Cast Iron to PVC Drain

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Plumbs Away

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I am removing all of the old 4-inch cast iron sewer pipe in my house and replacing with PVC. Is 3-inch PVC adequate, or should I keep it at 4? I have a kitchen sink, dishwasher, clothes washer and two bathrooms. Thanks in advance!
 

Terry

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If your 4" cast is 4-1/8" OD, then that side of the coupling would be copper sized. For that I get a copper x no-hub when I transistion to ABS or PVC.
Plastic is more like 4-3/8"
 

Plumbs Away

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Thanks for the information. The only cast iron that will be left when I finish this project is the vent stacks and the 4" 90° elbow where the exit point for the drainage system is located on the exterior wall. My plan is to use a donut at the 4" cast iron hub of the elbow to join my 3" PVC. This will be a vertical union. The previous owner already had the underground exterior drain lines replaced with PVC. There is a clean-out just outside the house where the main drain exits.
 
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