 30 in of the fixture connection.
Flushometer valve
A pressure-operated valve that flushes a closet or urinal; it needs high flow pressure and a large supply pipe.