Af,

Go with 1" will have less velocity which means less noise than 3/4". It also makes up for the fitting head loss thru the manifold. It is about the same cost as 3/4" pvc. JMHO Aloha

Thanks for the reply. I have two more questions concerning pipe size/diameter.
The diagram has 1" pipe as "feeder", "? pipes and 3/4 for the span that actually has the heads on them. Does this seem right? I hope I can explain what I mean by "feeder"..usually longer spans of pipe coming from the valve that leads to when the actual heads start their span.
My other question is sch40 or 200 pvc? I have researched and came up with so many different opinions. I live in zone 7 in lower North Carolina. Cost is not a big concern because they are nearly the same here.

Af,

1. The typical system has 1" piping from the source through the valve manifold to the first laterals. Pipe intersection evenly divides the flow into those laterals. Example main flow is 15 gpm, the intersection has three laterals, each lateral will have 5 gpm. Therefore the laterals do not need to be as large as the main. The 3/4" pipe are usually used for the laterals after the flow has been divided. There is nothing wrong with using the larger pipe as far as you want. It is just slightly cheaper (not by much). Larger pipe equates to slower flow and less noise when operating the system. 1 1/2" main through valve manifold to 1" pipe for laterals is okay just slightly more expensive but less noisy if the system is under normal (50-60 psi) pressure.
2. Which ever has the thicker wall, if they both cost the same, is best.

JMHO Aloha

That completely answers my questions in a language that I can understand, thanks