Actually, all major sprinkler controller manufacturers offer 2-wire systems specifically designed to drive long strings of 24 volt AC valves. The systems are in widespread commercial use and have been for many years. String length can be up to several thousand feet (some as much as 15,000 feet) long and include hundreds of valves.
A casual Google search on “2-wire irrigation” will yield lots of information.
Power is not an issue. Using some approximations:
Voltage drop at 3000 feet assuming 500 ma:
3000 ft * 6.3 ohm/1000 ft * .5 = 9.5 v.
24V – 9.5 = 14v. This is plenty for pull in for an irrigation solenoid. This also assumes 300 ma to power all solenoid and interfaces on the string. This estimate ignores inductance and AC effects. This link: https://rayshobby.net/wordpress/understanding-24vac-sprinkler-valves/ does a good job of explaining things in detail.
The National Electric Code does not define “low voltage” per se, but 30V is generally considered the limit. No issues with licensed electricians. Also note that in general, homeowners doing improvements on their private homes are not required to use licensed electricians as long as they adhere to applicable NEC codes and have the required permits and inspections. This varies by local jurisdiction.
So the implementation of a 2-wire system is not only practical, it has already been done – a lot.
I am starting implementation of such a system using ASK (amplitude shift keying) to bidirectionally communicate with many (100 is design goal) valves using two wires. The challenge with is approach is the current required by each ASK modem in the valve string. If the current per station is too high, I might have to change the comm approach.
I plan to design for cat5 wiring as it is cheap and low cost valve nodes are the goal. A node cost of $12 for control of 4 ball valves would be fine. Motorized ball valves are $15 each from China.
As I have undergrad and graduate degrees in electrical engineering with an emphasis on embedded software and digital electronics, I expect development to be straightforward.
I’ll let you know when I have the system finished. I might consider selling the components.