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Sanitary Piping - Drainage Fixtures Unit Loads

Maximum Drainage Fixture Unit - DFU - loads in sanitary piping systems.

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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 US gpm, ≈ 0.47 l/s).

Maximum Drainage Fixture Units (DFU) in Stacks and Horizontal Fixture Branches

Maximum Drainage Fixture Units (DFU)
Pipe SizeHorizontal fixture branchStacks3) less than three stories in heightStacks more than three stories high
Total for stackTotal 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 US plumbing codes.

Maximum  Drainage Fixture Units (DFU) in Building Drains and Building Drain Branches from Stacks

Maximum Drainage Fixture Units (DFU)
Pipe SizeSlope (in/ft (cm/m))
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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