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Current flowing through transmission lines induces a magnetic field that surrounds the lines of each phase and affects the inductance of the surrounding conductors of other phases. Because of the economic benefits of load sharing, what is electric cable wide area transmission grids may span countries and even continents. Local sources (even if more expensive and infrequently used) can protect the power supply from weather and other disasters that can disconnect distant suppliers. 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. High-voltage direct current (HVDC) systems require relatively costly conversion equipment that may be economically justified for particular projects such as submarine cables and longer distance high capacity point-to-point transmission. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits. Submarine HVDC systems are often used to interconnect the electricity grids of islands, for example, between Great Britain and continental Europe, between Great Britain and Ireland, between Tasmania and the Australian mainland, between the North and South Islands of New Zealand, between New Jersey and New York City, and between New Jersey and Long Island.

At the outset, cable systems only served smaller communities without television stations of their own, and which could not easily receive signals from stations in cities because of distance or hilly terrain. A twisted pair has two wires of a cable twisted around each other. At times of lower interest rates and low commodity costs, Kelvin's law indicates that thicker wires are optimal. Substation transformers reduce the voltage to a lower level for distribution to customers. When the shaft is depressed a metal piece attached to the center of the shaft (but insulated from the shaft) slides downward and electrically connects the upper post to the lower post. Usually the former refers to a single, solid metallic conductor, with or without insulation, while the latter refers to a stranded conductor or to an assembly of insulated conductors. While the price of generating capacity is high, energy demand is variable, making it often cheaper to import needed power than to generate it locally. Such facilities are nuclear, coal or hydroelectric, while other energy sources such as concentrated solar thermal and geothermal power have the potential to provide firm power. Because loads often rise and fall together across large areas, power often comes from distant sources.

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. Overall the electrical cables are important for efficiently transportation of electrical power in every sectors. The slowly varying portion of demand is known as the base load and is generally served by large facilities with constant operating costs, termed firm power. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. Interconnection of local generation plants and small distribution networks was spurred by World War I, when large electrical generating plants were built by governments to power munitions factories. World War II and TV research and production was put off for the next six years. GTE Automatic Electric/Microtel and Phillips were two of the major employers in Brockville - both major employers until they shut down some 50 years later. 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.

Voltage was stepped down to 100 volts using the Stanley transformer to power incandescent lamps at 23 businesses over 4,000 feet (1,200 m). 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. Transmitting electricity at high voltage reduces the fraction of energy lost to Joule heating, which varies by conductor type, the current, and the transmission distance. In that country, transmission operations and electricity markets are separate. A 15 kV transmission line, approximately 175 km long, connected Lauffen on the Neckar and Frankfurt. Red : Phase 2 or transmission wires. In the 19th century, two-phase transmission was used but required either four wires or three wires with unequal currents. The 20th century's rapid industrialization made electrical transmission lines and grids critical infrastructure. HVDC is necessary for sending energy between unsynchronized grids. 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.