SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Since you are trenching the system, the mainline can be any length. It's only pipe, and you can run the numbers from a pressure-loss table, to see what works best.

Please read through the entire tutor of Irrigation Tutorial First please!!!!

You are going to want to know your water meter size, and how many gallons per minute you are going to flow through each valve. The lower gallons per minute will give you less friction loss than the higher gallon per minute flow. I think you should either up size your pipe according to the charts of friction loss that were referenced earlier and or reduce the flow of the zones that are at the end of the mainline. Do the math!!! How many gallons per minute flow through this sprinkler head and this nozzle with this pressure and then add all the other heads in for that zone.

From my perspective, as an Irrigation DIYer, and someone who HAS tunnelled under a driveway in heavy clay mixed with alabaster rock, here's the points I think you need to consider:

1. For the typical home owner, a 1-1/4" Sch40 PVC pipe is only going to lose about 1 psi / 100' of pipe or less. So your pressure loss in a 300' pipe is going to be about 3psi. This assumes your flow rate isn't going to be greater than 14gpm. But even at 20gpm, the pressure loss only doubles to 6psi.

2. Paying someone to tunnel under your driveway is going to cost $$$. Depending upon the size of your drive and the room you've got to work with, my best educated guess is that the price would be in the gross neiborhood of $300-$500.

3. You can do the job yourself. I did so going under an 11' wide driveway with only 5' of room to work. But I would say that it took me 40 to 80 man-hours to get the job done. You can read about my saga HERE, and I can provide you with some suggestions based on my experience if you want to go this route.

So given that an extra 300' of mainline isn't a show-stopper with a 1-1/4" mainline, I would say it comes down to where you want to spend your time and money:

Little Time + Lots of Money = Pay Someone to do it.
Some Time + Some Money = dig a 300' trench.
Lots of Time + Little Money = tunnel under the driveway yourself.

The reason is because you're talking about using 1-1/4" pipe.

It all comes down to math.

The speed of a given flow of water (i.e. 12gpm) is proportional to the cross-sectional area of the pipe. But since the cross-sectional area is a function of the square of the diameter, when you double the diameter, you quadtruple the cross-sectional area. The net result is that for each step up in pipe size (1/2 -> 3/4 -> 1 -> 1-1/4) you are roughly doubling the cross-sectional area (even though is seems like a small increase in size).

When it comes to friction losses, the friction loss is a function of the square of the water speed. So if you double the speed of the water through a pipe, you quadtruple the friction losses.

The over-all result is that for a give flow, one step in pipe size changes the friction losses by a factor of 4.

So a 1-1/4" pipe @ 12pgm will lose 0.89psi/100', while a 1" pipe @ 12gpm will lose 3.36psi/100'. (It's not a pefect factor of 4 because a 1-1/4" is only 70% larger than 1"... so not quite a size factor of 2:1)