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Joule's first law states that energy losses are proportional to the square of the current. A 2024 report found the United States behind countries like Belgium and the Netherlands in adoption of this technique to accommodate electrification and renewable energy. 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. For a long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the cost of the required converter stations at each end. At higher voltages, where more than 2,000 kV exists between conductor and ground, corona discharge losses are so large that they can offset the lower resistive losses in the line conductors. Like transmission, subtransmission moves relatively large amounts of power, and like distribution, subtransmission covers an area instead of just point-to-point.

No fixed cutoff separates subtransmission and transmission, or subtransmission and distribution. Understanding the temperature distribution along the cable route became possible with the introduction of distributed temperature sensing (DTS) systems that measure temperatures all along the cable. In this respect, the same cable has more ampacity when in the air than when in a conduit. While subtransmission circuits are usually carried on overhead lines, in urban areas buried cable may be used. Because of the economic benefits of load sharing, wide area transmission grids may span countries and even continents. Adding transmission lines is difficult due to cost, permit intervals, and local opposition. For intermediate-length lines on the order of 100 kilometres (62 miles), the limit is set by the voltage drop in the line. Higher order phase systems require more than three wires, but deliver little or no benefit. Because of this phenomenon, conductors must be periodically transposed along the line so that each phase sees equal time in each relative position to balance out the mutual inductance seen by all three phases.

The mutual inductance seen by a conductor of the phase in the middle of the other two phases is different from the inductance seen on the top/bottom. Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance. 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. Also, it is difficult for a merchant transmission line to compete when the alternative transmission lines are subsidized by utilities with a monopolized and regulated rate base. Also, their insulation decreases the rate of cooling compared to bare wires. The cost of high voltage transmission is comparatively low, compared to all other costs constituting consumer electricity bills. In the 19th century, two-phase transmission was used but required either four wires or three wires with unequal currents. Currents that flow solely in reaction to these properties, (which together with the resistance define the impedance) constitute reactive power flow, which transmits no power to the load. Utilities add capacitor banks, reactors and other components (such as phase-shifters; static VAR compensators; and flexible AC transmission systems, FACTS) throughout the system help to compensate for the reactive power flow, reduce the losses in power transmission and stabilize system voltages.

Some jurisdictions, such as Minnesota, prohibit energy transmission companies from selling surplus communication bandwidth or acting as a telecommunications common carrier. New methods of reuse, such as echelon use of partly-used batteries, add to the overall utility of electric batteries, reduce energy storage costs, and also reduce pollution/emission impacts due to longer lives. Selection of electric cables must be based on the worst-case scenario and the highest ambient temperature. In these cases special high-voltage cables are used. 100, provided the same sized conductors are used in both cases. Just a short walk through the bush to the beach, and the same distance again from the township. 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. For overhead cables the fiber is integrated into the core of a phase wire. Overhead lines are normally in the air and cooled by streams of free air, whereas the underground cables are either in a conduit or buried underground. Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. When electrical energy is transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead.

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