Plumbing System Design

Plumbing System Design

Reference text for the plumbing system design area. Every rule below is stated from a printed line of

the 2024 International Plumbing Code (IPC 2024) that carries it, or from a line of the Georgia rule

110-11-1-.26 that the state prints beside it, and the address of the line is given with the rule.

Numbers appear only where the printed line carries them.

This area is the selection and sizing side of the trade: choosing a fixture, a valve, a pump, a water

heater, an interceptor or a drain system, and fixing its size and capacity. The code answers it in the

tables it prints for each subject — a flow rate and a pressure for a fixture supply, a drainage fixture

unit value for a fixture, a maximum load for a pipe size, a flow rate for a leader, a capacity for a

sewage pump, a retention capacity for a grease interceptor — and Georgia replaces two of those printed

values and adds a table of its own for water heaters. Section 11 names the outline items the pieces

filed here do not carry.

1. Sizing the water distribution system

The water service pipe is sized for the system it serves (IPC 2024, 603.1), and the distribution system

is designed with pipe sizes selected so that under conditions of peak demand the capacities at the

fixture supply pipe outlets are not less than the code's printed table of flow rate and flow pressure

for each fixture supply outlet; a fixture or appliance not listed in that table takes the minimum flow

rate and flow pressure of its manufacturer's installation instructions (IPC 2024, 604.3). The maximum

flow rates and consumption of the fixtures and fixture fittings themselves come from a second printed

table (IPC 2024, 604.4), which Georgia replaces with the state's own: the revised table keeps the

published maximum flow rate or quantity for each fixture and fixture fitting and carries the state's

note pointing at its own waiver from the high-efficiency fixture requirement (GA-rule, l. 736, GA-rule,

l. 820). A fixture supply pipe has a minimum size taken from the code's third table on the subject, the

supply pipe terminating not more than 30 inches (762 mm) from the point of connection to the fixture, a

reduced-size flexible water connector being of an approved type, the supply pipe extending to the floor

or wall adjacent to the fixture, and the minimum size of individual distribution lines in a gridded or

parallel system also coming from that table (IPC 2024, 604.5). In a parallel or gridded system, the hot

and cold water manifolds are sized in accordance with the manifold sizing table, the total gallons per

minute being the demand of all outlets supplied (IPC 2024, 604.10.1).

Where the pressures vary, the code gives the designer a rule rather than a value. Variable street

pressures are accommodated by one of the means the section prints (IPC 2024, 604.6); where the water

pressure is insufficient, an approved booster pump, elevated tank or other approved device supplied or

approved by the serving water purveyor is installed (IPC 2024, 604.7); where the static pressure within

a building exceeds 80 psi (552 kPa), an approved water pressure-reducing valve or regulator with

strainer is installed to reduce the pressure in the building distribution piping to not greater than 80

psi (552 kPa) static, with an exception for service lines to sill cocks and outside hydrants and for

main supply risers where pressure from the mains is reduced to 80 psi or less at individual fixtures

(IPC 2024, 604.8); a water-hammer arrestor is installed where quick-closing valves are used (IPC 2024,

604.9); and individual pressure balancing in-line valves are installed in a configuration that will

accommodate an individual pressure balancing in-line valve for each fixture fitting (IPC 2024, 604.11).

2. Selecting and sizing the valves

A valve selected for a system is compatible with the piping material installed in that system, conforms

to one of the standards the code's valve table lists or is approved, and a valve intended to supply

drinking water meets the drinking water standard (IPC 2024, 605.7). Where full-open valves and shutoff

valves are required, the code fixes their locations: the full-open valves from the building water

service pipe near the curb through the entrance into the structure, the discharge side of every water

meter, the base of every riser and the top of every down-feed pipe to the water supply pipe of every

water heater (IPC 2024, 606.1), and the shutoff valve locations printed in the section beside it

(IPC 2024, 606.2), with Georgia adding one — access on the same floor as a refrigerator for the shutoff

valve serving its automatic icemaker (GA-rule, l. 855). Access is provided to all of them (IPC 2024,

606.3), and they are identified: service and hose bibb valves by identification, and other valves not

adjacent to the fixture or appliance they serve by an identification indicating what they serve

(IPC 2024, 606.4). A safety valve is selected to its duty: a storage water heater operating above

atmospheric pressure has an approved self-closing pressure relief valve and temperature relief valve or

a combination conforming to the printed standard, not used to control thermal expansion (IPC 2024,

504.4), and the valve's relieving capacity equals or exceeds the heat input to the heater or tank it

serves, with a temperature setting of not more than 210°F and a pressure setting not exceeding the

manufacturer's rated working pressure or 150 psi, whichever is less (IPC 2024, 504.5). A backflow

preventer is chosen by application: which assembly belongs on which connection, at which degree of

hazard, is a printed table the application must conform to (IPC 2024, 608.1), and the minimum air gap

between an opening or outlet and a fixture's flood level rim is likewise a printed table (IPC 2024,

608.16.1). A storm drainage system is provided with backwater valves as required for sanitary drainage

systems (IPC 2024, 1101.9), and the conditions that require one are printed in the sanitary drainage

chapter: fixtures below the elevation of the next upstream manhole cover of the public sewer are

protected by a backwater valve installed in the building drain or the horizontal branch serving them

(IPC 2024, 714.1).

3. Sizing the drainage system

Horizontal drainage piping is installed at the slope the code's slope table requires (IPC 2024, 704.1).

The load a pipe may carry is counted in drainage fixture units taken from the fixture-unit table, and a

fixture not listed there is given its value by the code official on the basis of the discharge it

delivers (IPC 2024, 709.1, IPC 2024, 709.2); where a discharge is known only in gallons per minute or

litres per second, the fixture unit value is computed on the printed basis that 1 gpm (0.06 L/s) of flow

is equivalent to two drainage fixture units (IPC 2024, 709.3). The maximum number of fixture units

connected to a given size of building sewer, building drain or horizontal branch is determined from the

code's table of building drains and sewers, and the maximum connected to a given size of horizontal

branch or vertical soil or waste stack from the table of horizontal branches and stacks (IPC 2024,

710.1). Horizontal and vertical stack offsets in buildings of five stories or more follow the offsets

chapter (IPC 2024, 711.1, IPC 2024, 711.2). A fixture trap is sized to drain the fixture rapidly and is

not less than the size the fixture-unit table indicates for that fixture, and a trap is not larger than

the drainage pipe into which it discharges (IPC 2024, 1002.5).

A designer who sizes a system by computation rather than by table follows the chapter the code prints

for it: the design of the drainage system is by the methods of that chapter (IPC 2024, 713.1), the load

on the drainage system is calculated from the fixture discharge profiles and the drainage fixture units

of the fixtures served (IPC 2024, 713.2, IPC 2024, 713.2.1), and pipe sizes are selected from the pipe

wall roughness of the material and the slope of the horizontal drainage piping so that the pipe is sized

to prevent full-bore flow (IPC 2024, 713.3, IPC 2024, 713.3.1, IPC 2024, 713.3.2).

4. Sizing the vents

The minimum required diameter of stack vents and vent stacks is determined from the developed length and

the total of drainage fixture units connected to them in accordance with the code's table of size and

developed length, and in no case is it less than one-half the diameter of the drain served or less than

1 1/4 inches (32 mm) (IPC 2024, 906.1). A vent system designed as an engineered system follows that

section's requirements (IPC 2024, 919.1), and one designed by computation where the code permits it

follows the computerized vent design section (IPC 2024, 920.1).

5. Sizing roof drains, leaders, gutters and storm drains

A roof drain is selected on its published flow rate, which is based on the head of water above the roof

drain, and that rate is what the storm drainage system is sized to; the flow rate used for sizing is

based on the maximum anticipated ponding at the roof drain (IPC 2024, 1105.2). Vertical and horizontal

storm drain piping is sized on the flow rate through the roof drain, calculated by the code's rainfall

rate conversion method and checked against the roof drain manufacturer's published flow rate for the

specific model and size to verify that the selected roof drain will handle the anticipated flow, and the

flow rate in the piping does not exceed that of the storm drain pipe sizing table (IPC 2024, 1106.2,

IPC 2024, 1106.2.1). Vertical leaders are sized on the flow rate from the horizontal gutters or the

maximum flow rate through the roof drains, within the limits of the vertical leader sizing table

(IPC 2024, 1106.3). Where a vertical wall or a parapet wall is drained, the sizing follows those

sections (IPC 2024, 1106.4, IPC 2024, 1106.5), and a horizontal gutter is sized in accordance with the

gutter sizing table (IPC 2024, 1106.6). A secondary or emergency roof drainage system is sized in

accordance with the storm drain sizing section on the rainfall rate for which the primary system is

sized, its scuppers sized to prevent ponding deeper than the roof was designed for, an opening dimension

of not less than 4 inches (102 mm) in height and an opening width equal to the circumference of the roof

drain required for the area served, and the flow through the primary system is not counted when the

secondary system is sized (IPC 2024, 1108.3). The number of roof drains is fixed by the code: not less

than two roof drains in roof areas of 10,000 square feet (929 m²) or less and not less than four in

roofs over that area (IPC 2024, 1110.4).

6. Selecting a sump, a pump and a sewage ejector

A sump's pump, pit and discharge piping are selected and installed under the sump design section

(IPC 2024, 712.3), whose own subsections fix the pump, the pit, the discharge pipe and its fittings,

their materials and ratings, the maximum effluent level and the pump connection to the drainage system

(IPC 2024, 712.3.1, IPC 2024, 712.3.2, IPC 2024, 712.3.3, IPC 2024, 712.3.4, IPC 2024, 712.3.5). A

sewage pump or sewage ejector automatically discharges the contents of the sump to the building drainage

system (IPC 2024, 712.4) and is selected with the capacity and head the application requires: a pump

receiving the discharge of water closets handles spherical solids up to and including 2 inches (51 mm)

in diameter and other pumps up to and including 1/2 inch (13 mm), and the capacity for a given discharge

pipe diameter is not less than the value of the minimum capacity table — 21 gallons per minute for a

2-inch discharge pipe, 30 for 2 1/2-inch and 46 for 3-inch — with the printed exceptions for grinder

pumps and for a macerating toilet assembly serving a single water closet (IPC 2024, 712.4.2). A storm

sump pump is selected on the same principle: it is of a capacity and head appropriate to the anticipated

use requirements (IPC 2024, 1113.1.1).

7. Selecting a water heater

The water heater chapter governs the materials, design and installation of water heaters and their

related safety devices and appurtenances (IPC 2024, 501.1), and the selection side of it is what this

area asks for. The heater is located and connected so as to provide access for observation, maintenance,

servicing and replacement (IPC 2024, 501.4) and with the service space the code prints for an appliance

(IPC 2024, 502.5). Water from a tankless heater is not hotter than 140°F where intended for domestic

use, which does not supersede the requirement for protective shower valves (IPC 2024, 501.6). Where

leakage would cause damage, the heater stands in a pan at least 1 1/2 inches deep and large enough for

all dripping or condensate, drained by an indirect waste pipe at least 3/4 inch in diameter (IPC 2024,

504.7, IPC 2024, 504.7.1).

Georgia sizes the residential water heater in a table of its own. Water heaters installed in residential

occupancies are sized in accordance with the state's minimum capacities table or the manufacturer's

recommendations, and the water heater must at a minimum meet the First Hour Rating requirements of that

table (GA-rule, l. 494); the table prints a first hour rating in gallons for a residence by number of

bedrooms and number of bathrooms — 40 gallons for a one-bath residence with one bedroom, 40 for two

bedrooms and 48 for three; 47 gallons for a residence with two to two and a half baths and two bedrooms,

60 for three, 62 for four and 70 for five; and 60 gallons for a residence with three to three and a half

baths and three bedrooms, 67 for four, 70 for five and 72 for six (GA-rule, l. 568, GA-rule, l. 624,

GA-rule, l. 688) — and a tankless water heater is sized and installed per the manufacturer's

recommendations (GA-rule, l. 720). Georgia also states where the water heater stops

being a plumbing selection: the State's minimum requirements for boilers and water heaters and for

pressure vessels over 200,000 Btu/h (58.61 kW), 210 degrees Fahrenheit or 120 gallons capacity are

established by O.C.G.A. Title 25, Chapter 15 and the rules and regulations of the Office of Insurance

and Safety Fire Commissioner (GA-rule, l. 460). And a storage water heater supplied with cold water

passing through a check valve, pressure reducing valve or backflow preventer has a thermal expansion

control device connected to its cold water supply pipe downstream of all check valves, pressure reducing

valves, backflow preventers and shutoff valves, the tank located at least 18 inches of developed length

from the cold water inlet fitting, sized in accordance with the tank manufacturer's instructions and so

that the system pressure does not exceed the code's limit (GA-rule, l. 872).

8. Selecting an interceptor or a separator

The fixture and the flow decide the interceptor. A grease interceptor has the grease retention capacity

indicated in the code's capacity table for the flow-through rates indicated (IPC 2024, 1003.3.5.1), and

where a hydromechanical interceptor is selected its rate of flow control is governed by the section

beside it (IPC 2024, 1003.3.5.2). An oil separator is listed and labelled, or designed in accordance

with the code's oil separator design sections (IPC 2024, 1003.4.2). A sand or similar interceptor for

heavy solids is designed and located so as to have ready access for cleaning and has a water seal of not

less than 6 inches (152 mm) (IPC 2024, 1003.5). A clothes washer discharges through an interceptor

provided with a wire basket or similar device, removable for cleaning, that prevents the passage into

the drainage system of solids 1/2 inch (12.7 mm) or larger, string, rags, buttons or other materials

detrimental to the public sewage system (IPC 2024, 1003.6). Whichever is selected, it is designed so as

not to become air bound and is vented in accordance with one of the venting methods of the vents

chapter (IPC 2024, 1003.9), and access is provided to each interceptor and separator for service and

maintenance (IPC 2024, 1003.10).

9. Selecting fixtures, and what the number of them comes from

How many fixtures a building needs is a table question before it is a design question: plumbing fixtures

are provided in the minimum number shown in the code's minimum plumbing facilities table, based on the

actual use of the building or space, a use not shown in the table being considered individually by the

code official, and the number of occupants being determined by the International Building Code

(IPC 2024, 403.1). The calculation is printed: the total occupant load is divided in half to determine

the occupant load of each sex, the fixture ratios of the table are applied to the occupant load of each

sex, fractional numbers are rounded up to the next whole number, and for multiple occupancies the

fractional numbers are first summed and then rounded up (IPC 2024, 403.1.1). Georgia changes that table

twice: the service sink requirements are deleted entirely and without substitution (GA-rule, l. 249),

and the residential lines gain one kitchen sink per dwelling unit, one automatic clothes washer

connection per twenty dwelling units in an apartment house or per dwelling unit in a one- or two-family

dwelling, and not less than two exterior hose bibs, sill cocks or outside hydrants with one located on

the side or rear of the structure for a detached single-family, duplex or multi-family dwelling three

stories or less in height (GA-rule, l. 339, GA-rule, l. 364). Separate facilities, employee and public

toilet facilities, signage and drinking fountain location are the sections beside it (IPC 2024, 403.2,

IPC 2024, 403.3, IPC 2024, 403.4, IPC 2024, 403.5), and the state raises the threshold below which

drinking fountains are not required to an occupant load of 25 or fewer (GA-rule, l. 392).

Accessibility is selected to a standard rather than to a size: accessible plumbing facilities and

fixtures are provided in accordance with the building code's accessibility chapter (IPC 2024, 404.1),

and the accessible fixtures themselves are installed in accordance with the printed accessibility

standard (IPC 2024, 404.2). A specialty fixture is selected by its application and its material:

materials for specialty fixtures not otherwise covered in the code are stainless steel, soapstone,

chemical stoneware or plastic, or are lined with lead, copper-base alloy, nickel-copper alloy,

corrosion-resistant steel or another material especially suited to the application for which the fixture

is intended (IPC 2024, 402.2). The hospital, medical and prison fixtures the outline names are reached

by the code's provisions for plumbing in mental health centers (IPC 2024, 405.7), and its specialty

fixture and whirlpool provisions print the requirements those installations are selected against

(IPC 2024, 423.1, IPC 2024, 426.1, IPC 2024, 426.2). An emergency shower or eyewash station conforms to

the printed standard, needs no waste connection, and where hot and cold water are supplied to it the

temperature is controlled only by a temperature-actuated mixing valve complying with the printed

standard, a water heater supplying it directly complying with another printed standard (IPC 2024, 411.1,

IPC 2024, 411.2, IPC 2024, 411.3). A shower floor or receptor is selected with its support and its

lining: floors under shower compartments other than prefabricated receptors are lined and made

watertight with a material complying with the lining sections, the liner turning up on all sides not

less than 2 inches (51 mm) above the finished threshold level (IPC 2024, 421.5.2), and the receptor is

supported as the section requires (IPC 2024, 421.5.1). A pan under a water heater is the other pan the

outline names, and its size and drain are printed: the pan is large enough for all dripping or

condensate and its drain terminates over an indirect waste receptor or floor drain or to the exterior

not less than 6 inches and not more than 24 inches above the adjacent ground surface (IPC 2024, 504.7.1,

IPC 2024, 504.7.2).

10. Selecting a specialty waste system

A special or indirect waste system is required for the applications the code lists: food-handling

equipment other than in dwelling units, clear-water waste, humidifiers, dishwashing machines and

utensils, pots, pans and dishwashing sinks discharge through an indirect waste pipe, and fixtures not

required by that section to be indirectly connected are directly connected to the plumbing system as

the drainage chapter requires (IPC 2024, 802.1). The connection is made by an air gap or an air break of

the printed type (IPC 2024, 802.3.1, IPC 2024, 802.3.2), the receptor receiving the discharge is sized

for the maximum discharge of all indirect waste pipes it serves and installed to prevent splashing or

flooding (IPC 2024, 802.4.1), and a standpipe is individually trapped, extends not less than 18 inches

(457 mm) and not more than 42 inches (1067 mm) above the trap weir, and has access provided for rodding

(IPC 2024, 802.4.3). Corrosive wastes are not discharged into the plumbing system without a selected

device: corrosive liquids, spent acids or other harmful chemicals that destroy or injure a drain, sewer,

soil or waste pipe or create noxious or toxic fumes or interfere with sewage treatment are not

discharged without being thoroughly diluted, neutralized or treated by passing through an approved

dilution or neutralizing device, automatically supplied with enough diluting water or neutralizing

medium to make the contents noninjurious before discharge, and the nature of the waste and its treatment

method are approved prior to installation (IPC 2024, 803.1). The on-site reuse systems of the code's

nonpotable chapter, where one is selected, have their own design section, and Georgia adds to the

chapter the uses reclaimed water from a permitted treatment facility may supply (GA-rule, l. 1006).

11. What these pieces do not carry

(GA-bulletin, l. 478). The plumbing code does not size a boiler: what it states about one is the

protection of the potable water that feeds it — the potable supply to a boiler is equipped with a

backflow preventer with an intermediate atmospheric vent complying with the printed standards, and

where conditioning chemicals are introduced into the system the potable water connection is protected

by an air gap or a reduced pressure principle backflow preventer complying with the printed standards

(IPC 2024, 608.17.2). Above the printed threshold, Georgia hands the subject to another authority: the

State's minimum requirements for boilers and water heaters and for pressure vessels over 200,000 Btu/h

(58.61 kW), 210 degrees Fahrenheit or 120 gallons capacity are established by O.C.G.A. Title 25,

Chapter 15 and the rules of the Office of Insurance and Safety Fire Commissioner (GA-rule, l. 460). No

boiler capacity, rating or sizing method is stated in this text.

and vacuum piping (GA-bulletin, l. 481). Gas piping is not a subject of this code: the code's scope

states that the installation of fuel gas distribution piping and equipment, fuel-gas-fired water

heaters and water heater venting systems is regulated by the International Fuel Gas Code (IPC 2024,

101.2), and Georgia's general applicability section repeats that sentence as the first sentence of its

scope (GA-rule, l. 151). No gas sizing method, table or pressure is stated here.

storage systems chapter, whose nonflammable medical gas provision is the printed line this text can

point at (IPC 2024, 1202.1); the design and installation of nonmedical oxygen systems are carried by

the printed NFPA standards rather than by this code (IPC 2024, 1203.1). Nothing further is asserted

about industrial or specialty piping here.

first-hour rating that governs a residential water heater in Georgia is printed by the state's rule

and not by the model code (GA-rule, l. 494); the high-efficiency fixture and faucet requirements that

the code's maximum-flow table refers to are the state's own additions (GA-rule, l. 170, GA-rule, l.

404); and the requirement to disinfect a new or repaired potable water system is the state's revised

wording of the code's disinfection section (GA-rule, l. 892). For a tankless water heater the code

states no rating at all: it states the temperature limit of the water delivered, not hotter than 140°F

where intended for domestic use (IPC 2024, 501.6).