Shimano Ultegra Di2 EW-SD50 Electric Wire (300mm);

Check the size chart on this page for length, and don't worry about the rest. If you're running Ultegra 6770 or 6870, or Dura-Ace 9070, this E-Tube Di2 Wire will connect up all the moving parts. Its connectors are the same, regardless of application, so all you have to do is choose the length that's best suited.