Contractors will undoubtedly during their work, encounter cables in dwellings that are of considerable age and differ considerably to the modern flat PVC cabling now in general use.
Whilst the earlier rubber sheathed cables or vulcanised India rubber unsheathed cables will be instantly recognisable as of considerable age and in almost certain need of renewal, it is often the PVC sheathed cables dating from the 1950s and 1960s which may not always be recognised for what they are.
The British Standards Institution instigated a phased changeover from imperial to metric cabling, between the end of 1968 and late 1972. Trade in British electric cables to metric standards began in January 1970.
Due to the longevity of PVC as an insulating material many such pre-1970 imperial cables are still in use today and most may provide many more years service. The challenge for the contractor is to be able to identify imperial cables where necessary and be in a position to assess their suitability for any continued or modified usage.
Good practice when identifying such cables for re-use would be to clearly identify on any test certification or contract documentation. Bearing in mind the likely age of such cables should have a bearing on any suggested inspection or re-testing frequencies being made. Best practice is to renew such cabling during project work.
What to Look For
Imperial PVC sheathed cables used for domestic work, may be single stranded for the smallest size but in most cases are multi-stranded, having predominantly either 3 or 7 strands per conductor. Another distinguishing feature is that such cables – especially those made before about 1967 - when stripped back at terminations will often have a tinned copper (silver) appearance rather than the usual copper colour.
(These tinned copper conductors should not be confused with the solid aluminium conductor twin and earth cables, which made a brief and unwelcome appearance during the early 1970s!)
Externally the imperial cables may look similar to newer metric cables but due to the stranded construction of the conductors, the overall ‘feel’ of the cable will be more flexible compared to its stiffer metric counterpart. Cables marked ‘BS2004’ on their sheath will almost certainly be of imperial size.
Long-disappeared manufacturers’ names on cable sheathing such as ‘Greengates’, ‘Hackbridge Cables’, ‘Johnson & Phillips’, ‘Enfield Standard Cables’ often indicate cables dating from the 1960s.
What to Consider
Due to the different make up and cross sectional area of the imperial conductors within the older imperial cables, it should be recognised that the cables’ current carrying capacity and volt drop characteristics are different to their modern metric counterparts.
Therefore contractors must be fully satisfied that if re-using existing imperial cables they are still able to meet BS7671: 2008 with regards ratings and characteristics.
The table below can be used to compare imperial cables to their metric counterparts:
||Current Rating in 1966 IEE Regulations
(table 1: page 132 14th edition)
|Actual size in metric terms
||13 amps clipped direct
||0.97 mm sq
||16 amps clipped direct
||1.29 mm sq
||20 amps clipped direct
||1.94 mm sq
||25 amps clipped direct
||2.90 mm sq
||33 amps clipped direct
||4.51 mm sq
||41 amps clipped direct
||6.45 mm sq
||52 amps clipped direct
||9.35 mm sq
||67 amps clipped direct
||14.51 mm sq
It should be noted that the referencing given to the imperial cables relates to the number of strands and the diameter of each strand in inches. For example the 7/0.029 cable (often used for ring circuits) had seven strands each of 0.029 inch diameter, for the line and neutral conductors.
As with modern metric cables the cpc (if provided) was often correspondingly smaller.