There is no precise distinction between wire and cable, but the name cable normally describes two or more electric conductors, or cores, that are individually insulated and contained within a protective sheath. The cores are of stranded construction using twisted copper or aluminum wires. This configuration provides greater flexibility than a single solid conductor of similar rating.
Copper is the classic conductor material, but rising prices resulted in aluminum being widely used. Aluminum cables have a greater bulk but much less weight for a given current capacity. For equal electric resistance, an aluminum conductor has a cross section about 1.6 times that of a copper conductor.
Distribution systems
Electricity from generating plants is distributed at high voltage by means of overhead conductors carried on pylons (transmission towers). The conductors are not insulated themselves but are suspended from the pylons by strings of insulators.
Aluminum conductor, steel-reinforced (ACSR) cables have been used widely. They are comparatively inexpensive to produce but have a number of disadvantages: they are relatively heavy, the steel cores need to be galvanized and greased to resist corrosion, and they require complex terminations. More modern designs use either aluminum conductor, alloy reinforced (ACAR) or all aluminum alloy cable (AAAC) construction. The proportion of aluminum conductors in use increased after the development of improved jointing methods using an inert gas–metal deposition process.
The overhead distribution circuits are three-phase alternating current, with a separate conductor (or pair, or sometimes a group of four spaced conductors) for each phase of the power supply. The overhead system feeds at intervals into substations from which supplies are distributed to the surrounding area at lower voltages. Underground supply systems are still three-phased, but the three conductors are contained in one cable.
Insulation
Individual conductors are insulated in a number of ways according to the function of the cable. Rubber insulation was once used in lower voltage cables for domestic installation, but plastic (PVC) insulation is now used extensively.
In 1967, a fire in the power plant at La Spezia, Italy, spread through the large amounts of PVC-insulated wiring within the building. Alternative materials, which are less susceptible to combustion and do not produce large quantities of acrid smoke when they do burn, have been used since that time in areas where fire prevention is more important than cost, such as underground railways, aircraft, hospitals, and military installations.
Suitable materials include polytetrafluoro-ethylene (PTFE), silicone rubber, and a range of proprietary mixed and filled polymers. Extreme heat resistance is provided by the use of magnesium oxide powder as insulation. This substance is compressed around the conductors within a copper sheath, forming a cable that is impervious to moisture and resistant to attack by oil and gasoline. It will also carry large currents because the magnesium oxide is quite a good conductor of heat. Cable with this form of insulation is known as MICS (mineral-insulated, copper-sheathed) cable and is specially useful to industry for installations in boiler houses and other areas of high ambient temperature.
If the cable is for underground distribution at high voltage, oil-impregnated paper insulation is applied to the conductors using multihead lapping machines—each head carries several reels of paper that rotate around the conductor as it passes through the machine. For very high voltages, as many as 200 layers may be applied to one conductor. After the individual conductors have been insulated, further layers of paper are applied to the assembly. Following this procedure, a lead or, more often, an aluminum sheath is extruded over the cable to exclude moisture. The sheath may be armored with steel tape or steel wire and further protected by an outer wrapping of burlap or extruded PVC.
