The commonest conductor used in cables is copper . The olny other conductor used is aluminium . Copper was the earlier one to be used , although aluminium has the disadvantage of being much weaker than copper . Consequently BS 7671 states that the minimum permissible cross-sectional area is 16mm square . Aluminium 's greatest assets are that it is cheaper than copper , lighter , and that its price is less liable to fluctuations .
COnductors have usually been made exept for the smallest sizes , by twisting together a number of small cables , called strands , to make one larger cable . A cable made in this way is more flexible than a single cable of the same size and is consequently easierm to handle . Each layer is spiralled on the cable in the direction opposite to that of the previous layer ; this reduces the possibility that the strands will open under the influence of bending forces when the cable is being installed . 1mm square has a solid core , 1.5 mm square and 2.5 mm square is available as solid or stranded core ; sizes above these are available as stranded core olny .
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