The height of power lines are determined by the amount of voltage they carry and the type of traffic in the area. For example, areas with only foot traffic can get by with a shorter, standard height over areas where semi-tractor trailer trucks frequently travel.
The National Fire Protection Association provides written standards referred to as the National Electric Code, which is reprinted every three years. Article 225, Section 18 of the National Electric Code outlines height standards for power lines.
The minimum height around residential properties for power lines with less than 300 volts is 12 feet. Power lines with a voltage above 300 volts require a minimum height of 15 feet. Public and commercial areas, especially those that feature over-size truck traffic, need greater clearance for safety's sake and the National Fire Protection Association suggests a height of at least 18 feet.
The National Electric Code is not national law. Each state or city can adopt and implement their own standards if they choose. However, as the height of power lines is a safety issue more than anything, states can adopt parts of the code and amend other parts as they see fit.