This invention is an improved filling compound for preventing moisture penetration along the interior of high voltage electric power cables. Amorphous polypropylene, mixed with carbon where it must be semiconductive, is an extremely good filling compound because of its electrical characteristics and also because of its good flow characteristics when extruding. When the cable is to be used under conditions where it must withstand temperatures so low that the polypropylene becomes brittle, it can be blended with polyisobutylene rubber to withstand lower temperatures, but the blend must be kept within limits to prevent loss of the good flow characteristics of the amorphous polypropylene.