I'm going to replace my rusted-out iron pipes with copper. The plumbing is exposed in the basement so I'm not forced to use the existing design. I'm trying to figure out when I can run fixtures in series off of a main pipe, and when I should use parallel pipes. (sorry; I'm an electrical guy!)

As a specific example, let's look at a bathroom. The thick lines are 3/4" copper, the thin lines are 1/2":


I don't want the shower temperature/flow to change if the toilet is flushed. So, it seems that I can:

  1. Not worry about it; the layout is fine.
  2. Increase "A" to 3/4".
  3. Run a parallel pipe, as shown:


    (I know that I could solve this with a thermostatic mixer valve, but I'm looking for a generalized answer.)

I know that there must be guidelines for this sort of thing. Is this described in code somewhere? How do you decide when fixtures in series are OK, and when you should run pipes in parallel? This answer was illuminating but I'm looking for more details.

  • 1
    You could always run red A to the toilet as well to keep things balanced. Oooh. Warm seat.
    – isherwood
    Jan 26, 2018 at 21:48
  • @isherwood Or, perhaps, only the red :-)
    – bitsmack
    Jan 26, 2018 at 21:50

3 Answers 3


Usually if you have multiple users from the same line, that line should be bigger in order to avoid unbalancing

EG: you have a toilet a sink and a shower, you should run 3/4 pipe across all and connect each user with a 1/2 manifold

Another solution, if you can't replace existing with bigger pipes, is to use 'automatic pressure balancer' on the lines serving all, so if a big pressure drop is found on cold water, the same pressure drop will be applied to cold water so, at each end, the pressure isn't different.


I would go with your second diagram.

Keep the 3/4" as your main line and branch off of that to each fixture with 1/2".

  • 1
    The question asks how to determine sizing. I'm not sure we want to make it an opinion poll. :)
    – isherwood
    Jan 26, 2018 at 22:11

Without looking at your diagram, this is what I have done in my last 2 new houses and it works perfectly.Run all main lines in 3/4" and use no 1/2" tees anywhere.All tees are either 3/4"X3/4"X1/2" or 3/4"X1/2"x1/2". A 1/2" line supplies only 1 fixture or outlet not 2 or more. For example if the toilet has a 1/2" supply it can not also serve the sink or shower regardless of how close they are. Now, looking at your diagram, the 1st diagram (top) the blue "A" line would have a 3/4"X 1/2"x 3/4"tee and the 2nd tee would be a 3/4"X 1/2"X 1/2" tee and the red line would have a 3/4"X1/2"X1/2" tee.

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