Laying underground cable

[JohnD]

more than generous.

 

[securespark]

Methinks Keith is worried that installing a larger cable than is necessary, but not sure why.

 

[Pensdown]

Methinks it's because Keith is paying for it

 

[keith70]

HI
securesparkis right what are the implications of putting in a cable that is too large
i would think that putting in a cable that is larger than is needed is a much better option than putting one in that that is only just for the requirements stated
i appreciate cost would come into it but future proofing the job is a greater prioty for me rather than paying a few less quid for cable
i don't know what the cost comparision is between 6mm2 and 10mm2 SWA
perhaps someone could enlighten us
keith

 

[securespark]

securesparkis

Hey, I know I've been to Crete lately, but don't turn me Greek!!

There are no implications other than cost, really. Better to overdo it than underdo it.

There is a difference in cost:

6mm2 3 core SWA= approx 40.00

10mm2 3 core SWA= approx 60.00

per 25 metres.

 

[keith70]

hi securespark
sorry about that thanks for the reply glad i got it the right way round
keith

 

[GettinSadda]

10mm2 ?!?!?

The max current for 8m of 10mm2 SWA is 85A - do you really expect to ever use this? If so you will also need to get your supply uprated because a pound to a penny this is no more than a 100A supply and is also powering the whole house! Do it now if you really want to spend money on the off chance that you may one day set up some huge commercial electric kiln or similar!

6mm2 will do 62A over 8m and so I would think that even this gives plenty of scope for future use. Unless everything else in the installation is designed for >62A it is foolish to over-specify a single component.

Oh and if you really want to future-proof the installation lay any SWA you want, but put it in some sort of conduit so that you can pull a larger cable if required later... what's that you say? You are doing that already? Well, problem solved.

OK, now I have that off my chest I can ask what you actually did in the end - I understand that this thread is over a year old.

 

[petrolhead1]

I have a garage/shed about 80 metres from the house. Laying power cable to it is feasible in that we can dig a trench through the garden and lay a conduit but my main concerns are:

Safety : what type of sockets and junction should I use in the shed
Connection to the house supply. We have a modern circuit board which runs the lighting and power circuits of the house and has a second bank of fuses that previously ran a bank of storage heaters.

Cable gauge - my understanding is that 80metres is considered a long run and not necessarily approved is that correct or by choosing the right cable can I complete the project?

Where to find a cable supplier?

 

[briwire]

That is a long run Petrolhead1, but it can be done. Firstly you need to work out what load you will have in the garage/shed. Suppose it is lighting (1 Amp) and sockets (say 13 Amp-heater-drill). So that at any time you shouldn't be drawing more than 14 or 15 Amp. You can get a 16A Circuit Breaker for your house fuse board, but as it would be feeding a socket that could be used outside it must also have an RCCD in the your house fuseboard. You can get them combined.

Normally 2.5mm cable would be OK for 16A but there will be volt drop along your cable and this musn't exceed 4% (9.2volt at 230volt supply). I've done the calcs and you would need 10mm cable to supply a maximum of 16A for 80 meters.

You would have to use steel wire armoured cable (SWA). You would have to dig a narrow trench about 450mm deep. Remove any sharp stones s etc. Fill the bottom with 2 inch of sand, lay the cable and pack a couple of inches of sand around it and on top. On top of this, lay yellow electrical warning tape along the whole length, then fill with soil.

Inside the garage you could use a small Dis board to seperate the power from the lighting, with a 16A fuse for the power and 5A for the lighting. An alternative would be to go into a socket and spur off for the lighting but I don't think you would fit the wire into the terminations of a standard socket.

I would turn into an expensive job. The main problem being the distance involved. You would have to get the cable from a wholesaler, about £150, £30 for the RCD/MCB. I wouldn't envy you digging the trench though.

 

[petrolhead1]

Briwire,

Thanks very much for the advice. I'm very grateful.

You're right the digging is the biggest challenge. I have a strong son who is keen to dig however I also have a friend with a selection of diggers at his disposal and we could consider organising a trench digging party one weekend. Anyway I need to get fitter!

I just didn't want to tackle the monster trench dig without knowing an electrician would be prepared to connect the cable at both ends. Step 1 is to dig the trench and rather than lay cable straight away we'll put in a polypropylene rope to enable the cable to be pulled through later.

 

[Graham88]

Guys don't know if this is too late but I am an electrical engineer and in my experience; don't use ducting. A real steel wire armour cable has its own ducting, the stell armour that encases it, if your intent on using ducting then don't use SWA, a 2.5 twin and earth will be ample. Putting an SWA into a conduit will only cause it to trap in heat, which will reduce its service life. If you're competent -

1) Dig trench - min. depth: 20"
2) Lay 4mm SWA (Ample for a garage) in trench, get it in the house and under the floorboards up to your meter.
3) Get the other end in the garage, pin it up the wall and fit it into a consumer unit (3 or 4 gang is plenty, really easy to fit if you are reasonably competent in DIY)
4) You can fit the lights and sockets yourself to, very easy to get to grips with; lights should be on an independant 6A breaker, ring main on 20A or 32A breaker.
5) get a professional in to do the inspection and testing, once he gives you a cert, he can hook your project up to the supply.
6) Fill in the trench.

 

[riveralt]

Its tooo late- its dug up and filled in - three times probably by now

 

[petrolhead1]

Its tooo late- its dug up and filled in - three times probably by now

Actually its not too late. It's been delayed by lots of other projects that took priority. Thank you Graham88. I'm more encouraged to take this on at last.

 

[riveralt]

Its tooo late- its dug up and filled in - three times probably by now

Actually its not too late. It's been delayed by lots of other projects that took priority. Thank you Graham88. I'm more encouraged to take this on at last.

Hi Petrolhead1, it is unclear from Graham88's response whether he is referring to your project or the OP. I assumed he was referring to the original OP - Sorry.

While BS7671 has now moved the 17th Edition and the allowed voltage drop has increased to 5%, I'm sure that 4mm swa will not be sufficient for an 80m run more like 10mm (I haven't got my tables with me to double check)
You will already be aware this is a Part P issue and BC will need to be involved.
Graham88 is wrong regarding the inspection and testing. An electrician who is a member of a competent persons scheme cannot test other peoples work. He could supervise and monitor someone else doing the grunt work but he would be responsible for signing the certificate and I wouldn't unless I was involved in each stage of the process.

 

[RF Lighting]

Guys don't know if this is too late but I am an electrical engineer

Really??? What qualifications do you have?

and in my experience; don't use ducting.

Always always always use a ducting for ALL underground wiring. It costs virtually nothing on top of the cost of the cable, and saves so much work, time, cost and inconvenience should the cable ever need to be replaced at a later date.

A real steel wire armour cable has its own ducting, the stell armour that encases it,

No. The steel armour provides mechanical protection to the cable. It is NOT a ducting. These are entirely different things providing entirely different functions.

if your intent on using ducting then don't use SWA, a 2.5 twin and earth will be ample.

A twin and earth cable is never allowed to be buried underground, ducting or no ducting. It does not meet the requirements of regulation 522.8.10. Also, how can you possibly say 2.5mm² cable will be big enough without knowing the proposed load, or the length of the run? Also you specify a 4.0mm² later on. Why is this?

Putting an SWA into a conduit will only cause it to trap in heat, which will reduce its service life.

What utter nonsense.

1) Dig trench - min. depth: 20"

There is no depth specified. All that regulation 522.8.10 requires is that it "shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground"

2) Lay 4mm SWA (Ample for a garage) in trench, get it in the house and under the floorboards up to your meter.

How can you possibly say 4.0mm² cable will be big enough without knowing the proposed load, or the length of the run? As an example, I had to install a 16.0mm² supply to my garage.

3) Get the other end in the garage, pin it up the wall and fit it into a consumer unit (3 or 4 gang is plenty, really easy to fit if you are reasonably competent in DIY)

Is it plenty? What about glanding the SWA? Is this easy for a diyer?

4) You can fit the lights and sockets yourself to, very easy to get to grips with; lights should be on an independant 6A breaker, ring main on 20A or 32A breaker.

I presume you mean a ring final circuit? Regulation 433.1.5 specifies that all ring final circuits must be protected by either a 30A or a 32A protective device.

Also what about the mandatory RCD protection for all socket-outlets not exceeding 20A that are for use by ordinary persons and are intended for general use, as required by regulation 411.3.3?

5) get a professional in to do the inspection and testing, once he gives you a cert, he can hook your project up to the supply.

A member of a self certification scheme is NOT allowed to sign off someone elses work. You would to contact your local BCO BEFORE you start any work to enable full compliance with part P of the building regulations.

6) Fill in the trench.

Hooray, you got something correct.


Sorry but that is one of the most incorrect pieces of 'information' I've ever seen on this website, with virtually every point being incorrect.