How to Size Sanitary Drain Pipes in Canada: NPC 2020 Worked Examples
One fixture schedule, three different pipe checks. Follow the load from the washroom branch to the stack and building drain, with the governing limits shown at each step.
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A sanitary pipe is not sized from the number of washrooms alone. You need to know which fixtures discharge into each segment, their drainage fixture-unit load, the pipe's function and the installation constraints. A horizontal branch, a vertical stack and a building drain do not share one interchangeable capacity table.
This guide uses NPC 2020 as its reference edition and develops an original two-storey example. It is intended for mechanical designers checking a drainage layout. Confirm the edition and amendments adopted for the project location; the national model code is not automatically the local requirement everywhere.
For a connected fixture layout, the sanitary drainage design adds gravity branches, floor stacks, fixture-unit accumulation, slope and pipe-size review. Venting and other excluded checks still need separate design.
Water-closet count can govern before the FU limit
NPC 2020 Sentence 2.4.9.2.(2) requires NPS 4 for branch and building-drain sections downstream of the third water-closet fixture-drain connection. Sentence (3) sets NPS 4 as the minimum for a stack serving more than six water closets. These checks are independent of hydraulic capacity. Count one WC in a bathroom group, and confirm the actual connection arrangement.
| Illustrative segment | FU capacity alone | WC-count check |
|---|---|---|
| Branch downstream of 3 tank WCs: 12 FU | NPS 3 fits 27 FU | NPS 4 minimum |
| Stack serving 7 tank WCs: 28 FU total, 16 FU largest storey | NPS 3 fits 102 / 18 FU | NPS 4 minimum |
1. Draw the flow path before counting fixtures
Mark the fixture outlets, room branches, stack connections and building drain on the plan. Give each segment a label. When two branches join, the downstream segment receives both upstream loads. A branch on another floor does not belong in the current floor branch total, but it can belong in the stack total below their junction.
Record actual existing pipe sizes during renovation work. Even when a new fixture tally is small, the size of an incoming drain can constrain the downstream pipe. The calculator's listed fixture minima do not describe every existing pipe in the building.
Keep supply and drainage calculations separate
A fixture can have one water-supply demand and a different sanitary hydraulic load. The water-supply WSFU schedule cannot be pasted into a drainage table.
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Start 30-Day Free Trial2. Build the first-floor fixture schedule
Assume two flush-tank water closets, two domestic lavatories with NPS 1¼ traps and one domestic sink. All five fixtures discharge to the same floor branch. For this example, the domestic sink is counted on its own; no bathroom-group allowance or separately discharging dishwasher is added.
1.Match the fixture entry
A larger trap, another discharge arrangement or another fixture category may change the load or minimum. Check the precise entry instead of selecting the nearest name.
2.Avoid group double counting
If you use a bathroom-group entry, do not also count its included toilet, lavatory and bathing fixture. Additional fixtures need their own treatment.
3.Document exceptions
An emergency floor drain or a dishwasher connected to a domestic sink can have different load treatment from an ordinary separate discharge. Record why the selected entry applies.
| Fixture | Count | FU each | Load |
|---|---|---|---|
| Flush-tank water closet | 2 | 4 | 8 FU |
| Domestic lavatory, NPS 1¼ trap | 2 | 1 | 2 FU |
| Domestic sink | 1 | 1.5 | 1.5 FU |
| Floor-branch total | 5 | — | 11.5 FU |
3. Select and check the floor branch
For the 11.5-FU example, Table 2.4.10.6.-B provides 6 FU at NPS 2 and 27 FU at NPS 3. The smaller entry cannot carry the calculated load. The water closets also impose a minimum drain size, so NPS 3 is the table-based branch candidate.
That selection does not complete the branch design. Establish the actual slope, vent arrangement, trap-arm lengths, fittings and access for maintenance. In particular, do not treat the building-drain slope input in a calculator as verification of the branch's installation slope.
11.5 FU ≤ 27 FU branch capacity → NPS 3 candidate
This compares the stated branch load with the relevant table. It is not an equivalent GPM calculation or a statement that any NPS 3 layout is acceptable.
4. Check total stack load and the busiest storey
Now assume the upper storey contributes another 8.5 FU to the same stack. The portion below both entries carries 20 FU. Assume the additional 8.5-FU schedule has no water closets, so this stack still serves two WCs. The greatest load from one storey remains 11.5 FU. Compare both values with Table 2.4.10.6.-A: an NPS 3 stack has listed limits of 102 FU total and 18 FU from any one storey, so it satisfies these two checks.
Consider a different arrangement with the same 20-FU building total, but 19 FU entering from one floor. The NPS 3 total limit still passes while its one-storey limit fails. That is why the calculator requests the busiest storey rather than just the number of floors.
| Case | Total stack FU | Largest storey FU | NPS 3 table check |
|---|---|---|---|
| Original example | 20 | 11.5 | Both limits pass |
| Concentrated floor load | 20 | 19 | One-storey limit fails |
Offsets need separate attention
A stack with a qualifying nominally horizontal offset needs the additional applicable horizontal check. A vertical-stack result should not be applied blindly through an offset.
5. Check building-drain slope and connection minimum
At a slope of 1:50, the NPS 3 building-drain capacity in Table 2.4.10.6.-C is 27 FU, which accommodates the 20-FU example. However, for the applicable section connected to a public sewer downstream of the main cleanout, the NPS 4 minimum governs. A larger size can therefore be required even when the load alone fits a smaller table entry.
Do not disable a calculator's public-sewer minimum simply to obtain a smaller result. First identify the exact segment being sized and whether the condition applies. Site servicing, utility requirements and existing connection elevations also need coordination.
10 m ÷ 50 = 0.20 m = 200 mm of fall
This is the geometric fall for a 10 m horizontal run at 1:50. Confirm that the available elevations leave room for fittings, cover and the required connection.
6. Issue a schedule someone else can review
A useful drainage schedule lists segment ID, upstream fixtures, drainage FU, minimum incoming size, pipe function, selected table and resulting NPS. Add slope for horizontal runs and the controlling storey load for stacks. This lets another designer reproduce the calculation without reverse-engineering the drawing.
Recalculate downstream sections when a fixture changes. Replacing a fixture can affect the branch, the lower stack and the building drain; changing only the local symbol misses those accumulated loads. Keep the calculation and drawing revision together.
- Check the full route for reductions in drainage pipe size.
- Coordinate vents, cleanouts, trap protection and stack offsets.
- Treat pumped and continuous discharges with their applicable methods.
- Check local adoption and amendments before final issue.
Try the example, then check your own section
The free sanitary calculator includes a Load worked example button. Change one floor's load and compare the branch, stack and building-drain outputs before transferring the reviewed result to your drawing.
Frequently asked questions
How many toilets can a 3-inch pipe carry in Canada?
Under NPC 2020, branch and building-drain sections downstream of the third WC fixture-drain connection require at least NPS 4. A stack serving more than six WCs also requires NPS 4. A smaller WC count does not guarantee NPS 3 is adequate: check FU capacity, storey loading, slope and other minimums.
Is DFU the same as WSFU?
No. Drainage fixture units describe sanitary hydraulic loading. Water-supply fixture units are used in domestic water demand methods. Do not substitute one for the other.
Can I use the calculator for a complete permit design?
It supports table-based checks, but does not design venting, trap arms, cleanouts, offsets or the entire drainage network. Complete the applicable project and code checks.
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Start 30-Day Free TrialPrimary references
Use the edition and method accepted for your project. These authoritative resources provide further context; this article is educational and is not a code-compliance determination.