Structures and facilities

There are many different types of transmission line structures used throughout ATC’s service area. Learn about the wires and circuits.

We use both wood and steel structures depending on the conditions of the land and surrounding areas, the height needed for the poles, the price of steel and other factors, such as:

  • construction
  • operation
  • maintenance
  • cost
  • electrical capacity and voltage of the transmission line
  • physical electrical clearances
  • environmental impact

About the wires

The wires used in large transmission lines are mainly aluminum. Aluminum costs less and is lighter than copper, which was primarily used until the 1950s. A single-circuit transmission line will have three wires; a double-circuit line will have six wires. An additional wire (or two), called a shield wire is connected to transmission line structures at the top to protect the main conductors (wires) from a direct lightning strike. If lightning strikes, it will hit the shield wires rather than the main wires. If lightning were to strike the main wires, a short circuit to the ground might occur, which could cause a momentary power outage that may not even be noticed by the general public.

Transmission lines are connected to the structures by insulators that are usually made of porcelain. They must be strong enough to support the weight of the transmission lines while preventing a contact between the wires and the structure. Contact between the two would cause a short circuit and disrupt supply.

345-kilovolt, double-circuit on single poles

Higher voltage lines require taller poles and wider rights-of-way.

H-frame wood structure

This type of design allows for longer spans (the distance between structures). In general the height of the structures is less than single-pole structures.

Double-circuit, 138-kilovolt steel lattice structure

Lattice structures are strong (rigid) compared to their weight. The strength is based on the design and not necessarily because it is a truss system.

Double-circuit, 138-kilovolt line build on galvanized steel poles

This type of design is most often used to route two circuits out of a substation to keep the right-of-way narrow.

138-kilovolt, single-circuit line on weathering steel

This type of maintenance-free structure will weather (rust) over time to give the appearance of wood and is generally used in wooded areas.

138-kilovolt steel H-frame

Similar to wood h-frames, this design allows for longer spans (distance between structures).

Double-circuit, 138-kilovolt transmission line built on wood structures

Local electric distribution lines, cable and telephone lines are sometimes carried on the same structures to make efficient use of space. Taller poles are needed to do this.