If you already own a containment system for your pet and are looking to expand your coverage then you only need to purchase extra boundary wire/accessories. Twisted wire is used to open up a safe zone for your pet to cross. The twisted wire cancels out the signal. Use 18 gauge (thicker) wire for the most robust signal.
Innotek P-Wire 500 Feet Boundary Wire 18 Gauge YCDRFDXVK