<P> A lack of electrical energy storage facilities in transmission systems leads to a key limitation . Electrical energy must be generated at the same rate at which it is consumed . A sophisticated control system is required to ensure that the power generation very closely matches the demand . If the demand for power exceeds supply, the imbalance can cause generation plant (s) and transmission equipment to automatically disconnect or shut down to prevent damage . In the worst case, this may lead to a cascading series of shut downs and a major regional blackout . Examples include the US Northeast blackouts of 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 shut downs occur . Transmission companies determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure that spare capacity is available in the event of a failure in another part of the network . </P> <P> High - voltage overhead conductors are not covered by insulation . The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands . Copper was sometimes used for overhead transmission, but aluminum is lighter, yields only marginally reduced performance and costs much less . Overhead conductors are a commodity supplied by several companies worldwide . Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits . Conductor sizes range from 12 mm (#6 American wire gauge) to 750 mm (1,590,000 circular mils area), with varying resistance and current - carrying capacity . For normal AC lines thicker wires would lead to a relatively small increase in capacity due to the skin effect (which causes most of the current to flow close to the surface of the wire). Because of this current limitation, multiple parallel cables (called bundle conductors) are used when higher capacity is needed . Bundle conductors are also used at high voltages to reduce energy loss caused by corona discharge . </P> <P> Today, transmission - level voltages are usually considered to be 110 kV and above . Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads . Voltages less than 33 kV are usually used for distribution . Voltages above 765 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages . </P> <P> Since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed to maintain safety . Adverse weather conditions, such as high wind and low temperatures, can lead to power outages . Wind speeds as low as 23 knots (43 km / h) can permit conductors to encroach operating clearances, resulting in a flashover and loss of supply . Oscillatory motion of the physical line can be termed gallop or flutter depending on the frequency and amplitude of oscillation . </P>

How many amps are in a transmission line