Need more wire? The Ultra-Wire Boundary Extension Kit gives you 500 feet of the special Ultra-Wire, 50 Brightly Colored Flags and 10 Solderless, waterproof wire connectors. This wire was specially designed and manufactured for one and only one purpose, electronic dog fence boundary wire. It has electrical and mechanical properties optimized for your dog containment application exceeding even 18 gauge wire performance. The wire is solid core copper designed for optimum electrical impedance. It has an extra thick jacket made of a specially formulated insulation material that is especially resistant to Ultraviolet sunlight, moisture, rot and corrosion. It's 40% thicker overall and about three times stronger than standard 18 gauge wire, greatly reducing the risk of wire breaks. This is hands down the best dog containment fence wire you will find anywhere. It's guaranteed to last at least 20 years!