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P3005.4 Drain Pipe Sizing

Residential Code 2021 of Colorado > 30 Sanitary Drainage > P3005 Drainage System > P3005.4 Drain Pipe Sizing
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Drain pipes shall be sized according to drainage fixture unit (d.f.u.) loads. The size of the drainage piping shall not be reduced in size in the direction of flow. The following general procedure is permitted to be used:
  1. Draw an isometric layout or riser diagram denoting fixtures on the layout.
  2. Assign d.f.u. values to each fixture group plus individual fixtures using Table P3004.1.
  3. Starting with the top floor or most remote fixtures, work downstream toward the building drain accumulating d.f.u. values for fixture groups plus individual fixtures for each branch. Where multiple bath groups are being added, use the reduced d.f.u. values in Table P3004.1, which take into account probability factors of simultaneous use.
  4. Size branches and stacks by equating the assigned d.f.u. values to pipe sizes shown in Table P3005.4.1.
  5. Determine the pipe diameter and slope of the building drain and building sewer based on the accumulated d.f.u. values, using Table P3005.4.2.
P3005.4.1 Branch and Stack Sizing
Branches and stacks shall be sized in accordance with Table P3005.4.1. Below grade drain pipes shall be not less than 11/2 inches (38 mm) in diameter. Drain stacks shall be not smaller than the largest horizontal branch connected.

Exceptions:
  1. A 4-inch by 3-inch (102 mm by 76 mm) closet bend or flange.
  2. A 4-inch (102 mm) closet bend connected to a 3-inch (76 mm) stack tee shall not be prohibited.
TABLE P3005.4.1
MAXIMUM FIXTURE UNITS ALLOWED TO BE CONNECTED TO BRANCHES AND STACKS
NOMINAL PIPE SIZE (inches) ANY HORIZONTAL FIXTURE BRANCH ANY ONE VERTICAL STACK OR DRAIN
11/4a, b — —
11/2b 3 4
2b 6 10
21/2b 12 20
3 20 48
4 160 240
For SI: 1 inch = 25.4 mm.
  1. 11/4-inch pipe size limited to a single-fixture drain. See Table P3201.7.
  2. Water closets prohibited.
P3005.4.2 Building Drain and Sewer Size and Slope
Pipe sizes and slope shall be determined from Table P3005.4.2 on the basis of drainage load in fixture units (d.f.u.) computed from Table P3004.1.
TABLE P3005.4.2
MAXIMUM NUMBER OF FIXTURE UNITS ALLOWED
TO BE CONNECTED TO THE BUILDING DRAIN,
BUILDING DRAIN BRANCHES OR THE BUILDING SEWER
DIAMETER OF PIPE (inches) SLOPE PER FOOT
1/8 inch 1/4 inch 1/2 inch
11/2a, b — Note a Note a
2b — 21 27
21/2b — 24 31
3 36 42 50
4 180 216 250
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.
  1. 11/2-inch pipe size limited to a building drain branch serving not more than two waste fixtures, or not more than one waste fixture if serving a pumped discharge fixture or food waste disposer discharge.
  2. No water closets.
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