I've got a wire around about 4ac; it's attached just above ground level to a perimeter fence for the most part, and barely under ground where it crosses an unfenced area. In that section, the wire goes across, jogs over about 10 feet, comes back, jogs over 10 feet more, and crosses one more time, so there's an entire zone of "do not cross" - I found a single strand wasn't sufficient to keep a running cat or dog from crossing the line as they'd be at the wire before the warning even went off. With three strands they've got time to reconsider if they really want to get across...
This system has worked for me for seven years now; I'm on my second controller as the first died from who-knows-what. They sent me a replacement which unfortunately only has a light come on when there's a loop signal; if there's no loop, the light goes out and no alarm sounds, so I need to look at the unit every once in a while to make sure it's still working (original unit sounded like a smoke detector if the loop was broken).