I am putting in new irrigation and supply lines for my remodeled backyard and I've done my homework on water pipe sizing, but I'm left wondering if I'm installing too big of a pipe onto the main. The main is 3/4" at about 110psi from the street and 65psi after the pressure regulator. I guess I could connect either before or after the bell.

It's 160 feet from the main to my pool. Along this 160' 3 different hose bibs are stubbed off. At the pool I feed (1) a 3/4" PVC pool filler, (2) a 3/4" supply line to an outdoor shower, (3) 6x 3/4" PVC drip irrigation lines that travel up to 100' away but need pressure to weep out, and (4) 2x more hose bibs.

As I write this, it sounds like overkill on paper, but I actually want to leave some room for expansion including adding an outdoor kitchen or more irrigation lines someday.


I have studied the tables and my math came up with around 44 Water Supply Fixture Units (WSFUs), but there was some guessing with the drip irrigation. That works out to an undersized 1-1/4" PVC line (purple) for the main feed line and 3/4" PVC lines (blue) for the supply lines, all potable. My plan was to use 1-1/2" potable PVC since I need more than the chart called out and I want to be able to expand later. The problem is that the tables don't have 3/4" main and 1-1/2" supply as an option.

Am I looking at this the right way? Can you have too big of a supply line coming off of the main? Any help or insights would be greatly appreciated.

1 Answer 1


You can have any size line coming off - the likely issue being that your 3/4 main will be the limiting factor, or choke point. But by using larger pipe you ensure that it's the chokepoint, as opposed to having greater losses all over the system caused by your distribution pipes also being small. And you are ready if the main is ever increased to meet your use, as might be required.

Looked at another way, with a 3/4" main supply, 1" is probably enough upsizing to ensure that the limiting factor is the supply. If you know the approximate length of your 3/4" main, and the pipe material, you can calculate the flow that would drop it from 110 PSI (supplied) to 65 PSI (desired) and once you flow that much water, you will start to lose pressure (but you might be able to accept somewhat less than 65 PSI delivered, too, which would buy you a bit more.)

i.e. I calculated (with a web-based calculator that has the following choices for pipe type) on an assumed 100 feet of plastic (smooth) pipe and got 20.23 GPM for 103.95 feet of head (the 45 PSI between supply and regulated, times 2.31 feet of water head per PSI.) New Steel pipe, 16.16GPM. Old steel pipe, 14 GPM. Corroded steel, 10.7 GPM. Those lower numbers have to do with more friction being caused by the internal surface of the pipe not being smooth.

To the extent that your uses are not simultaneous, you may need less pipe than you calculate - and for irrigation with a residential-sized supply, you almost always are going to need a means (distributor valve, can't recall the usual term right now) of running only a part of it at any given time, not the whole thing at once.

  • 1
    If this is part of an irrigation system (not just home owner running lines) the manufacturer or company managing it will give guidelines on pressure and waterflow. You would be surprised how little water many irrigation systems use - they are just not a open faucet. Also the pool filler and the bibs are not in need of water, you just accept what comes out. If this is a house that is running a car wash service that needs 3 bibs to rinse cars... that's a different question. But 1" is more than enough for a typical residential home.
    – DMoore
    Oct 7, 2021 at 19:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.