Most power lines are made of aluminum wire wrapped around a steel core. This combination, known as ACSR (Aluminum Conductor Steel Reinforced), dominates overhead power transmission worldwide because aluminum carries electricity well while weighing far less than copper. The steel center provides the mechanical strength needed to span long distances between towers without snapping.
Why Aluminum Instead of Copper
Copper is actually a better electrical conductor than aluminum, but it lost the power line competition decades ago. Aluminum has 61 percent of copper’s conductivity, yet only 30 percent of its weight. That tradeoff works out remarkably well: a bare aluminum wire that matches a copper wire’s electrical performance weighs half as much. For lines that need to stretch hundreds of feet between towers, that weight difference is enormous. It means smaller towers, less structural stress, and significantly lower material costs.
Copper also can’t match the mechanical strength requirements of large spans on its own. You’d still need to reinforce it, which adds even more weight. Today, copper is rarely used in overhead transmission lines.
How Standard Overhead Lines Are Built
The standard ACSR cable has a layered design. At the center sit several strands of galvanized steel wire twisted together. These carry the mechanical load, keeping the cable taut across spans that can exceed 1,000 feet. Wrapped concentrically around that steel core are multiple layers of aluminum wire (specifically an alloy called 1350-H19) that handle the electrical current. The steel core can be coated with zinc, aluminum, or aluminum-clad layers to resist corrosion, which is especially important in coastal or humid environments.
This isn’t a single solid wire. Both the steel core and the aluminum outer layers are made of many individual strands twisted together, which makes the cable flexible enough to handle wind, ice loading, and temperature swings without cracking.
Transmission Lines vs. Distribution Lines
The massive towers you see along highways carry transmission lines, operating at 115,000 to 500,000 volts. These long-distance lines use the heaviest ACSR cables with thick aluminum layers and robust steel cores, since they need to carry enormous amounts of power across miles of open terrain.
The smaller wooden poles running through your neighborhood carry distribution lines, typically operating between 2,000 and 35,000 volts. These lines still use aluminum conductors, but they’re lighter and simpler in construction because the spans are shorter and the loads are smaller. Distribution lines in residential areas sometimes include a thin insulating layer, though many remain bare like their high-voltage counterparts.
The Insulators Holding Everything Up
The wires themselves are only part of the system. Where cables attach to towers and poles, insulators prevent electricity from flowing into the structure and down to the ground. These disc or bell-shaped components are traditionally made from porcelain, which is manufactured from clay, quartz, and feldspar, then covered with a smooth glaze that sheds rain. Toughened glass is another classic option, offering higher resistance to electrical breakdown.
Many utilities have shifted toward composite polymer insulators for certain applications. These use a central rod of fiberglass-reinforced plastic with an outer shell of silicone rubber. They’re lighter than porcelain, less likely to shatter, and easier to install, though porcelain and glass remain common on high-voltage transmission towers.
Underground Cables Use Different Materials
Underground power cables face a completely different challenge. Since they can’t use open air as insulation, they need multiple protective layers wrapped around the conductor. The conductor itself is still typically aluminum or copper, but it’s surrounded by a thick layer of cross-linked polyethylene (XLPE), a specialized plastic that withstands high temperatures and resists electrical breakdown. XLPE has become the dominant insulation material for underground high-voltage cables because it can be easily formed during manufacturing and then chemically cross-linked into a tough, heat-resistant network.
Beyond the insulation, underground cables include semiconducting shields on either side of the XLPE layer to manage electrical stress, a metallic sheath (often lead or corrugated aluminum) for moisture protection, and an outer jacket of heavy-duty plastic. The result is a cable several inches in diameter that can safely carry high voltage through soil, under roads, or beneath waterways.
Newer Composite Core Conductors
A newer design replaces the steel core with a carbon fiber composite wrapped in a protective fiberglass sheath. These cables, called ACCC (Aluminum Conductor Composite Core), still use aluminum on the outside for conducting electricity, but the lighter core allows 25 to 30 percent more aluminum to be packed into the same diameter without adding weight. More aluminum means the line can carry more current.
The carbon fiber core also expands far less with heat than steel does. When power lines get hot under heavy electrical load, they sag. Less thermal expansion means less sag, which lets utilities push more power through existing corridors without rebuilding towers. These composite conductors are increasingly used when utilities need to upgrade capacity on existing infrastructure.
The Wire on Top You Might Not Notice
Look at a transmission tower and you’ll often spot a thinner wire running along the very top, above the main power-carrying conductors. This is the ground wire, or shield wire, designed to intercept lightning strikes before they can damage the energized lines below. It’s typically made of steel or aluminum-clad steel.
Many utilities now use a version called optical ground wire (OPGW), which serves double duty. Inside, a stainless steel or aluminum tube houses 8 to 48 glass optical fibers for telecommunications. That tube is then stranded with layers of steel and aluminum wire, making it look identical to a standard cable from the outside. The steel provides strength and lightning protection, the aluminum provides conductivity, and the fiber optic core carries data, letting utilities monitor their grid and lease bandwidth to telecom companies.