From what little I know about metal cables, I wouldn't think our soft lead pellets and slugs would hurt a power line. I did do a little bit of looking on the internet, and found some posts by employees/former employees of power companies up north, and they were saying that they would go around using shotguns to shoot the ice off of the power lines so they wouldn't break under the weight of the ice. One even said there was a time the power company handed out boxes of shotgun shells asking the public to help. Now, not all the cables up there are for power. The power cables that run from pole to pole are usually uninsulated and made of aluminum with a steel or aluminum alloy core (for strength). For most of us, our low power, soft lead projectiles are harmless. The phone and cable TV lines are a different story. They are insulated, and much more likely to suffer from a pellet or slug's impact. They are typically black, rather than the bare metal of power lines. All the farm fields I ever hunted only had power lines, usually to carry power to a barn or water pump. You are not likely to see a phone or internet cable going to a barn on the back side of a farm. So, like so many other situations in our lives, it just depends. Are you sure it's just a power line, and not a phone or TV cable? Is the range and your skill set such that a hit on the line is very unlikely? I personally wouldn't hesitate to shoot a bird on a power line, if the bird was a valid target and the area behind the bird was clear, and the range/wind conditions were within my abilities to put the pellet on target.
James