Wiring

[adwt2004]

This is my electric set up that i'll inherit when i move house. I recognise most the components but the brown box on the left flummoxes me, it has 5VA written on it. Any ideas chaps?

The wiring in the house is the pvc type, not rubber thank god. but the lights will need earths (The light switch i checked had no earth in it.) Can i continue to use this wiring, it has thin strips of steel which are coiled together so it is not copper.

I'm going to get the consumer unit changed for an mcb style one too, not rcd so is this possible?

Untitled

• adwt2004
• 12 Dec 2008
• 2

Untitled

• adwt2004
• 12 Dec 2008
• 1

 

[RF Lighting]

It's a doorbell transformer.

If you have no earth to the lights, and tinned copper wiring, then you really need to bite the bullet and have the place rewired.

The lights will rewiring anyway if you want to have any metal switches or light fittings.

With wiring of that age, you will find there are no way near enough sockets for modern needs.

You also have no idea who has been tinkering with the wiring. The 45A MCB in your fusebox is an immediate sign of bad installation.

Don't wast your time and money fitting a new fusebox.

You have not yet decorated or furnished the house, so now is the very best time to have the work done.

 

[ban-all-sheds]

And if you can have it done before you even move in, so that the electrician doesn't have to spend his time, and your money, restoring/maintaining power, replacing floorboards each night, moving furniture about, sweeping up etc and can use power tools to chase the walls, then it'll take him less time, and in theory he should therefore want less of your money.

 

[conny]

"and in theory he should therefore want less of your money."

What a wonderful thing 'Theory' is!

 

[adwt2004]

The 45amp mcb was added by an electrician, a good one who's worked with me before. The cable is correctly sized too and new colours so that's fine. The lights shouldn't be too bad to alter, agree on a rewire for those due to wanting metal sockets (missus wants em!). If the tinned copper wiring is in good nick, which it is, why change it elsewhere? The sockets are all earthed etc and copper pipework is bonded, will be a nightmare to alter the two rings.

Makes sense about the doorbell transformer. Wondered where that was powered from. It is directly linked to the CU. May remove it and get a wireless me thinks. Can a new style CU using mcbs not be installed with the following changes made above? Sockets are plenty to be honest. Only three need moving somewhere else.

Appreciate your advice

 

[RF Lighting]

The 45amp mcb was added by an electrician, a good one who's worked with me before. The cable is correctly sized too and new colours so that's fine.

No it isn't.

Those CU's are rated to a MAXIMUM of 30A per way. 45A MCBs are the very best way to set these CUs on fire. It happens all the time.

There is also no RCD to this circuit, which is gash, and depending on when it was done, breaches the regs too.

The lights shouldn't be too bad to alter, agree on a rewire for those due to wanting metal sockets (missus wants em!).

Good.

If the tinned copper wiring is in good nick, which it is, why change it elsewhere?

How do you know? Have you inspected ALL of it, and carried out insulation resistance tests?

The sockets are all earthed etc and copper pipework is bonded, will be a nightmare to alter the two rings.

It'll be even worse when its carpeted and decorated.

Makes sense about the doorbell transformer. Wondered where that was powered from. It is directly linked to the CU. May remove it and get a wireless me thinks.

It'll be connected to one of the lighting circuits.

Can a new style CU using mcbs not be installed with the following changes made above?

Possibly. Impossible to say without seeing the job.

Sockets are plenty to be honest. Only three need moving somewhere else.

Oh.

Appreciate your advice

Even though you will not take it?

 

[adwt2004]

I don't want to change all the wiring, it's a hellish job to be honest, i chased and run all the cables in my current house, i'm very proud of it but it is very hard work. If i don't need to i won't but if it all needs rewiring then fair enough, up come the floor boards and out comes the sds drill.

I didn't realise that CU only accepts up to 30amps or so. The 45amp shower fuse was off so i'll go along with that, but the electrican is timed served and did my house excellently, the previous occupant of the house i'm buying was very tight so i will speak to the electrician and get his advice too on what he thinks. The cooker circuit is 30amp and with diversity it comes out needing about 28 amps. Not that happy about that so the CU will have to be uprated to accept a second 40amp radial. Are you saying that if the cabling to the sockets is sound and tests come out ok that it can be re-used? that is the only part i am now considering keeping if i can.

 

[RF Lighting]

What size is the CPC in the 7/.029 socket cables?

Some old socket cables have an undersized CPC, and are too small for the adiabatic equation.

 

[adwt2004]

Not sure of the cable size, will have to take a look tomorrow. Is the equation you refer to in lehmans terms the wires ability to hold on to it's heat and not give off too much heat for it to cause a fire hazard, something along the lines of thermodynamics?

It's a good point about the cpc's though.

You are convincing me to re-wire entirely it has to be said. Can new cables be pulled through existing conduits? If this could be done then chasing wouldn't be needed as much. The sockets locations are fine as they are so it's only the conduit that would be a problem. It'll be metal but would that be an issue? The light switches need moving mostly due to re-hanging doors so chasing is mandatory. God knows how the wires get upstairs from the cu. It's all under the stairs. cheers for your advice so far.

 

[RF Lighting]

Not sure of the cable size, will have to take a look tomorrow. Is the equation you refer to in lehmans terms the wires ability to hold on to it's heat and not give off too much heat for it to cause a fire hazard, something along the lines of thermodynamics?

Basically you can use a CPC smaller than the line and neutral conductors as it is designed to only carry a load under fault conditions.

The CPC is sized to carry the fault tor a maximum of 5 seconds, in which time the fuse / MCB will have automatically disconnected the supply (providing the curcuit has benn properly designed).

As the CPC is only carrying the load for a very short time it can carry a big current without getting hot enough to damage the insulation of the cable (or anything else).
The adiabatic equation is used to work out the CSA of the CPC by taking into account how long the fault will last, the amount of heat generated, and the wires ability to disperse the heat.

Hope that makes sense

Can new cables be pulled through existing conduits? If this could be done then chasing wouldn't be needed as much. The sockets locations are fine as they are so it's only the conduit that would be a problem. It'll be metal but would that be an issue?

If there is any you can try and reuse it. Providing there are no nasty bends or anything you should be ok. If it is in capping you may get away with it, but it'll be difficult.
If the cables are buried straight in the plaster youre knackered. [/quote]

 

[adwt2004]

Ok mate, think it would be best to re-wire then. I have no idea how any of it is wired and like the fact i understand my circuits here at home. So there goes, 2 ring mains, 2 light circuits, two radials (cooker and shower) and a boiler spurred fused at 3amp from a local socket. Doesn't sound much here does it?!

I take it lights are 6amp, sockets 32amp, cooker and shower probably 40-45amp?

 

[securespark]

What size is the CPC in the 7/.029 socket cables?

Some old socket cables have an undersized CPC, and are too small for the adiabatic equation.

Not sure if it's 3/.029 or 3/.036. Any older sparks care to confirm?

However, RF, it's my understanding that an undersized cpc protected by a 3036 is in danger, but to make it safe all you need do is fit a B type breaker. Is that correct?

 

[ban-all-sheds]

The adiabatic equation is used to work out the CSA of the CPC by taking into account how long the fault will last, the amount of heat generated, and the wires ability to disperse the heat.

I thought that the adiabatic equation assumed that the conductors lost no heat? That's what it should assume, given what the thermodynamic term "adiabatic" means...

What size is the CPC in the 7/.029 socket cables?

Some old socket cables have an undersized CPC, and are too small for the adiabatic equation.

Not sure if it's 3/.029 or 3/.036. Any older sparks care to confirm?

I'm not an older spark, but AFAIK it was 3/.029, which is 1.28mm², i.e. "undersized", particularly as when originally used it would have been protected by a 3036...

However, RF, it's my understanding that an undersized cpc protected by a 3036 is in danger, but to make it safe all you need do is fit a B type breaker. Is that correct?

You should probably do the calculation....