March 25, 2026
Guide to Sanitary Drainage System Sizing Using FUs NPC Exam Guide
Plumbing Study Guide
March 25, 2026
Plumbing Study Guide
Sanitary drainage sizing using Drainage Fixture Units (FUs) is one of the most important topics in the Canadian plumbing trade. It appears frequently on Red Seal exams because proper sizing directly affects:
In real life, poor FU sizing causes slow drainage, blockages, siphonage, and failed inspections — which is why the National Plumbing Code of Canada (NPC) places strong emphasis on hydraulic loading and sizing rules.
The NPC provides technical requirements for the design and installation of plumbing systems and is adopted (often with amendments) by provinces and territories, so always verify your local edition.
In the NPC structure, sanitary drainage sizing mainly appears in:
Exam tip: Questions rarely ask for memorized article numbers — they test your ability to APPLY table values and rules logically.
These terms are critical for both exams and job-site communication.
Fixture Unit (FU)
The unit of measure is based on: 1. rate of discharge, 2. time of operation and 3. frequency of use of a fixture that expresses the hydraulic load that is imposed by that fixture on a drainage system. Traditionally, the time and frequency waste value of 1 fixture unit is 1 cubic foot per minute discharge rate.
The section of pipe between a fixture trap and the branch connection.
A horizontal drain that receives the discharge from fixture drains and conveys it to a stack or building drain.
A vertical pipe carrying discharge from fixtures on multiple floors.
The lowest piping inside the building that receives discharge from stacks and conveys it toward the building sewer.
The piping outside the building carries flow to the municipal sewer or private treatment.
Indicates the load placed on a section of drainage pipe by one or more fixture units, expressed in fixture units.
The actual centerline length of the pipe, including fittings, is relevant when assessing venting and, sometimes, system layout.
No more than 2 water closets on 3” horizontal drainage piping, and no more than 6 water closets on 3” vertical piping. In both instances, the pipe size would be increased to 4” where the 3rd and 7th water closets connect. (Code reference 2.4.9.2)
No connections within the first 1.5m of the horizontal section of piping on stacks that receive 30 fixture units or more, or receive discharge from fixtures located on 2 or more storeys. This is when vertical stacks turn horizontal. (Code reference 2.4.2.1(2)).
Means a pipe that connects the waste opening of a fixture to the trap serving the fixture.
Minimum size for stacks
No section of piping shall be sized smaller than the largest pipe connected upstream of that section, regardless of fixture units; stacks may not reduce in size.
Fixtures do not discharge simultaneously at full flow.
FUs allow engineers and plumbers to:
This probabilistic approach is why FU tables are central to NPC drainage design.
Connections to drainage systems 2.4.2
Location of fixtures 2.4.3
Treatment of sewage and waste 2.4.4
Traps 2.4.5
Separate system 2.4.6.
Cleanouts 2.4.7
Minimum slope and length of drainage pipes 2.4.8
No reduction in size 2.4.9.1
Serving water closets 2.4.9.2
Sizing of fixture outlet pipes 2.4.9.3 & Sizing table 2.4.9.3
Sizing of building drain and building sewer 2.4.9.4
Total load on a pipe 2.4.10.1
Hydraulic loads for fixtures 2.4.10.2
Table 2.4.10.6-A (loads to stacks), Table 2.4.10.6-B (loads to branches), Table 2.4.10.6-C (loads to sanitary building drain or sewer)
Table 2.4.10.9 (storm building drain, storm building sewer or a combined building sewer)
Multi-stack systems are common in:
Instead of a single stack serving all fixtures, multiple stacks collect fixture groups and combine them downstream.
Key concept:
FUs increase cumulatively as you move downstream.
*Important to note: Must start at the top and work your way down floor by floor to size properly.*
Using the NPC fixture unit table:
Example
Write FUs beside every fixture BEFORE sizing any pipe.
Each horizontal branch must be calculated separately.
Example:
This total determines the minimum pipe size using horizontal drainage sizing tables.
NPC limits for horizontal pipes are stricter because:
Must size branches based on the biggest pipe attached to the branch. Only when the connection is made with a bigger pipe must the branch main become that size. However, there are limitations on how many fixture units each pipe size can handle before you must upsize to the next pipe diameter.
Rule of thumb:
Horizontal = usually larger than vertical for the same load.
Add fixture loads floor by floor.
Example:
Stack total at base = 24 FUs.
Vertical stacks can carry more FUs because gravity flow is assisted.
When two stacks join:
Stack A = 24 FU
Stack B = 18 FU
New downstream load = 42 FU
Resize the pipe at that point using horizontal drainage tables (2.4.10.6-B).
The building drain carries:
This is often the largest pipe inside the building (size using 2.4.10.6.-C).
Answer: 4” (42 FU is above the capacity of a 3” pipe, regardless of slope. Must be 4”.).
Using the wrong table is one of the biggest exam mistakes. Once you upsize, you cannot reduce pipe size (2.4.9.1).
When a vertical stack offsets and becomes horizontal:
Horizontal sizing rules may apply. Upsizing the pipe may be required.
*Important to note: once you upsize your pipe, you cannot reduce the pipe back down once going vertical again.*
Some fixtures (ex, water closets) trigger minimum pipe sizes regardless of FU totals.
You never size the whole system at once.
You size each individually:
1. Forgetting to add loads downstream
2. Using a vertical table for a horizontal drain
3. Ignoring minimum WC pipe sizes
4. Missing FUs added at offsets
5. Not identifying separate stack segments
When you see a complex FU problem:
1. List fixtures first
2. Write FUs beside each fixture
3. Total for each branch
4. Total each stack level
5. Add stacks at every intersection
6. Size each section independently
*Look for when stacks go from vertical to horizontal, may have to upsize pipe*
Real system success is the ability to understand and apply the following all at once:
Oversizing can be nearly as bad as undersizing because flow velocity drops.
Three-storey building:
Each floor:
Per floor = 13.5 FU
Stack total:
13.5 × 3 = 40.5 FU
Now:
Variations of this type of question are present on the C of Q.
A horizontal sanitary branch in a building receives discharge from the following fixtures on the same floor:
The branch is installed at a slope of 1 in 50 (2%).
What is the minimum permitted pipe size for the horizontal branch according to the National Plumbing Code?
A. 1½″
B. 2″
C. 3″
D. 4″
Correct Answer
C — 3″
1. Use Fixture Unit Values
Under the National Plumbing Code
Fixture unit values are found in:
Table 2.4.9.3.A — Fixture Unit Ratings
Typical values:
Calculate total fixture units:
4+1.5+1+1= 7.5 FU
Total = 7.5 fixture units
2. Size Horizontal Branch Using Fixture Units
Use:
Table 2.4.10.6.B — Size of Horizontal Fixture Branch and Stack
For a horizontal branch at a 1 in 50 slope:
So 2″ is too small.
Next size:
3″ → allowed.
3. Why is 3″ required
Because:
Relevant code:
4. Why are other answers wrong
A — 1½″
Too small
Cannot carry the WC discharge
B — 2″
Only allowed up to 6 FU.
D — 4″
Allowed but not minimum
The exam asks for the minimum permitted.
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