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Thus, reducing the current by a factor of two lowers the energy lost to conductor resistance by a factor of four for any given size of conductor. The Western Interconnection has two primary interchange voltages: 500 kV AC at 60 Hz, and ±500 kV (1,000 kV net) DC from North to South (Columbia River to Southern California) and Northeast to Southwest (Utah to Southern California). Two DC links originally implemented as market interconnectors, Directlink and Murraylink, were converted to regulated interconnectors. HVDC links stabilize power distribution networks where sudden new loads, or blackouts, in one part of a network might otherwise result in synchronization problems and cascading failures. One grid connects most of continental Europe. It also featured Siemens alternators and 2.4 kV to 100 V step-down transformers - one per user - with shunt-connected primaries. It was powered by a 2 kV, 130 Hz Siemens & Halske alternator and featured several Gaulard transformers with primary windings connected in series, which fed incandescent lamps. These networks use components such as power lines, cables, circuit breakers, switches and transformers. The voltage level is changed with transformers.

Voltages above 765 kV are considered extra high voltage and require different designs. 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. Single-phase AC is used only for distribution to end users since it is not usable for large polyphase induction motors. These were induction motors running on polyphase current, independently invented by Galileo Ferraris and Nikola Tesla. These included single phase AC systems, poly-phase AC systems, low voltage incandescent lighting, high-voltage arc lighting, and existing DC motors in factories and street cars. Widespread use of such motors were delayed many years by development problems and the scarcity of polyphase power systems needed to power them. New power lines can take 10 years to plan, permit, and build. The optimum size of a conductor for a given voltage and current can be estimated by Kelvin's law for conductor size, which states that size is optimal when the annual cost of energy wasted in resistance is equal to the annual capital charges of providing the conductor. The US Northeast faced blackouts in 1965, 1977, 2003, and major blackouts in other US regions in 1996 and 2011. Electric transmission networks are interconnected into regional, national, and even continent-wide networks to reduce the risk of such a failure by providing multiple redundant, alternative routes for power to flow should such shutdowns occur.

Since the power flow in a DC link is controlled independently of the phases of the AC networks that it connects, this phase angle limit does not exist, and a DC link is always able to transfer its full rated power. For large conductors (more than a few centimetres in diameter), much of the current flow is concentrated near the surface due to the skin effect. Electricity is transmitted at high voltages to reduce the energy loss due to resistance that occurs over long distances. Efficient long-distance transmission of electric power requires high voltages. Bundle conductors are used at high voltages to reduce energy loss caused by corona discharge. It is uneconomical to connect all distribution substations to the high main transmission voltage, because that equipment is larger and more expensive. If demand exceeds supply, the imbalance can cause generation plant(s) and transmission equipment to automatically disconnect or shut down to prevent damage. A sophisticated control system is required to ensure that power generation closely matches demand. In frequency signaling, the generating units match the frequency of the power transmission system.

Interconnecting multiple generating plants over a wide area reduced costs. Thus, multiple parallel cables (called bundle conductors) are used for higher capacity. High-voltage overhead conductors are not covered by insulation. Cable and excavation costs are much higher than overhead construction. This extends the repair period and increases costs. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to further lower costs. At times of lower interest rates and low commodity costs, Kelvin's law indicates that thicker wires are optimal. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads. Different classes of loads (for example, lighting, fixed motors, and traction/railway systems) required different voltages, and so used different generators and circuits. Alternating current's economies of scale with large generating plants and long-distance transmission slowly added the ability to link all the loads. This restricted the distance between generating plant and loads. The first long distance AC line was 34 kilometres (21 miles) long, built for the 1884 International Exhibition of Electricity in Turin, Italy. The study measured the electric field strength at the edge of an existing right-of-way on a 765 kV transmission line.

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