When to rewire?

HappyHippyDad · Oct 10, 2024

Good morning all.

I have just done a small job, during which I noticed all the cable is imperial cable (meaning 50+ years old)

No green goo. Global IR test 0.1Mohms (L/N - E).

When do you guys suggest a rewire?

The 0.1Mohms may well just be rust/rodent damage/damp etc etc.

If I fix the fault and the IR tests are >1Mohm, I would then suggest to the customer that they start considering a rewire, rather than saying they really need one.

Is this similar to what everyone else suggests?

Murdoch · Oct 10, 2024

0.1Mohms could be the sign of things plugged in IMHO

Could you do a circuit by circuit IR to narrow down the issues?

I would mention the word rewire to them face to face and see how they react

mort2376 · Oct 10, 2024

I always say that between 40 to 50 years is the expected life expectancy for cable.

It's down to so many factors so there is no definite answer.

SPECIAL LOCATION · Oct 10, 2024

HappyHippyDad said:
Good morning all.

I have just done a small job, during which I noticed all the cable is imperial cable (meaning 50+ years old)

No green goo. Global IR test 0.1Mohms (L/N - E).

When do you guys suggest a rewire?

The 0.1Mohms may well just be rust/rodent damage/damp etc etc.

If I fix the fault and the IR tests are >1Mohm, I would then suggest to the customer that they start considering a rewire, rather than saying they really need one.

Is this similar to what everyone else suggests?

I would start by saying that you have noticed the installation is quite old and there is no label at the CU identifying when it was last inspected and tested to confirm its safety for continued use... (re: 514.12.1)

Then ask them when it was last fully tested and do they still have copy of the inspection report?

If they either have no idea or it is known to be longer than 10years...
I would be recommending a comprehensive inspection and test of the whole installation, including IR's on individual circuits, not just global IR, ASAP.. as there could be potential problems..
(Also point out this could easily be a full days labour on an average domestic dwelling with no previous certificates / documentation available.)

Once you have some accurate tests results for the whole installation circuit by circuit, you are then in a position to identify what areas are due for remedial works / replacement to ensure the safety of persons using the installation.

I would never be suggesting anything needs rewiring without first reviewing some accurate records of circuit composition and test readings.. It is not uncommon for older installations to have various circuits, (parts of circuits), with wiring that is much newer following earlier building alterations etc.. and so does not require replacement yet.

If I ever come across IR test readings in single figures on an individual circuit that tends to start alarm bells in my book!
Even on some very old wiring I have know double digit 20 or 30meg+ IR @ 500v across all conductors.

Also don't forget older installations without RCD's at the CU may well have socket outlets with RCD's built in further down the circuits which can mess up your global IR readings.

What was the IR for the circuit you were doing your minor works on?

HappyHippyDad · Oct 10, 2024

Aabsolutely

Murdoch said:
0.1Mohms could be the sign of things plugged in IMHO

Could you do a circuit by circuit IR to narrow down the issues?

I would mention the word rewire to them face to face and see how they react

Hi Murdoch.
Yes, that's the plan. Standard fault finding for an IR fault. Hopefully it's just something plugged in, or an outside light with some water in it etc.
I've already broached the subject of a rewire and they are open to discussion. I just wanted to gauge dome ideas of when you guys generally suggest a rewire when there is imperial cable.

7029 Dave · Oct 10, 2024

If you've got to chase around splitting down circuits length by length to find the low resistance on a 50 plus year cable it might be beneficial to rewire it

binky · Oct 10, 2024

it's not the age of the cable. it's the condition that matters, Cable used at full load 24/7/365 can be worn out in as little as 5 years, cable hardly used can last an awful lot longer. UK cable went metric circa 1970, that doesn't mean every house was wired in metric from 1970 onwards as no doubt people used up old imperial stock. So you really need to test the IR of the cables so ascertain just how 'satisfactory' they still are.

HappyHippyDad · Oct 10, 2024

SPECIAL LOCATION said:
I would start by saying that you have noticed the installation is quite old and there is no label at the CU identifying when it was last inspected and tested to confirm its safety for continued use... (re: 514.12.1)

Then ask them when it was last fully tested and do they still have copy of the inspection report?

If they either have no idea or it is known to be longer than 10years...
I would be recommending a comprehensive inspection and test of the whole installation, including IR's on individual circuits, not just global IR, ASAP.. as there could be potential problems..
(Also point out this could easily be a full days labour on an average domestic dwelling with no previous certificates / documentation available.)

Once you have some accurate tests results for the whole installation circuit by circuit, you are then in a position to identify what areas are due for remedial works / replacement to ensure the safety of persons using the installation.

I would never be suggesting anything needs rewiring without first reviewing some accurate records of circuit composition and test readings.. It is not uncommon for older installations to have various circuits, (parts of circuits), with wiring that is much newer following earlier building alterations etc.. and so does not require replacement yet.

If I ever come across IR test readings in single figures on an individual circuit that tends to start alarm bells in my book!
Even on some very old wiring I have know double digit 20 or 30meg+ IR @ 500v across all conductors.

Also don't forget older installations without RCD's at the CU may well have socket outlets with RCD's built in further down the circuits which can mess up your global IR readings.

What was the IR for the circuit you were doing your minor works on?

Thanks for that comprehensive and interesting answer. I shall take some of your points on board.

I'm away from the test results at the moment, but I remember being completely happy with the IR results for my circuit, somewhere between 50 - 100M ohms.

7029 Dave · Oct 10, 2024

binky said:
it's not the age of the cable. it's the condition that matters, Cable used at full load 24/7/365 can be worn out in as little as 5 years, cable hardly used can last an awful lot longer. UK cable went metric circa 1970, that doesn't mean every house was wired in metric from 1970 onwards as no doubt people used up old imperial stock. So you really need to test the IR of the cables so ascertain just how 'satisfactory' they still are.

1970 was still imperial cables went metric around 1976-77

binky · Oct 10, 2024

7029 Dave said:
1970 was still imperial cables went metric around 1976-77

https://electrical.theiet.org/wirin...mating-the-age-of-an-electrical-installation/

7029 Dave · Oct 10, 2024

binky said:
https://electrical.theiet.org/wirin...mating-the-age-of-an-electrical-installation/

Mmm I started in 1979 yes metric was well in by then, done some work round my aunties which was a council house it was rewired around 1974 the cables were PVC but the lighting was wired in 3/029 and the sockets 7/029.
However if thats what the document says I stand corrected.

7029 Dave · Oct 10, 2024

binky said:
it's not the age of the cable. it's the condition that matters, Cable used at full load 24/7/365 can be worn out in as little as 5 years, cable hardly used can last an awful lot longer. UK cable went metric circa 1970, that doesn't mean every house was wired in metric from 1970 onwards as no doubt people used up old imperial stock. So you really need to test the IR of the cables so ascertain just how 'satisfactory' they still are.

This is a big factor

UNG · Oct 10, 2024

HappyHippyDad said:
I would then suggest to the customer that they start considering a rewire, rather than saying they really need one.

IMO there is never a good time to suggest or attempt to sell a rewire to a customer unless they have just purchased the property and haven't properly moved in, there will always be at least one or two rooms that have been recently redecorated

7029 Dave said:
1970 was still imperial cables went metric around 1976-77

7029 Dave said:
Mmm I started in 1979 yes metric was well in by then, done some work round my aunties which was a council house it was rewired around 1974 the cables were PVC but the lighting was wired in 3/029 and the sockets 7/029.
However if thats what the document says I stand corrected.

When I started in 1976 I was told by the older guy's that most things had gone metric 4 - 5 years earlier which is generally confirmed by a number of sources
With a rewire circa 1974 I would not automatically assume that the stranded cable is imperial as around that period there was a lot of aluminium twin and earth which had stranded cores, there are houses close to where I live that were built in the early 1970's that are awful to work on because they were wired with aluminium T&E which can be quite brittle now it has aged

7029 Dave · Oct 10, 2024

This was stranded tinned copper to my knowledge they never made twin and earth stranded in metric?

UNG · Oct 10, 2024

7029 Dave said:
This was stranded tinned copper to my knowledge they never made twin and earth stranded in metric?

They did back then and still do now https://www.buymaterials.com/produc...GXX5zydUOfgcHwSBuAfl41JaY9Oa8Fe7RES7Dcr9lKua1 it is quite pricey though which is probably why it is not used that much

Used loads of it in 1980/1 when a new glass plant was built locally and they had it specc'd for the office wiring every fortnight on a Friday afternoon for a couple of months I went up to AEI cables in Leigh and collected about 4000m each trip

7029 Dave · Oct 10, 2024

UNG said:
They did back then and still do now https://www.buymaterials.com/produc...GXX5zydUOfgcHwSBuAfl41JaY9Oa8Fe7RES7Dcr9lKua1 it is quite pricey though which is probably why it is not used that much

Used loads of it in 1980/1 when a new glass plant was built locally and they had it specc'd for the office wiring every fortnight on a Friday afternoon for a couple of months I went up to AEI cables in Leigh and collected about 4000m each trip

Just looked solid core cpc, all the presumed imperial cables cpc were stranded what ive seen, can you confirm.

Phoenix · Oct 10, 2024

UNG said:
They did back then and still do now https://www.buymaterials.com/produc...GXX5zydUOfgcHwSBuAfl41JaY9Oa8Fe7RES7Dcr9lKua1 it is quite pricey though which is probably why it is not used that much

A lot of local authority buildings, schools especially, round here have it in, put in during the 1990, I think it was probably when they started accepting T&E in their commericial buildings and not inisiting on singles in containment, so it would pull down drops to accessories, etc. I knew it definatly was 2.5 because it was on jobs not hold enough to have imperial in them, plus I came across a part drum with the label on it once. The way to tell the difference is that it still has a solid core 1.5mm, just L+N that are stranded, where-as 7/029 had a 3/036 CPC.

I'm pretty sure the imperial metric changeover was 1970/1971 ish. Because I once did same work in a school (in a different county) that was built around then, in one point a ring was wired so far around in 7/029 w/ 3/036 cpc, then it became 2.5/1.

2.5/1 persistsed for about 10 years till 1980/1981 until they realised that it technically fails the adiabatic on a 30A rewireable (But would generally be fine on a B32 MCB or RCBO if the PEFC was below 3kA) No good on 30A Type 4 loadmasters that were from the same era as the 2.5/1 though... most of those are probably due for changing out though

Murdoch · Oct 10, 2024

I’d be looking for an RCD socket or rcd FCU as those can give off very odd readings

Fleeting · Oct 11, 2024

To suggest a rewire you must support it with evidence. My house was wired in 1971 and it is perfectly sound.

binky · Oct 11, 2024

True story time. Had a customer contact me about rewiring her smallish 3 bed semi. 2 other companies had told her she needed a full rewire costing either £12k or £14k , and this was around 8 years ago. I asked if they had done a full inspection, they hadn't. So I said we would do that and see if it was really needed. It needed a few things sorting out,a new board, and in short cost £1400 for the works in total. She gave me a 4 out 5 star review stating she thought we didn't offer good value for money. Worst review I ever got, no pleasing some folk

When to rewire?

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