Classification by operating voltage

High- and medium-voltage power lines in Łomża, Poland

Overhead power transmission lines are classified in the electrical power industry by the range of voltages:

Low voltage (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.

Medium voltage (MV; distribution) – between 1000 volts (1 kV) and to 69 kV, used for distribution in urban and rural areas.

High voltage (HV; sub-transmission less than 100 kV; sub-transmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.

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

Ultra high voltage (UHV) – higher than 800 kV

Overhead power transmission lines are classified in the electrical power industry by the range of voltages:

Low voltage (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.

Medium voltage (MV; distribution) – between 1000 volts (1 kV) and to 69 kV, used for distribution in urban and rural areas.

High voltage (HV; sub-transmission less than 100 kV; sub-transmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.

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

Ultra high voltage (UHV) – higher than 800 kV

Structures for overhead lines take a variety of shapes depending on the type of line. Structures may be as simple as wood poles directly set in the earth, carrying one or more cross-arm beams to support conductors, or "armless" construction with conductors supported on insulators attached to the side of the pole. Tubular steel poles are typically used in urban areas. High-voltage lines are often carried on lattice-type steel towers or pylons. For remote areas, aluminum towers may be placed by helicopters. Concrete poles have also been used. Poles made of reinforced plastics are also available, but their high cost restricts application.

Each structure must be designed for the loads imposed on it by the conductors. The weight of the conductor must be supported, as well as dynamic loads due to wind and ice accumulation, and effects of vibration. Where conductors are in a straight line, towers need only resist the weight since the tension in the conductors approximately balances with no resultant force on the structure.

Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands. Each structure may be stabilized considerably by the use of guy wires to counteract some of the forces applied by the conductors.

Power lines and supporting structures can be a form of visual pollution. In some cases the lines are buried to avoid this, but this "undergrounding" is more expensive and therefore not common.

A grounded cable called a static line is sometimes strung along the tops of the towers to provide lightning protection. An optical ground wire is a more advanced version with embedded optical fibers for communication.