Check out wikipedia for some facts: http://en.wikipedia.org/wiki/Overhead_power_line. Note in our case we are facing:

  • Extra high voltage (EHV; transmission) – over 230 kV, up to about 800 kV, used for long distance, very high power transmission.

230 kV designs are always over 100 feet high and often towards 150 feet and need a clear cut under those line 200+ feet across.  Many have observed that they hummm all the time.