Solid 18 gauge wire is preferred. If you have stranded wire, turn power off, twist and pull the end of the wire so the wire strands are aligned. Push and hold the release button to the side of the terminal that you plan to attach the wire to while inserting the wire. Release the button and gently, but firmly, pull the wire to verify good connection. If the wire doesn't go in smoothly and hold firmly, you may need to connect the ends of the stranded wire to a short length of solid wire with wire-nuts.