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 |
|---|---|---|---|---|
| 1/.044 | 1.118 | 0.981 | 0.981 | 1 |
| 3/.029 | 0.737 | 0.426 | 1.278 | 1.5 |
| 3/.036 | 0.914 | 0.657 | 1.970 | 1.5/2.5 |
| 7/.029 | 0.737 | 0.426 | 2.983 | 2.5 |
| 7/.036 | 0.914 | 0.657 | 4.597 | 4/6 |
| 7/.044 | 1.118 | 0.981 | 6.867 | 6/10 |
| 7/.052 | 1.321 | 1.370 | 9.591 | 10 |
| 7/.064 | 1.626 | 2.075 | 14.528 | 16 |
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
