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Tuesday, October 5th 2010, 5:29pm

Question About GPM

Okay, so I tapped into my 1 inch copper line just 3 feet from my 5/8 inch meter, installed 1 inch brass curb stop and then about 50 ft of 1-1/4 shc. 40 pvc to the 1 inch PVB. Did the 5 gallon test and couldn't believe I was getting 25 gpm especially since the hose bib last year gave me about 12 gpm. Yea had to repeat it two more times before I was convinced. Absolutely thrilled since that will reduce the number of required zones from my original design at 12 gpm. My question is how much of the 25 gpm could I use? From the PVB the mainline will travel about another 150 ft. to the furthest valve and the furthest heads will be spray heads maybe 175 ft from the PVB. I was hoping to use 18 of the 25 which will be just 72% of the gpm. Also 1-1/4 for all piping.

Central Irrigation

Supreme Member

Posts: 306

Location: Central Minnesota


Tuesday, October 5th 2010, 6:16pm

Need to know what your static water pressure is. 25 GPM is what you can technically flow through 1 1/4" pipe without causing any damage. However, we need to know what your water pressure is in order to determine how much of that 25 GPM you can use in order to have adequate pressure at the heads.


Tuesday, October 5th 2010, 7:05pm

Need to know what your static water pressure is. 25 GPM is what you can technically flow through 1 1/4" pipe without causing any damage. However, we need to know what your water pressure is in order to determine how much of that 25 GPM you can use in order to have adequate pressure at the heads.
Ok, Thanks. That's what I thought. How accurate are those $10 gauges they sell at HD?


Supreme Member

Posts: 395

Location: Houston, Texas


Tuesday, October 5th 2010, 7:48pm

I would still use 12 GPM as a bechmark and maybe be at 15 GPM Max. You want to be conservative with it. It is still only a 5/8" meter. You don't want to break or stress the meter. I may get some feedback or static on this.

Better safe if conditions change or more houses are built on the water supply.

My 2 Cents

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Supreme Member

Posts: 395

Location: Houston, Texas


Tuesday, October 5th 2010, 7:54pm

I checked my design software on a 5/8 inch meter with all the pressure possible and different variations of pipe size, I could not get it to recommend more than 13 GPM on a 5/8 Inch meter. This is the recommendation based on Velocity, Flow, and Pressure.

Good Luck

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Active Member

Posts: 33

Location: east coast


Tuesday, October 5th 2010, 8:38pm

GPM and pipe size

The meter size is certainly something you have to consider in this design. hi.todd is correct that you may be taxing that meter too much if you go beyond approx 13gpm. Usually with a 1" service you have at least a 3/4" meter installed. That would help in giving you more safe flow. But just as important, as Central said, you need to know your static pressure so you know how much pressure you can safely loose due to friction losses and still have enough to operate the sprinklers. As flow increases, so does your friction loss. How long is the 1" copper service line from the city main to your point of connection? I'll assume that the 1" copper service is Type "K'. If you look at a friction loss chart for type "k" you will see that as you get up to 16 GPM and beyond that the water velocity starts to creep up and of more significance is the amount of pressure loss per 100' at the higher flows.
I would look into exchanging the exist meter to a 3/4" and staying in the 14-16 gpm range.


Advanced Member

Posts: 67

Location: South Jersey


Tuesday, October 5th 2010, 9:24pm

I also have a 1" copper supply line with a 5/8" meter. My water company charges more for a 3/4" meter even though you use the same amount of water. Anyway, my supply line is about 45ft long from the street and I tapped into the meter about 2ft after it. I'm using 1" Sched 40 for my application. I have about 30ft of 1" Sched 40 PVC before it exits the house and hits a 1" Wilkins PVB. If I open the shut off valve I get a huge amount of flow compared to the measley 7.5GPM from my hose bib. My static PSI is 51. The problem is that the meter is rated for a maximum flow rate. For instance my Badger brass meter is only rated for 20GPM max if it is a continuous flow of water. Even though the manufacturer says 20GPM that still feels awful high to me.

With that high flow rate you may get a huge amount of flow when it is free flowing but you may only be getting a few psi of pressure once it exits the 1.25" PVC. Designing for 25GPM means you won't have any pressure to lift the spray heads or throw the water more than a few feet if you max out your flow. Hopefully, I worded that elegantly enough to be understood. You have a lot of pressure loss calculations you'll need to do. I designed my entire system based off the information at If you haven't already I suggest reading the entire site multiple times until you understand what he is saying and then start designing your plumbing and zones. Since you already have 1.25" PVC in place you won't need to redo that part of it. It should be more than enough.

And and even more important is that if you start flowing a high GPM once that zone valve snaps closed you could be looking at a huge pressure surge in the irrigation system as well as throughout the house. This is commonly called water hammer. Ice maker and toilet bowl lines are usually the first thing to blow out. At 25GPM the water will be moving at about 6.5 ft/sec through the 1.25" PVC pipe. And even faster through the 1" copper house supply and meter. The rule of thumb is not to exceed 5 ft/sec of water velocity.

For what it's worth, my largest zone is only using around 9GPM. But I am being very cautious with my design.

This post has been edited 1 times, last edit by "ReddHead" (Oct 5th 2010, 9:29pm)


Supreme Member


Wednesday, October 6th 2010, 8:29am

...I designed my entire system based off the information at If you haven't already I suggest reading the entire site multiple times...

I'm going to give a huge big "DITTO" to reading (and re-reading) this web site. It has a wealth of information... but it takes a couple of reads before you can start to understand it all (and even then, some of it's still confusing).


Supreme Member

Posts: 3,884

Location: Metro NYC


Wednesday, October 6th 2010, 11:14am

I suspect the 5/8-inch meter the OP has is one of the newer 3/4 x 5/8 models that have a higher flow capacity in the smaller package. 15 gpm could be had from one of those with no worries.


Wednesday, October 6th 2010, 4:53pm

OP here

OP here. First of all thank you for all the input. It is greatly appreciated. I want to respond to some of the issues you have raised.
1) I made my connection on the 1" copper service line about 3 feet from the 5/8 meter. So not much copper pipe from the meter to my connection. It's 50 ft of 1-1/4 sch. 40 pvc from there to 1" Febco PVB.
2) I have a Neptune 5/8" meter (model T10) which is rated at 150psi and a normal operating rangeof 1/2 to 20 gpm. So looks like this bad boy could handle 18 gpm. Besides if it wears out, isn't that on the water company since I was operating within the normal range? Also looked into larger meter. They are willing to do it but a 3/4 inch is about 5-10 dollars more per month. Not gonna bankrupt me, but why upgrade if the 5/8 can handle 18 gpm?
3) If the technicalities work out I am not looking to use the full 25 gpm but 18. However, will do what is recommended.
4) Once again, if "worse head analysis" etc... allows it, looks like 18 gpm through 1-1/4 sch. 40 moves at 3.85 ft/sec. so well under the 5 and psi loss of just 1.88/100 (From the Hunter catalog).
5) Thank you for recommending Jess Stryker's website. Yes, very familiar with that as I found it a few years ago and have read it all the way through at least once. I will refer back to it and follow the steps for "worst head analysis" once I know what my static pressure is.
6) Does anyone have an opinion on the $10 orbit water pressure gauge at that orange store? If you say thumbs down, then what do you recommend that won't cost an arm and a leg? Have a plumbing supply about 3 miles away that supplies most if not all of the irrigation professionals in the area so I guess I could go there and ask them. Once again, THANK YOU!

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