DWV Sizing

[Spaceman Spiff]

I'm sure my plumber is correct, which is why I'm asking this; to educate me, an electrician. (IPC 2003)

Stack A:
2nd floor - 7 Fixture Units - 3" (w/c)
1st floor - 13 FU - 3" (w/c)
Basement - 5 FU - 3" (w/c)
TOTAL 25FU 3"

Stack B:
1st floor - 4FU - 2"
Basement - 6FU - 2"
TOTAL 10FU 2"
Feeds into "Building drain" (Correct term?) 3" from stack A

Stack C:
2nd floor - 10FU - 3" (w/c)
TOTAL 10FU 3"

building drain A&B and drain C tie together at a 3x3x4 and the main goes to septic in a 4".

I thought the first time that stuff from "A" was limited to 20FU according to table 710.1(2) but the pipe under the slab is actually a "Building drain" and is capable of 36FU in 3" 1/8" slope according to 710.1(1). Then when we add in B the total is 35FU. Add in C and the total is 45FU where the slope needs to increase to 1/2" or pipe needs to increase to 4" for 1/8" slope.

Branch=pipe between fixture and stack or fixture and building drain.
Stack=vertical part of DWV system.
Building drain=between stack and point 30" outside of foundation wall.
Sewer main=30" outside of foundation to septic system.

Am I following correctly what my plumber is doing? Thanks!

[hj]

In general terms the 3" is adequate up until the fourth toilet is connected to the pipe. At that point most codes require a 4" pipe, they do not allow the 3" to be "upgraded" regardless of its slope. However if it is the IPC, it will allow a lot of things that have been traditionally "illegal".

[RioHyde]

While I happen to do plumbing in a state which follows IPC, I wouldnt size the building drain as 3" no matter how much slope is there or what the book says is minimum. True that according to table 710.1(1) you can put 50 dfus on a 3" building drain with 1/2" slope, but is it really good plumbing practice to do so? In my own opinion (and we know what those are worth) I really dont think it is. Go with a 4" building drain at 1/4" per foot slope. There's no way you'll ever put enough in the house to hit the limit of 216 dfus, but its nice to know you can add any amount of plumbing to the building drain in the future and still be within acceptable limits.

Good luck!