Garage electrics

[ban-all-sheds]

Now for the cable no way to bury in the Tarmac drive

Nonsense.

How would you react if your electricity supply cable broke, and the DNO said "Sorry Mr Pease, I'm afraid you'll just have to manage without electricity from now on, as the cable is buried under a tarmac pavement so there's no way to get at it"?

... wealth and safety dept ...

Wrong attitude.

 

[ban-all-sheds]

... Buried cables, conduits and ducts shall be at sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground.

The highlighted term is crucial.

Is it to be reasonably foreseen that a gravel strip right next to a house wall would be subjected to disturbance so severe and uncontrolled that an armoured cable a few cm down would be damaged?

IMO, no, it is not a reasonably foreseeable event.

Spades? Nobody will dig in such a gravel strip. They might use a spade to scrape gravel out of the way, but they aren't going to put their boot on top of the blade and thrust it down.

It's a common sense / risk assessment exercise.

 

[Andypease1]

Said in jest . Built my own house and everything is certificated and correct. I do not break the rules just question them sometimes. Even got a cert from Thames water because my bath is larger than normal.

The reason I went on the forum was to ensure I got good advice about the rules. I said I could not bury under Tarmac

Have followed your responses for a long while and you seem to be amongst the best would value your opinion on my original posting

Cheers Andy

 

[ban-all-sheds]

Am revamping my garage electrics have installed a 4 way Consumer unit 2 x 6amp mcb ( lighting ) 1 x 16amp ( socket and garage door) 1 x 32amp (ring main 4 sockets and 1 spur.

I agree with the earlier comment about not needing a ring.

Are you really going to need more than 20A or 25A out of that circuit? If you are, I hope you've thought carefully about the distribution of the loads so as to comply with 433.1.103.

Propose a 40 amp RCBO in my house consumer unit

I think taking it off the house CU and supplying the garage via a switchfuse on the meter tails is better.

Now for the cable no way to bury in the Tarmac drive

What you mean is you want to avoid the disruption and cost of digging up and repairing the drive.

so will have to put about
75mm down in the gravel strip that rounds around the house. Can use armoured or twin and earth both in plastic conduit.

SWA laid in the gravel will be fine.

Plastic conduit would be pointless.

Steel conduit would be a PITA

T/E in conduit not a good idea.

should the cable be 4mm or 6mm2

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20070327014113/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html <- Specify 2-core cable and PVC, even if that is not what you are using or it will make invalid assumptions.
  

 

[Andypease1]

Much obliged to all who helped me

Over and out

Cheers
Andy

 

[JohnW2]

... Buried cables, conduits and ducts shall be at sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground.

The highlighted term is crucial. Is it to be reasonably foreseen that a gravel strip right next to a house wall would be subjected to disturbance so severe and uncontrolled that an armoured cable a few cm down would be damaged? IMO, no, it is not a reasonably foreseeable event.

It's hard to disagree with that, and I've already said that, if it were my house, I would personally be perfectly happy and comfortable with that arrangement, regardless of what regulations said.

However, there seems to be some inconsistency in interpretation of that regulation when people are being advised here. For example, those proposing to bury SWA an inch or three below paving stones are usually told that it's a 'no-no', even though I would again think that it was not 'reasonably foreseeable' that the cable would suffer mechanical damage. I know (or, at least, very strongly suspect on the basis of strong circumstantial evidence) that some (inherited) SWA travels 'just under' her paved patio, and I am in no way uncomfortable about that.

Kind Regards, John

 

[ban-all-sheds]

For example, those proposing to bury SWA an inch or three below paving stones are usually told that it's a 'no-no', even though I would again think that it was not 'reasonably foreseeable' that the cable would suffer mechanical damage.

Actually I can see some sense in that - when paving is removed, it (more often then not) is done with breakers/diggers/picks, so it is reasonable to foresee.

 

[JohnW2]

Actually I can see some sense in that - when paving is removed, it (more often then not) is done with breakers/diggers/picks, so it is reasonable to foresee.

I can see that in some contexts, but I think the situations we most commonly get asked about relate to patios or garden paths consisting of paving slabs loosely laid on sand, gravel or or whatever (like the cable I described), in which I case I do not really think that damage is 'reasonably foreseeable'.

Kind Regards, John

 

[mfarrow]

It could also be argued that it's reasonably foreseable that ivy may grow up a wall where SWA has been attached and then someone will have a go at it with loppers.

Cables have often been 'buried' at a depth and then covered with some form of additional mechanical protection where it can't be at the recommended 600/1200/etc depth of cover. This is especially true where cable is run over bridges or graded up to the surface.

 

[JohnW2]

It could also be argued that it's reasonably foreseable that ivy may grow up a wall where SWA has been attached and then someone will have a go at it with loppers.

Yes, that could be argued. Mind you, the DNOs (PVC/PVC singles, no armour in sight!) overhead feed to my house was installed (before my time) on the wall behind a ~100-year old wisteria that covers the back of my house, which frequently requires extensive (and sometimes 'vicious') pruning! There are various events there which I would say were 'reasonably foreseeable' but which, fortunately, have not yet occurred! Of course, DNOs do not work to BS7671

Kind Regards, John

 

[DaftPunk]

I was once called to repair a pyro which had need chopped through during an ivy trimming session.

 

[mfarrow]

At times it is difficult to think of an edition of the wiring regulations the DNOs do comply with!

To add to what was said earlier, T&E should not be left in contact with water in accordance to the product standard. For me that means not running it through conduit etc where water ingress is unknown.

Also I'm not entirely comfortable with steel conduit as mechanic protection underground as: it's usually quick to get rusty at the ends where there is no galvanising, you'll never test it for continuity as it doesn't have to act as a cpc therefore you'll never know if it's still in reasonably one piece, and it's not as thick even as the 3mm plating recommended to protect cables buried in walls. On the surface it's fine but underground I'd prefer extra depth or cover with something less susceptible to weathering.

 

[bernardgreen]

The DNO seem happy with 11 Kv cables being less that 12 inches below the finished surface of the foot path of a bridge that is being re-built.

Untitled

• bernardgreen
• 20 Oct 2014
• 1

Untitled

• bernardgreen
• 20 Oct 2014
• 1

 

[ban-all-sheds]

Presumably (IMO reasonably) they think it unlikely that anybody will be hacking at the footbridge without making proper enquiries about concealed utilities.

 

[bernardgreen]

It is not just a foot bridge, it is a two lane road over a railway line.

It would take a very unlikely ( but not impossible ) accident to damage those cables after the bridge is completed.