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My house has a 1/2" underground copper pipe providing the water supply. I am not quite sure how long the run is back to the meter/utility supply, but it's likely ~75-100ft. I actually got the chance to measure the static pressure and the pressure at 2.5GPM flow (the showerhead is the only source of which I know the flow rate). The static pressure is 65psi, which drops to 56psi at 2.5GPM flow.

I will be redoing the plumbing soon, and I'm curious whether it's worth using 3/4" for the interior manifold plumbing. Similarly, downstream of the manifold, if the pressure drop is minimal, I plan on using 1/2" or 3/8".

Does it make sense to run any 3/4" pipes at all? After all, it seems a majority drop would occur in the long run of 1/2" pipe. My plumbing segments will all be less than 40ft total. Similarly, is there a guideline for acceptable pressure drop along a length of pipe?

And lastly, if I may, does the water pressure need to be higher than 0psig at a faucet or fixture?

EDIT: I know now that the underground run is just about 100ft. It actually lines up pretty well with tabulated values for head loss in 1/2" pipe at 60°F.

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    Last question: well, yes, if you want more than a dribble for peak flow. As to the rest: you can never get a higher flow rate than the source pipe (and source pressure) allow, so pretty much the only reason to go to larger pipes inside yr house is if that simplifies coupling to various end items. – Carl Witthoft Feb 3 '17 at 15:19
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    At some time in the future the 1/2" underground supply may be replaced with a larger one. So it may make sense to use 3/4" inside the house in some places. – Jim Stewart Feb 3 '17 at 16:11
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    I own a rental property with a 1/2" copper line coming in through the floor. I assumed that it ran back to the city shutoff, but I recently learned that there's actually 1" copper underground to just outside the foundation. As Jim says, plan for a future upgrade and plumb according to modern standards. – isherwood Feb 3 '17 at 18:38
  • @CarlWitthoft, I thought that the larger diameter plumbing inside would help reduce pressure loss inside, allowing slightly greater flow than if I just used another 50ft of 1/2" PEX (meaning 1.5x pressure drop that I measure at the hose bib). – Hari Ganti Feb 3 '17 at 21:42
  • @JimStewart, that's also part of my rationale, but I wasn't sure. – Hari Ganti Feb 3 '17 at 21:43
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I would run the line to the manifold in 3/4" and then the branch circuits in 1/2" like you said. The cost increase is minimal and as long as you have the room for the extra 1/4" of pipe width, you won't second guess yourself. If you put in the 1/2" and they upsize you main in a year, you're going to wish you had installed the larger diameter.

Yes, you want more than 0 psi at the point of use. Most faucet manufacturers will say you need ~30 psi minimum for proper operation of the fixture.

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