Main water line size

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Gman7

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Hi All,
We are in the process of starting to build a 5 bedroom house on a piece of property in North Carolina and wondering what size water line we may need.

Here is all the pertinent information I could think of:

The house will sit on a 3/4 mile dead end road fed by a 16" main line from the intersection
Once you turn on the road, the first 1700' is a 4" main water line, then next 1400' is 3" line, last 800' is 2" line
14 - number of other houses on the road

110' - distance from end of the main water line to my driveway where the water meter would be placed
550' - distance from water meter to where the line would enter the house
125' - additional distance the water line would travel to get to the opposite corner of the house
+15' - change in elevation from the water meter to the house (plus an additional 20' from ground level to the second floor shower head)

55psi and 78psi - water pressure from my two closest neighbors on the opposite side of the street close to where the main water line ends

86 - Fixture units, including hose bibs for a pole barn

The water utility department tells me they will run a 1" main water line 110' from the end of their line to my meter.
The price difference between a 3/4 and 1" water meter is a few thousand dollars.

From what I can figure out, I would need a 2" water line from the meter to the house due to the fixture units, distance and elevation.
I am not sure what effect having a 1" line running 110' to the meter would have on the line from the meter to the house.

Any and all thoughts are appreciated, thank you!
 

wwhitney

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Couple possible issues with your data:

First, 55 psi and 78 psi are surprisingly different. Are those really both static pressure measurements (taken when the particular neighbor making the measurement isn't using any water)? What are the elevations at which those were each taken, relative to your future house elevation?

Second, 86 water supply fixture units seems implausibly large. Each bathroom, for example, would only be 3.6 wsfu. Could you share your accounting? Are there any sprinklers, either landscape or fire?

Cheers, Wayne
 

Gman7

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Couple possible issues with your data:

First, 55 psi and 78 psi are surprisingly different. Are those really both static pressure measurements (taken when the particular neighbor making the measurement isn't using any water)? What are the elevations at which those were each taken, relative to your future house elevation?

Second, 86 water supply fixture units seems implausibly large. Each bathroom, for example, would only be 3.6 wsfu. Could you share your accounting? Are there any sprinklers, either landscape or fire?

Cheers, Wayne
Hi Wayne,
Thanks for the response. The houses on the street vary in age, the one with the higher water pressure was just redone, and I’m thinking maybe they put in a new main water line, whereas the older house may have galvanized pipe. I took both measurements on the same day when no one was home at either house. Both of the houses are at the same elevation as the beginning of my driveway, give or take a few feet. My house site is 14’ higher.

As far as the fixture units, it will be a 5 bedroom, 6 1/2 bath house with a laundry on the first and second floor, bar with sink, scullery (and second sink), sink in garage, 4-5 hose bibs at the house and 4 more at the pole barn. I used an online calculator and that is the number I got when I plugged it all in. I realize we would never be using everything at once.


No sprinklers will be coming off this line, we have a pond that we will use for that.

Thank you
 
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wwhitney

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Thanks for the response. The houses on the street vary in age, the one with the higher water pressure was just redone, and I’m thinking maybe they put in a new main water line, whereas the older house may have galvanized pipe. I took both measurements on the same day when no one was home at either house.
For static pressure, the galvanized pipe issue is a non-factor and doesn't affect the measurement. So are you sure that the measurements were with no flow? No irrigation sprinklers going off? If the meters are outside and you have access to them, you can confirm that there is no flow while you take the measurement.

If those are both static measurements, then either the utility pressure out at the 16" line varies quite a bit, or the water main on your street is undersized, and the change in other houses' usage between the two measurements accounts for the variation, or some combination of the two. Absent further information it would be prudent to use 49 psi or so (55 psi - 6 psi head loss) as your minimum static pressure. [Although if the pressure reading is so low because of the effect of an undersized water main on your street, the situation is worse than being directly connected to a 16" water main that would give you 49 psi static.]

On the WSFUs, assuming normal tank-type WCs, I get 6*3.6 (full baths) + 2.9 (half bath) + 2*1.4 (washing machines) + 4*1.4 (kitchen and other sinks) + 1.4 (1 dishwasher?) = 34.3 WSFUs. I don't think anything is attributable to the hose bibbs themselves, at least not per 2018 IPC Appendix E Table E103.3(2). Which gives a peak flow rate of 25 gpm per Table E103.3(3), vs 39 gpm or so for 86 WSFU. Even 25 gpm seems high; per the computation method, a 7 apartment building with 1 kitchen (1.4) and 1 bath (3.6) per unit, no dishwashers or washing machines, would be 35 WSFUs, and it would presumably come a lot closer to demanding 25 gpm than a single family house would.

Anyway, a 2" private lateral (after the meter) is probably a good idea, assuming the upcharge from 1-1/2" is not much of a concern. If it is, you could do some further calculations. E.g. if your demand is 25 gpm, then 550' of plastic pipe with a 2" ID would give you a dynamic pressure drop of 3 psi; while with a 1.5" ID it would be 13 psi. But for a more reasonable flow of say 12 gpm, those numbers are 1 and 3 psi, respectively.

Of course, that's just the 550' from the meter to your house; you have no control over the pressure loss before your meter. But if you go with a 2" line from meter to house, you can be pretty confident based on the information provided that if the interior piping is properly sized, and you do see dramatic pressure swings, it is attributable to the water supply before the meter.

Cheers, Wayne
 

Jeff H Young

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You can only have 85 fixture units on 1 inch meter and 2 inch pipe . even with a 1 1/2 meter you need 2 inch to the house Id go with only a 1 inch meter If they would let me, but I would not concider 3/4 way undersize
 

wwhitney

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The water utility department tells me they will run a 1" main water line 110' from the end of their line to my meter.
The price difference between a 3/4 and 1" water meter is a few thousand dollars.
Jeff's post reminds me I forgot to comment on that.

The difference for you between a 3/4" meter and a 1" meter is that the 1" meter will drop less pressure at a given flow rate. For example, my water company uses these meters:


For them, at 25 gpm, a 3/4" meter drops about 5 psi, while a 1" meter drops 1 psi. While at 35 gpm, the numbers are about 10 psi and 2.5 psi.

If the water utility will run a large water line for that 110' if you go with a 1" meter, that may be worth it. Otherwise the 1" meter is only worth it if you believe the 86 WSFU and according 39 gpm demand. Or perhaps if you expect the water line in the street is going to need upgrading and you want to be able to lobby for that while demonstrating that there's no chance your variations in water pressure are attributable to anything specific to your house.

Cheers, Wayne
 

Jeff H Young

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I think 3/4 meter will work fine but dosent meet standards so go small and swap meter later could be an option . As a Pro I wouldnt concider it but on my own home I very well might go cheap and take a chance you might even get the utility to use an oversized anglestop or pay for one yourself . differance in price could be substantial bettween 3/4 and 1 inch meter both upon installation and in the monthly bill.
1 inch meter generally calls for 1 inch pipe coming in . if you could get them to run 1 1/2 and put a 3/4 meter on the cheap Id be all over that of cource I dont know if you have codes or engineering standards or what determines that stuff there , just that my code it aint going to fly. Ive had inspectors make me rework piping over 1 fixture unit but generally I oversize a bit .
 

Gman7

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Jeff's post reminds me I forgot to comment on that.

The difference for you between a 3/4" meter and a 1" meter is that the 1" meter will drop less pressure at a given flow rate. For example, my water company uses these meters:


For them, at 25 gpm, a 3/4" meter drops about 5 psi, while a 1" meter drops 1 psi. While at 35 gpm, the numbers are about 10 psi and 2.5 psi.

If the water utility will run a large water line for that 110' if you go with a 1" meter, that may be worth it. Otherwise the 1" meter is only worth it if you believe the 86 WSFU and according 39 gpm demand. Or perhaps if you expect the water line in the street is going to need upgrading and you want to be able to lobby for that while demonstrating that there's no chance your variations in water pressure are attributable to anything specific to your house.

Cheers, Wayne

Wayne, thanks for the response.
I pulled up the 2018 IPC chart and agree with your 34.3 WFSUs. I also agree we would never come close to using everything at any given time. I’m curious why the other online calculators are so far off though.

Having said that, using 45-49 psi, if the county insists on running 1” line 110 feet to the water meter, would you go with a 3/4” vs a 1” meter? And at that point would a 1 1/2” or 2” water line to the house make more sense?

thank you !
 

wwhitney

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Having said that, using 45-49 psi, if the county insists on running 1” line 110 feet to the water meter, would you go with a 3/4” vs a 1” meter? And at that point would a 1 1/2” or 2” water line to the house make more sense?
Let's take as a baseline a 110' 1" public lateral, a 3/4" water meter, and a 550' 1-1/2" private lateral. And let's take 25 gpm as a conservative (high) peak demand.

Then for the private lateral, we saw a 1-1/2" ID private lateral gives 13 psi pressure drop; 2" ID private lateral gives 3 psi pressure drop. [Note the ID may exceed the nominal size, this comparison would best be done with the actual IDs of the actual pipes you are considering, be it PVC Schedule 40 or PE SDR pipe or whatever.] That's a 10 psi difference at 25 gpm.

For the water meter, we saw that at 25 gpm a 3/4" meter might drop 5 psi, while a 1" meter might drop 1 psi. That's a 4 psi difference.

And for the public lateral, 1", 110' and 25 gpm drops 18 gpm; 1-1/4" drops 6 psi; and 1-1/2" drops 2.5 psi.

So the performance value of improvements, in decreasing order, without regard to cost:

Upsize 1" public lateral (110') to 1-1/4" = 12 psi
Upsize 1-1/2" private lateral (550') to 2" = 10 psi
Upsize 3/4" meter to 1" meter = 4 psi
Upsize 1-1/4" public lateral (110') to 1-1/2" = 3 psi

For piping pressure drops at 20 gpm, multiply all the above numbers by 2/3. Or for any other flow rate Q, multiply them by (Q/25)^1.852. Pressure drop calculations per:


Cheers, Wayne
 
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