Re-Wiring

Does your property need a full rewire?

As mentioned in the ‘replacing consumer units’ section, the single best thing you can do to improve the safety of your electrical installation is to make sure you consumer unit is up to date.  So if you suspect you may need a re-wire, then contact an electrician and ask them to take a look at your consumer unit first.  Even if you do need a re-wire, a short term solution is to replace your consumer unit.  This will not mean you don’t now need a re-wire but it may buy you a little time to save some money etc.  As part of any works to replace you consumer unit the electrician will be able to tell you if indeed you do require a re-wire.

The Main Reasons for Re-wiring;

Most people expect to need a rewire every 30 years or so.  Whilst this may be true it is not just the age of the wiring itself which points to the number 30.  It can depend on many other factors;

  • The type of cable used

Traditionally, in domestic properties, what electricians refer to as ‘Twin & Earth’ cable is used.  This is two conductors to carry the load which are separately insulated with an earth conductor in the centre all wrapped in an outer sheath.  It is usually grey, but can be other colours, and flat in profile.  It is available in many sizes and configurations depending on which circuits it is to be used on.  Modern twin & earth cable contains blue and brown coloured conductors.  Older twin & earth cable contains Black and red cable.  Because we know that this change was implemented as part of the 3rd amendment to the 16th edition wiring regulations in 2004 (Now up to the 18th edition amendment 1), if you have any older black and red twin & earth then we can safely say that your wiring is at least 16 years old.

Going back further, there were many other types of Twin & Earth cable used.  Imperial Twin & Earth cable was used up until the changes made to the 14th edition of the wiring regulations which was amended in 1970 to introduce metric sizes.  So if you have any of the following types of cable we can safely say your installation is at least as old as 50 years;

– 7/029 T&E – 7 strands of 0.29 inch guage cable [Mainly used for sockets]

– 3/029 T&E – 3 strands of 0.29 inch guage cable [Mainly used for lighting]

– 1/044 Unstranded – This was used for two way lighting circuits and was double insulated

As well as the conductor sizes the type of material on the outer sheath also varied.  It is now predominantly made out of PVC for standard cabling but Ashathene and Rubber were both previously used.  Whilst the Ashathene sheaths lasted a long time the rubber did not.  ANY rubber cabling in your installation is a sure sign you need a rewire as a matter of urgency. 

Whilst all of the above types of cabling can be used to try and date your wiring, it is still not the case that if it is older than 30 years it needs to be re-wired, although it would be expected that more frequent inspections were made to the installation in order to mitigate the risks of the older cabling.  It does however, give a good insight into the condition of the installation generally.  It is also the case that some of the older types of cabling did not always utilise a separate earth conductor in the cabling.  It was often the case that some cabling contained only the two load carrying conductors.  These were mainly used in lighting but can be found in other circuits.  Because this is outside the regulations for other reasons, this type of wiring does need to be replaced, or at least a separate earthing conductor needs to be added providing a good earth to every appliance or fixture or ALL fixtures using this type of wiring must be CLASS II (no metallic surfaces).  It is often less hassle to simply re-wire the property than to have separate earths installed.

Other types of older wiring can include aluminium stranded twin and earth cables.  In this case, the aluminium needs to be far thicker than an equivalent of copper in order to carry the same load.  This is due to the conductivity and resistivity of copper versus aluminium.  As such the aluminium cable can still be left in an installation but generally it was used at a time where installations were not designed to carry as much load so caution is recommended.  At the time of use, there was a shortage of copper.  If the aluminium cores are protected with rubber then this is not the case and they should be replaced because of the rubber sheath.  It can be easy to confuse aluminium cables with tinned copper cables which were much more common.  Cloth Covered Wiring is also obsolete and definitely needs replacing. 

– Cloth covered wiring – Pre 1950

– Cables with a Lead Sheath – Used Pre 1950

– Cables with a Rubber sheath (TRS or Tough Rubber Sheath) – Used between 1943 and 1975

– PVC Cables with no CPC (Circuit protective conductor) or earth – Used between 1950 and 1965

– PVC Black and Red Twin & Earth – Used between 1970 and 2004

Above dates are approximations

  • The amount of additions or ‘add ons’ to the installation

When an installation is first completed, the sockets will be arranged in RING MAINS.  As the name suggests there are RINGs of sockets or LOOPs.  Wiring is taken from the fuse board to the first socket, then to the second, third and to all the other sockets in turn and then, crucially, a final cable is wired back to the fuse board to complete the RING. 

This is all about load carrying capacity of the cabling and the most efficient and cost effective way of wiring the circuit.  If the Sockets were not connected in a RING then the size of the cable used would need to be increased and it would be much harder to terminate the socket.  Especially if additional sockets are SPURRED from the original. 

So if the sockets are not connected in a RING but rather in a chain then we call this a RADIAL circuit.  There is no final return leg back to the fuse board. As shown below;

So these are how the circuits look when they are first installed, but over time additional sockets need to be installed or Added.  It is impractical to install a whole new circuit from the fuse board and often impractical to split the original RING and add a socket as this involves major changes to the original wiring which is now often plastered in walls and ceiling etc.  We can however, add a socket as a SPUR.  In this case we take an additional cable from a socket in the existing RING or by jointing a cable in the existing RING.  This is shown below;

This is far easier to cable than adding the socket into the RING itself but is not as good electrically.  If we imagine that every plug top can potentially use a full 13amps of power (As this is the highest fuse you can get for a plug top).  This means that a double socket can potentially use 26amps of power.  2.5mm Twin & Earth cable, used for RING MAINS, is rated above 26amps [this does depend on exactly how it is installed and the type of cable used].  So as soon as we install an extra socket on a SPUR then the maximum load required on the cable which is used to create the SPUR is 26amps.  If we now require a second Socket to be added then we cannot SPUR this from the 1st SPURRED socket as this would overload the cable used for the first SPUR.  Therefore we cannot add the socket as shown below;

Instead we would need to create an entirely separate SPUR from the RING MAIN as shown below;

So it can be seen that wiring of RING becomes very complicated very quickly once sockets are added over time.  It does not take long before an electrician cannot tell easily what is a SPUR and what is in the RING.  By mistake SPURS are then Installed from existing SPURS which once again is not allowable due to the maximum load any one cable in the circuit can carry.  There becomes a point when the whole circuit should be rewired.