How large a copper water line for 150' run to a guest house? Pressure regulator?

I would think that 3/4" should handle it. However, I strongly suggest you check the local authorities before beginning as they may have specifications for this.

You wont reduce the available pressure by using a smaller pipe, you will only reduce the available flow volume in Gallons per minute.

Static pressure will be at the 80 psi from the time you turn on the faucet until the flow reaches a constant volume, dependant on pipe friction loss and supply pressure--then and only then will it drop to a lower pressure at the other end.

IOW, you will turn on the faucet and get a big spurt at 80 psi for a few seconds, and then it will equalize at a lower pressure and flow.

To reduce pressure you would put a pressure regulator on the line, suggest at the end of the run.

I would use 1 inch pipe and probably pvc schedule 40. I would not reduce the pressure by mechanical means unless your going to be using CPVC inside. The pressure and volume will be enough reduced by adding the 150' of pipe into the system.

Rule of thumb pressure drops, PVC pipe (psi/100 ft): 1/2 3/4 1"

Copper pipe will be about 0.2 to 0.5 psi/100 ft more loss.

You could get by with 3/4" but I'd favor 1"

RB

Precisi>

hear in the us we or i should say most would use black pollyelean ken uhrick