Estimated Age Of Wiring System

Estimated Age Of Wiring System

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When completing electrical inspections as part of an Electrical Installation Condition Report, one of the boxes asks for the estimated age of wiring system.

Whilst this may seem like an easy, quick check, it can often be more difficult to accurately estimate the age of the wiring than you would imagine.

Dating old wiring can be made difficult due to the fact that different parts of the installation are often updated independently of other parts.

For example, it is common that the consumer unit or fuse board is often newer than the actual cables themselves. This is due to the fact that these items are usually upgraded before the wiring would be considered to be due for upgrade.

Dating Old Electrics

The first thing to really notice when you are working out the estimated age of wiring system is that neither the service head, nor the fuse board/consumer unit are a reliable indicator of age.

The best clue to the age of the wiring is the cable itself. This is the main bulk of the wiring system and gives the best indicator of age.

When dating old electrics always inspect the cables installed at the property and use them as your first clue to the age of the installation.

Other items such as the consumer unit, earthing & bonding and accessories can also be used to give indications of age. You can use these together with the information gained from inspecting the cable to help narrow down a window of the age.

Age of Old Wiring & Cables

The cable itself gives a number of clues about it’s age. This can be in the form of colours, or also in the type and construction of cable.

The most obvious modern cable is PVC Twin & Earth. This contains 2 equal sized insulated conductors (Live & Neutral) and a bare central CPC which is slightly smaller (in line with adiabatic equation).

First up is the colour of the insulated conductors. Blue & brown insulated conductors for L&N were first introduced in 2004. There was a small crossover period where stocks of red & black T&E cable were allowed to be used up.

Prior to this, Live was red & Neutral was black in UK installation wiring. If you come across black and red cables, you can be sure that they are at least 15 years old by now.

Going back further still, before metric cable, T&E wiring was imperial sizes. This is easiest to identify at sockets where instead of 2.5mm solid copper conductors, there are 3 strands of ????? copper for each conductor.

This looks similar to that in the picture below. This particular cable dates from before 1971. Therefore this cable is at least 49 years old!

This type of cable is nearing the end of it’s life. IF you are inspecting an installation with this type of cable, be sure to pay particular attention to both insulation resistance values & also loose conductors as the conductor becomes softer.

Estimated Age Of Wiring System - At Least 49 Years!!
Estimated Age Of Wiring System – At Least 49 Years!!
how old is my wiring - pre wired conduit
Estimated Age Of Wiring System = 40 years

Another type of cable you may come across is ‘pre wired conduit’. This was popular in the 70’s and was often used in concrete council flats.

It is basically PVC wiring within a tougher, outer sheath, often coloured to indicate it’s particular size and intended application.

Again, this type of cable indicates that the estimated age of wiring system is 40+ years old and thus should be given careful consideration for upgrade as it is pretty much at the end of it’s life.

Insulation resistance testing and careful visual inspection will reveal if the cable is safe for continued use.

This is not one that crops up very often anymore, it has usually been upgraded by now during the course of refurbishment works.

When trying to work out the estimated age of wiring system, be sure to check that the cable you are checking makes up the bulk of the installation. We have come across many instances where modern cable has been used to install a single circuit over the years, however the rest of the installation may well be 30-40 years old.

More modern requirements (such as the need for RCD protection of cables) has stopped this practice to some extent, it has happened plenty of times in the years gone by. However, a small number of homeowners (or even electricians!) fail to follow the latest regulations and will install a new circuit to an old BS3036 fuse board.

Earthing & Bonding

Both the earthing & the bonding can give some clues when dating old electrics.

Unfortunately this does not give a definitive answer as both the earthing and bonding can often have been upgraded over the life of the installation.

That being said these do give a couple of clues which can be used to narrow down the age of the installation if combined with a few other observations from this guide

The number one thing to look for with earthing and bonding to give clues to be able to date the electrical installation is whether the conductors are either green or green & yellow.

  • Pure green earthing or bonding conductors date from before 1977
  • Green & Yellow earthing and bonding conductors date from 1976 onwards. It is worth nothing that older installations could have green/yellow cables as they have been upgraded over the years. This could leave the rest of the installation older than the colour of the earthing cable suggests.

Other clues which may give you some ideas of the timeframe it was installed include the presence, or lack of equipotential bonding.

Total absence of main protective bonding conductors generally predates 1966, although we have seen examples of much newer installations with simple poor workmanship which have been missing the correct bonding.

Note this method is not very reliable either as bonding can have been removed inadvertently during building works, works can have taken place to renew other sections of cable but the bonding can be the same as original.

Because of these reasons, we don’t recommend relying on the bonding conductors as an accurate way of telling the estimated age of wiring system

Fuse Board / Consumer Unit

When wanting to know the age of older electrical systems, it is possible to gain some input from the age of the fuse board / consumer unit.

It should be taken into account, however, that the main board can often have been upgraded since the installation of the actual wiring system.

For this reason you should never rely solely on this as an accurate method of dating the electrics.

Estimated Age Of Wiring System - Between 30-40 years
Estimated Age Of Wiring System – Between 30-40 years

Main Service Head

This part of the installation is possibly the least reliable in terms of dating the age of the wiring system.

Main service heads can have been in place for many, many years and still provide a reliable service. The installation could have been rewired more than once and still be on a fairly old head.

Conversely, if there has been an issue with the supply network, the distribution network owner (DNO) can replace the service head with a newer model without any of the installation having been altered.

For this reason alone, we don’t recommend using this when estimating the age.

For reference purposes however, we will give an idea of the age of certain types of main service head

Double Pole Fused, Cast Iron Service Head – fitted approx 1940s – Not a great indicator of the estimated age of wiring system

We’ll be updating the ‘Estimated Age Of Wiring System’ post over the coming weeks. If you have any pictures of any particularly old wiring then we’d love to share them with our readers!

2 Responses

  1. Hi, the image of wooden frame consumer unit is older than the ‘30-40 years’ indicated. Suggest this is more like 50 years as a minimum.

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