<P> In the United States, power transmission is, variously, 230 kV to 500 kV, with less than 230 kV or more than 500 kV being local exceptions . For example, the Western System has two primary interchange voltages: 500 kV AC at 60 Hz, and ± 500 kV (1,000 kV net) DC from North to South (U.S. - Canada border to U.S. - Mexico border). </P> <P> The 287.5 kV (Hoover to Los Angeles line, via Victorville) and 345 kV (APS line) being local standards, both of which were implemented before 500 kV became practical, and thereafter the Western System standard . </P> <P> Transmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending on the specific conductors, the current flowing, and the length of the transmission line . For example, a 100 mi (160 km) span at 765 kV carrying 1000 MW of power can have losses of 1.1% to 0.5% . A 345 kV line carrying the same load across the same distance has losses of 4.2% . For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor . For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the I 2 R (\ displaystyle I ^ (2) R) losses by a factor of 100, provided the same sized conductors are used in both cases . Even if the conductor size (cross-sectional area) is reduced ten-fold to match the lower current, the I 2 R (\ displaystyle I ^ (2) R) losses are still reduced ten-fold . Long - distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV . At extremely high voltages, 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 . Measures to reduce corona losses include conductors having larger diameters; often hollow to save weight, or bundles of two or more conductors . </P> <P> Factors that affect the resistance, and thus loss, of conductors used in transmission and distribution lines include temperature, spiraling, and the skin effect . The resistance of a conductor increases with its temperature . Temperature changes in electric power lines can have a significant effect on power losses in the line . Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance . The skin effect causes the effective resistance of a conductor to increase at higher alternating current frequencies . </P>

What percentage of electricity is lost during transmission
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