Long underground AC cables have significant capacitance, which reduces their ability to provide useful power beyond 50 miles (80 kilometres). Alternating current's economies of scale with large generating plants and long-distance transmission slowly added the ability to link all the loads. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads. Commercial electric power was initially transmitted at the same voltage used by lighting and mechanical loads. As the current carrying capacity requirement of electric cables increases, more strands are added, and accordingly, more reinforcement is necessary. In some metropolitan areas, cables are enclosed by metal pipe and insulated with dielectric fluid (usually an oil) that is either static or circulated via pumps. They operate at high temperatures and must be well insulated to retain heat. However, cables must be insulated. DC cables are not limited in length by their capacitance. Types intended for special purposes, such as in a computer uninterruptible power supply, may be rated by manufacturers for discharge periods much less than one hour (1C) but may suffer from limited cycle life. Underground lines are limited by their thermal capacity, which permits less overload or re-rating lines.

Other studies, however, reported statistical correlations between various diseases and living or working near power lines. Overhead transmission wires depend on air for insulation, requiring that lines maintain minimum clearances. Copper was sometimes used for overhead transmission, but aluminum is lighter, reduces yields only marginally and costs much less. Overhead conductors are supplied by several companies. The remaining or peak power demand, is supplied by peaking power plants, which are typically smaller, faster-responding, and higher cost sources, such as combined cycle or combustion turbine plants typically fueled by natural gas. Renewable energy sources, such as solar photovoltaics, wind, wave, and tidal, are, due to their intermittency, not considered to be firm. Energy is usually transmitted within a grid with three-phase AC. In what became a universal system, these technological differences were temporarily bridged via the rotary converters and motor-generators that allowed the legacy systems to connect to the AC grid. This practical demonstration of a transformer and alternating current lighting system led Westinghouse to begin installing AC systems later that year. Higher voltage is achieved in AC circuits by using a step-up transformer. The first three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt.

The first long distance AC line was 34 kilometres (21 miles) long, built for the 1884 International Exhibition of Electricity in Turin, Italy. The first transmission of single-phase alternating current using high voltage came in Oregon in 1890 when power was delivered from a hydroelectric plant at Willamette Falls to the city of Portland 14 miles (23 km) down river. A common dry cell is the zinc-carbon battery, sometimes called the dry Leclanché cell, with a nominal voltage of 1.5 volts, the same as the alkaline battery (since both use the same zinc-manganese dioxide combination). Hydro and wind sources cannot be moved closer to big cities, and solar costs are lowest in remote areas where local power needs are nominal. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to further lower costs. Transmission efficiency is improved at higher voltage and lower current.

Electric poles and supporting structures come in different forms and sizes, mainly based on the voltage of the power they transmit. If an electric fault damages the pipe and leaks dielectric, liquid nitrogen is used to freeze portions of the pipe to enable draining and repair. This extends the repair period and increases costs. The temperature of the pipe and surroundings are monitored throughout the repair period. Underground cables take up no right-of-way, have lower visibility, and are less affected by weather. Electric power can be transmitted by underground power cables. In the late 1880s and early 1890s smaller electric companies merged into larger corporations such as Ganz and AEG in Europe and General Electric and Westinghouse Electric in the US. These companies developed AC systems, what is electric cable but the technical difference between direct and alternating current systems required a much longer technical merger. Joule's first law states that energy losses are proportional to the square of the current.