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Drain pipe size for DFU
Smallest IPC-permitted drain pipe size for a given DFU load. Distinguishes horizontal branch vs vertical stack since each has its own table.
Recommended drain pipe size · IPC 710.1(2)
4"(horizontal)
Carries up to 160 DFU at the rated slope.
Horizontal branch caps assume 1/4″/ft slope. At 1/8″/ft (large pipes only) caps are slightly lower — check IPC 710 directly. Building drain + sewer table is separate (710.1(1)); for buried drain past the foundation, consult the engineer.
Reference: IPC Table 710.1(2)
Horizontal branch DFU caps (at 1/4" per foot slope): 1-1/2" = 3 DFU; 2" = 6; 2-1/2" = 12; 3" = 20; 4" = 160; 5" = 360; 6" = 620; 8" = 1400. Vertical stack caps (one story above lowest interval): same shape but slightly higher at upper sizes.
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Forward runs DFU pipe sizing, water-supply GPM sizing, water-heater sizing, and the rest of the NEC + IBC + IMC + IPC catalog over text.