Relays of mechanical nature capable of 15-20 amp loads are going to be of substantial size, solid state relays will reduce the physical size substantially for that type of load, but then will be more costly. Just out of curousity, why would you want to install relays at utilization points? In that case you would use the existing wire for your supply yet still have to pull in controls wires back to your processor? Do you need 15-20 amps for the devices you are actually going to use?
I haven't read to many of the posts on this topic in here, but I have done some automation wiring in my own house. I used a central plc, AB 504 with a few Flex I/O Blocks located closer to where I wanted to use them at. This allowed me to just run "Blue Hose" from processor to remote locations and greatly reduced my required wiring for devices, and gave me the flexibility to add on to the system at a later time very easily.
I actually had a friend who automated his sliding patio door with a couple photo eyes so that his dog could let itself out instead of having to constantly get up and down to let the dog in and out, haha. Same friend also had a fish pond in his yard that the birds kept going in and eating his fish on him, he hooked up a motion sensor to a water sprinkler and water valve to spray them whenever they tried to go into his pond, no more problems with birds eating his fish again. I guess just a couple of simple things that can be done with home automation.
Good Luck with your project, it is an interesting thing to actually do.
