Sizing of drain pipe from house to septic tank inlet

[Midwest DIY'r]

Trying to educate myself on pipe sizing according to IPC and I'm either missing or not interpreting the code correctly, likely a little of both is my guess. We are building a 3 bedroom home and have detached garage with a 3/4 bath. I have a 3" drain coming out of the shop with a cleanout, the house is not built but will be later this year. The septic contractor is installing the system designed by a local soil scientist, the drawing doesn't specify pipe sizing but does specify an effluent loading rate of 0.3 GPD per sq ft.

How is the pipe running between the house and septic inlet sized appropriately? I've read several posts on using 3" vs 4" but I'm unclear which is correct and variables to consider. We appreciate any assistance that can be shared, thanks.

 

[GReynolds929]

It depends on how many DFU's and toilets there are.

 

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[John Gayewski]

Usually outside of the house or building people use 4 inch. But pipe can now carry 5 toilets on 3" in the upc as toilets don't use as much water.

Your septic tank will likley have a 4" inlet. I would go with 4 inch from the house to the septic and size the piping inside of the house according to the load.

 

[SuperCoolHVAC]

Last system we did we used 4” to the house then reduced. Could use 3” but 4” seems to be more future proof/ clog resistant. UPC also lets you go 1/8 slope instead of 1/4. Only concern would be if you can’t get pipe deep enough and you have dump trucks driving over but then you would have other issues to deal with regardless of pipe size.