Sanitary Piping - Drainage Fixtures Unit Loads
The relationship between volume flow and fixture units is not constant, but varies with the number of fixture units. A Fixture Unit is not a flow rate unit but a design factor based on the rate of discharge, time of operation and frequency of use of a fixture. A fixture unit is equal to one cubic foot of water drained in an 1 1/4 pipe over one minute (≈ 7.48 U.S. gpm, ≈ 0.47 l/s).
Maximum Drainage Fixture Units (DFU) in Stacks and Horizontal Fixture Branches
| Pipe Size | Maximum Drainage Fixture Units (DFU) | ||||
|---|---|---|---|---|---|
| Horizontal fixture branch | Stacks3) less than three stories in height | Stacks more than three stories high | |||
| NPS (inches) | DN (mm) | ||||
| Total for stack | Total for one story | ||||
| 1 1/20) | 40 | 3 | 4 | 8 | 2 |
| 20) | 50 | 6 | 10 | 24 | 6 |
| 2 1/20) | 65 | 12 | 20 | 42 | 9 |
| 3 | 80 | 201) | 481) | 722) | 201) |
| 4 | 100 | 160 | 240 | 500 | 90 |
| 5 | 125 | 360 | 540 | 1100 | 200 |
| 6 | 150 | 620 | 960 | 1900 | 350 |
0) No water closet permitted
1) Maximum two water closets
2) Maximum six water closets
3) Stacks - vertical runs of plumbing waste or drainage lines
The values are based on the major U.S. plumbing codes.
Maximum Drainage Fixture Units (DFU) in Building Drains and Building Drain Branches from Stacks
| Pipe Size | Maximum Drainage Fixture Units (DFU) | ||
|---|---|---|---|
| Slope (in/ft (cm/m)) | |||
| NPS (inches) | DN (mm) | ||
| 1/4 (2.1) | 1/2 (4.2) | ||
| 20) | 50 | 21 | 26 |
| 2 1/20) | 65 | 24 | 31 |
| 3 | 80 | 421) | 501) |
| 4 | 100 | 216 | 250 |
| 5 | 125 | 480 | 575 |
| 6 | 150 | 840 | 1000 |
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