See: Crafting Guide
HV-Cables have an extremely high distance-related energy loss. They can have 3 different levels of insulation. Considering that a shock from a poorly insulated HV cable will most likely prove fatal, it is recommended to insulate them well unless the cable is located in a completely unreachable area. Even then, insulation should be applied to reduce the massive energy losses.
HV cables lose 0.8 EU every block, so 1 EU is lost after 1.25 blocks.