ATC uses a variety of tower and structure designs throughout our service area. 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

The composition of the land the transmission lines will cross is considered during the planning phase along with legal restrictions, such as zoning and land-use regulations, and the appearance of the structures. The width of a right-of-way depends on the voltage of the line and the height of the structures, but may be 75-150 feet or more depending on the type of facilities. Special consideration is given to the impact the transmission line and construction will have on native vegetation and the environment.

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Double-circuit, 138-kV line 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.

Double-circuit, 138-kV line on galvanized steel poles This type of design is often used when the line will carry heavy electric loads. Higher voltage lines require taller poles-—sometimes 100 feet or more.

Single-circuit 138-kV line on weathering steel This type of maintenance-free structure will weather (rust) over time to give the appearance of wood.

H-frame wood structure This type of design allows for shorter spans (distance between structures). In general, the height of the structures is less than single-pole structures.

Double-circuit 138-kV line on steel lattice tower Lattice structures are very strong, relatively light and can be erected without the need for heavy equipment and major access roads.

138-kV steel H-frame Similar to wood h-frames, this design allows for shorter spans (distance between structures).

345-kV, double-circuit line on single poles Higher voltage lines require taller poles and wider rights-of-way.

About the conductors
The conductors (wires) used in large transmission lines are mainly aluminum. Aluminum has replaced copper lines, because of the lower cost and lighter weight. A single-circuit transmission line will have three conductors; a double-circuit line will have six. An additional wire (or two), called a shield wire is connected directly to the transmission line towers at the top to protect the main conductors from a direct lightning strike. If lightning strikes it will hit the shield wires rather than the conductors. If lightning were to strike the main conductors, a short circuit to the ground might occur, which could result in a wide-spread power outage.

Transmission lines are connected to the towers 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 tower. Contact between the two would cause a short circuit and disrupt supply.