Chapter: Plumbing System Design

Learning Objectives

By the end of this chapter, you will be able to:

4.Apply the 2018 Georgia Plumbing Code (IPC with GA amendments) to design safe, code-compliant plumbing systems for various occupancy types.

5.Calculate fixture unit loads, sanitary drainage sizing, vent sizing, and water supply demand using the correct tables and methodologies.

6.Identify the responsibilities of the master plumber as the licensed designer and installer, including permit, inspection, and coordination duties.

7.Navigate the code structure efficiently to locate design-related requirements during the exam.

8.Recognize common design errors and exam traps related to fixture counts, pipe sizing, and system interconnections.

1.1 The Master Plumber’s Role in System Design

As a master plumber in Georgia, you are not merely an installer—you are the responsible licensed professional who ensures the design meets the minimum standards of the 2018 Georgia Plumbing Code (based on the 2018 IPC with state amendments). The code is a performance and prescriptive document; it tells you what minimum requirements are, and in many cases how to achieve them. Your design must account for:

Public health and safety: preventing contamination, cross-connections, and unsanitary conditions.

System functionality: adequate pressure, flow, drainage slope, and venting.

Maintainability: access for cleaning, repair, and replacement.

Coordination with other trades: structural, electrical, mechanical, and fire protection.

The exam tests your ability to apply code logic, not memorized text. You must know where to find the rule and how to apply it to a scenario.

1.2 Code Navigation: Where to Find Design Rules

For the closed-book exam, you cannot bring the code. However, understanding the code’s internal organization helps you recall the logic. The 2018 IPC is divided into chapters; the Georgia amendments modify specific sections. Key chapters for design:

Topic	IPC Chapter / Section	Notes
Definitions	Chapter 2	Know key terms: fixture unit, developed length, etc.
General Regulations	Chapter 3	Trap requirements, cleanouts, protection of piping
Fixtures, Faucets, Fittings	Chapter 4	Minimum number of fixtures (Table 403.1)
Water Heaters	Chapter 5	Sizing, temperature/pressure relief valves
Water Supply and Distribution	Chapter 6	Sizing (Table 603.1), pressure requirements
Sanitary Drainage	Chapter 7	Sizing (Table 710.1), slope (Table 704.1)
Vents	Chapter 9	Vent sizing (Table 916.1), vent types
Traps and Interceptors	Chapter 10	Trap seal protection, grease interceptors
Storm Drainage	Chapter 11	Roof drainage, conductors
Fuel Gas (IFGC)	Separate code	For gas piping design, not in IPC

Georgia Amendments: The state may modify fixture counts, venting rules, or add specific requirements for certain occupancies. Always check the amendment sections in your study materials—they often appear as exam questions.

1.3 Fixture Units and Demand Calculations

The foundation of all sizing is the fixture unit (FU). A fixture unit is a dimensionless number representing the rate of water consumption or drainage load. The code assigns values based on the type of fixture and whether it is public or private.

1.3.1 Drainage Fixture Units (DFU)

Use Table 710.1 (IPC) to assign DFU values. Key examples:

Lavatory (private): 1 DFU

Lavatory (public): 2 DFU

Water closet (private, 1.6 gpf): 3 DFU

Water closet (public, 1.6 gpf): 4 DFU

Bathtub: 2 DFU

Kitchen sink (domestic): 2 DFU

Floor drain: 2 DFU (but not counted for total load unless connected to a sanitary system)

Trap: Do not double-count fixtures. A water closet has an integral trap; a floor drain does not count as a fixture for minimum count purposes but does add load.

1.3.2 Water Supply Fixture Units (WSFU)

Use Table 603.1 (IPC) for cold and hot water. The values differ from DFU. For example:

Private lavatory: 1.0 WSFU (cold), 1.0 WSFU (hot)

Public lavatory: 1.0 WSFU (cold), 1.0 WSFU (hot) per faucet

Water closet (tank): 2.2 WSFU (cold only)

Shower head: 1.5 WSFU (cold), 1.5 WSFU (hot)

Important: For a water closet, you only count cold water. For a shower, you count both hot and cold separately, but the total demand is based on the sum of all WSFU, then converted to gallons per minute (gpm) using Table 603.1 (the conversion table) or the appropriate curve.

1.3.3 Converting WSFU to Flow (gpm)

The code provides a table (IPC Table 603.1) that converts total WSFU to expected demand in gpm. For example:

10 WSFU ≈ 6 gpm

20 WSFU ≈ 9 gpm

50 WSFU ≈ 18 gpm

100 WSFU ≈ 30 gpm

Exam Trap: Do not add fixture units linearly to gpm. The relationship is non-linear due to probability of use. A system with 100 WSFU does not require 100 gpm—it requires about 30 gpm.

1.3.4 Minimum Number of Fixtures

IPC Table 403.1 specifies the minimum number of fixtures for various occupancies (assembly, business, educational, factory, etc.). For example:

Business (office): 1 water closet per 25 employees (male and female separate), 1 lavatory per 2 water closets (or per 40 employees).

Assembly (theater): 1 water closet per 125 males, 1 per 65 females (or per 125 for male, 65 for female).

Georgia Amendment: Check for any state-specific changes to fixture counts, especially for food service or public assembly.

Exam Trap: The code requires separate facilities for males and females in most occupancies. Also, unisex or family restrooms may count toward the total only under specific conditions (e.g., for assembly occupancies with a certain capacity).

1.4 Sanitary Drainage System Design

1.4.1 Pipe Sizing

Sanitary drainage pipe is sized by DFU load and slope. IPC Table 710.1 gives the maximum DFU load for a given pipe size and slope. For example:

2-inch pipe at ¼ inch per foot slope: maximum 21 DFU

3-inch pipe at ¼ inch per foot: maximum 42 DFU (but 3-inch is the minimum for a water closet)

4-inch pipe at ¼ inch per foot: maximum 216 DFU

Rule: The minimum diameter for a water closet drain is 3 inches. A 2-inch drain cannot serve a toilet.

1.4.2 Slope Requirements

IPC Table 704.1 sets the minimum slope:

2-inch pipe: ¼ inch per foot

3-inch pipe: ¼ inch per foot

4-inch pipe: ⅛ inch per foot (minimum) or ¼ inch per foot if required by local conditions

6-inch and larger: ⅛ inch per foot

Exam Trap: For pipes 4 inches and larger, you may use ⅛ inch per foot, but only if the pipe is not subject to solids that would not be carried. In practice, ¼ inch per foot is safer for 4-inch.

1.4.3 Horizontal Branch and Stack Sizing

A horizontal branch is sized by the total DFU it carries, using Table 710.1.

A soil or waste stack is sized by the total DFU and the number of branch intervals. A branch interval is a floor level where horizontal branches connect. For a stack with more than three branch intervals, the stack must be sized larger than a single branch.

Example: A 3-inch stack can carry 42 DFU if it has 3 or fewer branch intervals; with 4 or more, the capacity drops to 30 DFU (per Table 710.1). This is a common exam trap.

1.4.4 Cleanouts

IPC Section 707 requires cleanouts:

At the base of every stack (at the lowest horizontal run).

Every 100 feet in horizontal runs (for pipes 4 inches and larger).

At every change of direction greater than 45 degrees (for 4-inch and larger).

For 3-inch and smaller, cleanouts at every change of direction greater than 45 degrees may be required if the run is long.

Design Point: Cleanouts must be accessible. A cleanout in a ceiling or behind a wall without an access panel is a violation.

1.5 Vent System Design

1.5.1 Purpose and Types

Vents protect trap seals by allowing air into the drainage system. Types include:

Individual vent: serves one fixture.

Common vent: serves two fixtures (e.g., back-to-back lavatories).

Wet vent: serves as both a drain and a vent for other fixtures (limited to certain configurations).

Circuit vent: serves multiple fixtures on a horizontal branch (max 8 fixtures).

Relief vent: protects a stack from pressure fluctuations.

1.5.2 Vent Sizing

IPC Table 916.1 relates vent size to the drain size and the developed length of the vent. The developed length is measured from the fixture connection to the vent terminal (usually through the roof). For example:

A 2-inch drain with a 2-inch vent can have a maximum developed length of 60 feet.

A 3-inch drain with a 2-inch vent: maximum length 30 feet (if the vent is smaller than the drain, the length is reduced).

Rule: The vent must be at least half the diameter of the drain it serves, but not less than 1¼ inches.

1.5.3 Vent Termination

Vents must terminate:

At least 6 inches above the roof (measured from the high side).

At least 10 feet horizontally from a window, door, or air intake (unless the vent is 2 feet above the opening).

Not under an overhang (unless the vent is at least 6 inches above the roof and the overhang is not a source of contamination).

Georgia Amendment: Some jurisdictions require a minimum of 12 inches above the roof for snow load. Know your local amendment.

1.5.4 Wet Venting Rules

A wet vent can serve a water closet and a lavatory (or bathtub) on the same floor, provided:

The wet vent is at least 2 inches in diameter.

The water closet drain connects to the wet vent within a certain distance (per code).

The wet vent receives discharge from the lavatory (which acts as the vent) and the toilet.

Exam Trap: A wet vent cannot serve fixtures on more than one floor. Also, a sink with a garbage disposer cannot wet-vent a toilet.

1.6 Water Supply System Design

1.6.1 Pressure Requirements

IPC Section 604 requires:

Minimum pressure at any fixture: 8 psi (but 15–20 psi is recommended for operation).

Maximum static pressure: 80 psi. If higher, a pressure-reducing valve (PRV) is required.

The water service must be sized to deliver the required flow at the point of use.

1.6.2 Sizing the Service and Distribution

Use Table 603.1 to convert WSFU to gpm. Then, size the pipe based on:

Available pressure (from the water main or booster pump).

Friction loss (use the Hazen-Williams formula or code tables).

Velocity limits: maximum 8 feet per second for cold water, 5 feet per second for hot water (to prevent noise and erosion).

Practical Method: The code allows a simplified method using the longest run and pressure drop. For the exam, know that a ¾-inch service is the minimum for a single-family home, but a 1-inch service is often required for a home with more than 3 bathrooms.

1.6.3 Hot Water Recirculation

If the distance from the water heater to the farthest fixture exceeds 100 feet (developed length), a recirculation pump is required (per IPC 607.2). The return pipe must be sized to maintain a minimum temperature of 110°F at the fixture.

1.6.4 Backflow Prevention

IPC Chapter 6 requires backflow protection:

An air gap or reduced pressure principle (RP) assembly for hazardous cross-connections.

A double-check valve for non-hazardous connections.

Hose bibbs must have a vacuum breaker.

Georgia Amendment: Some local jurisdictions require RPZs on all commercial services. Know your local requirements.

1.7 Traps and Interceptors

1.7.1 Trap Requirements

Every fixture must have a trap (except those with integral traps, like water closets). Requirements:

Trap seal: minimum 2 inches, maximum 4 inches (for floor drains, the seal can be 2 inches but must be maintained).

Trap diameter: not less than the drain diameter of the fixture.

Prohibited: traps that depend on moving parts, or traps that are not self-cleaning (e.g., bell traps).

1.7.2 Interceptors

Grease interceptors are required for fixtures that discharge fats, oils, or grease (e.g., commercial kitchens). Sizing is based on the flow rate and the number of meals served. The code (IPC 1003) requires:

Grease interceptors to be located where they are easily accessible for cleaning.

Not to be installed in a location where they can freeze.

Exam Trap: A grease interceptor cannot be used as a floor drain for a general-purpose area. It must be dedicated to the grease-producing fixtures.

1.8 Storm Drainage Design (Chapter 11)

Storm drainage handles rainwater from roofs, paved areas, and yards. Key points:

Roof drains: sized by the horizontal projected roof area and the rainfall rate (in inches per hour). Georgia uses a 1-hour, 100-year storm event (typically 4–6 inches per hour depending on location).

Conductors and leaders: vertical pipes that carry water from roof drains to the storm sewer or a splash block.

Sizing: Use IPC Table 1106.2 for vertical conductors and Table 1106.3 for horizontal piping.

Example: A 4-inch vertical conductor can handle about 1,800 square feet of roof area at a 4-inch-per-hour rainfall rate. A 6-inch horizontal pipe at ⅛ inch per foot slope can handle about 3,000 square feet.

Design Point: Storm and sanitary systems must be separate. No cross-connections are allowed.

1.9 Practical Field Points for the Master Plumber

153.Permits and Inspections: You are responsible for pulling the permit and scheduling inspections. The code requires a rough-in inspection before covering, and a final inspection before occupancy.

154.Coordination with Structural: Notching or boring of joists and studs must follow the building code (not the plumbing code). You must verify the location of beams and columns before running pipes.

155.Water Heater Safety: Every water heater must have a temperature and pressure (T&P) relief valve, rated at 150 psi and 210°F, with a discharge pipe that terminates 6 inches above the floor (not threaded, not connected to the drain).

156.Testing: Drainage systems must be tested with water or air. Water test: 10 feet of head (or 5 psi) for 15 minutes. Air test: 5 psi for 15 minutes.

157.Accessibility: Fixtures must be installed to meet the accessibility requirements of the building code (ADA), but the plumbing code sets the technical requirements for clearances and heights.

1.10 Common Exam Traps and How to Avoid Them

Trap	Correction
Using DFU for water supply sizing	Use WSFU for water, DFU for drainage.
Adding fixture units linearly to get gpm	Use the conversion table (non-linear).
Sizing a stack without considering branch intervals	Check the number of branch intervals; reduce capacity if >3.
Forgetting the minimum trap seal depth	2 inches minimum, 4 inches maximum.
Using a 2-inch vent for a 4-inch drain without checking length	Vent must be at least half the drain diameter; length limits apply.
Assuming a floor drain counts as a fixture for minimum count	Floor drains do not count toward minimum fixture requirements.
Ignoring the Georgia amendment on fixture counts	Always check state amendments for specific occupancies.
Allowing a wet vent to serve a toilet on a lower floor	Wet vents are limited to the same floor.
Not providing a cleanout at the base of a stack	Required at the lowest horizontal run.
Sizing roof drains using the 10-minute rainfall rate	Use the 1-hour, 100-year rate for Georgia.

1.11 Summary

Plumbing system design under the 2018 Georgia Plumbing Code requires a systematic approach: determine the occupancy, count the fixtures, assign fixture units, size the drainage and venting, then size the water supply. The master plumber must also ensure the system is testable, maintainable, and compliant with state amendments. On the exam, focus on the logic of the tables and the relationships between fixture count, pipe size, and slope. Practice converting between fixture units and flow, and memorize the key minimums (e.g., 3-inch toilet drain, 2-inch wet vent, 6-inch roof vent termination).

Remember: The code is a minimum standard. Your design should always aim for safety, durability, and serviceability—that is the hallmark of a true master plumber.

Plumbing System Design