Prior to modern cables, whose size (e.g. 2.5mm²) refers to the cross sectional area (csa) of the main conductors, cables were described by the number of strands/diameter of each strand in inches.
So 7/.036 was a cable whose main conductors had 7 strands, each of which was 0.036” in diameter.
The table below shows what the sizes are when expressed as mm², and what the modern replacement cable would be. If in doubt, go up in size rather than down.
|Cable size||Strand dia in mm||Strand csa in mm²||Total cable csa||Nearest equivalent|
Tables for current-carrying capacity and voltage drops of Imperial-sized British cables from the IEE Wiring Regs. 14th Edition, 1966.
Table 3: PVC sheathed twin & multi-core, copper
Table 9: Butyl & silicone twin & multi-core, copper
For comparison, here is a table from the previous Regs. This is taken from the ASEE Guide to the 13th Edition, 1955, as amended to December 1963:
Table 13: Twin & multi-core rubber/polythene/PVC insulated cables