Using existing galvanized steel conduit

[plinc]

Hi,

I want to sort out the wiring in my garage, it is currently disconnected as it was spurred off a socket in the dining room.

When we had the house rewired they said I would need to have the cable replaced to the garage and the garage re-wired too. As we could not afford the lot at this time I just asked them to disconnect it for now.

The old disconnected cable is currently running through a galvanized steel conduit, it looks like multicore 2.5mm T+E.

One electrician told us we could run 6mm from the meter to the garage and re-use the steel conduit so wouldn't I want to have to dig up the garden.

Another electrician said we should dig up the garden and run armoured cable from the meter to the garage.

Are they both correct and it's more personal choice ?

Many thanks for any help.
Pat.

PS I won't be doing the work I just want to know what the options are.

 

[ericmark]

Pulling cables through conduit requires the conduit to have been installed correct to start with. All too often the conduit is basically assembled around the cables rather than pulled in.

Also twin and earth does not like being fed through conduit with singles no real problem however often it needs a little "Yellow99" to help it slide through without damage to cables.

Also the conduit does need to be sound and not rusted through and earth or water filling the void.

Often worth trying but one has to accept that it may fail.

For an electrician to give an estimate when he has no idea if the conduit is good or bad is impossible which means you have either to have open cheque book or select a method where he can have a good idea of the cost.

If the conduit fails then you dig the trench while paying the electrician to watch or help you so to abandon the conduit and dig the trench anyway could work out cheaper if conduit fails.

So yes both electricians are right in a way. Once old cable is pulled out to pull in new behind it if any thing goes wrong then your stumped.

 

[ban-all-sheds]

All too often the conduit is basically assembled around the cables rather than pulled in.

Since you can buy pre-wired conduit (OK - flexible, not steel) in other countries in the EU, I've never understood why that's necessarily a Bad Thing.

 

[JohnW2]

All too often the conduit is basically assembled around the cables rather than pulled in.

Since you can buy pre-wired conduit (OK - flexible, not steel) in other countries in the EU, I've never understood why that's necessarily a Bad Thing.

I don't think it (assembling conduit around cable) necessarily is a bad thing if one views it as a once-only exercise. However, if one has hopes of being able to pull new cables through the conduit in the future, the fact that it has been satisfactorily done (after any necessary re-adjustment of the conduit!) at initial installation raises the chances/hopes that it will work when one tries to re-do it in the future.

Kind Regards, John

 

[ban-all-sheds]

Agreed, but the regs are a "must" on this, IIRC.

 

[JohnW2]

Agreed, but the regs are a "must" on this, IIRC.

Yes, I have a feeling that you're right (but who will ever know? ). Maybe the regs fear that assembling conduit around a cable might damage the cable? (I would personally have thought that pulling it through would probably be more likely to do that!).

Kind Regards, JOhn

 

[EFLImpudence]

I'm sure the regulations are more concerned with commercial and industrial installations for future maintenance rather than burying something in the garden.

 

[JohnW2]

I'm sure the regulations are more concerned with commercial and industrial installations for future maintenance rather than burying something in the garden.

Yes, I imagine that you're probably right. As I said a couple of messages back, I'm sure that pulling the first cable through (and doing any 'snagging' of the conduit installation at that time) maximises the chances that subsequent 'pulling through' in the future will work. Whether that's necessarily a 'safety' issue (which is largely what the regs are about) is perhaps another matter.

Kind Regards, John

 

[RF Lighting]

I'm sure I was taught it is to prevent damage to the cables if you're trying to assemble conduit around them.

 

[plinc]

Thanks for all the responses.

I haven't received the quotes as yet but I'm tempted to go with the armoured cable

If I dig the trench and remove the steel it should reduce the quote a fair amount. I did ask for an itemised quote for that reason.

The only thing I had against the armoured is routing it through the house is going to be a real pain.

Is it possible to swap to 6mm twin and earth once in the house?

 

[JohnW2]

Is it possible to swap to 6mm twin and earth once in the house?

Yes, it's certainly possible to change to T&E once in the house.

Kind Regards, John

 

[plinc]

Great thanks.

I've been doing a bit of research, coz I'm tempted to suggest I do all the donkey work - route all the cables and dig all trenches. Then ask for another quote to do the connections and provide the paper work.

I was told by them both I'd need 6mm SWA or T+E as it was a long run, 25m in total from the CU. This was to supply one light and two 13A sockets - these would be used for things like an electric drill or router, no kettles or electric fires.

So if the T+E is 6mm and the SWA is 6mm, would it be safe to join these as the earth in the 6mm T+E is only 1.5mm and it's 6mm in the SWA.

 

[plinc]

I wish I was home when the sparks came to measure up, I have more questions now than before they came.

I may get another quote and make sure I'm home and prepared.

After more digging it seems I may be able to use the existing 2.5mm cable running through the steel conduit.

If I have it spurred off the ring main with a 13A switched fused connection unit, then have a double polled switch in the garage feeding a socket and then on to another SFCU to a light fitting, this would be ok with regs and cheap as chips.

The only part I need to check is should the steel conduit be earthed at both ends, and can you just use a standard earth strap and take it t the SFCU's ?.

Sorry for the barrage of questions but some less scrupulous sparks will tell you that you need so much when you really don't.

 

[ericmark]

Volt drop, Current carrying capacity, and earth system all have to be considered and at 25 meters likely the electrician would use a TT earth rather than export from house but this is a personal thing.

The problem with any metal in the ground is electrolysis and with boats we use sacrificial anodes made of zinc to stop the steel being eaten away and so likely the earth rod would need to be zinc coated not copper coated to stop erosion.

Using the house earth would depend on the earth already used in the house both for the erosion problem and where the conduit is exposed clearly you would not want the two earth systems to be able to be touched together.

There is a method used with boats where a pair of diodes are inserted into the earth to give a 1.2 volt difference in earth voltage and using one of these between conduit and earth rod may be the answer but they are not cheap.

Again down to the electrician doing the paperwork to what he would accept.

With SWA the erosion is not a problem so either TT or exported TN earth could be used without considering if the tube will be eaten away. Depending on what earth was used to start with the tube may already be eaten away.

 

[plinc]

Hi Eric,

It would be 25m to the consumer unit from the garage, it's only 10m from the garage to the downstairs ring - so I guess the volt drop for a 13A spurred socket and a 5A light fitting would be fine.

I understood, perhaps wrongly, that the steel conduit would need to be earthed.