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Powering an outbuilding


 
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wjm

from United Kingdom

Joined: 23 Jan 2003
Posts: 26
Location: United Kingdom

PostPosted: Wed Sep 03, 2003 2:37 pm Reply with quote

I am at the planning stage for an outbuilding which will be about 15/20m from the house.


What is the best way to get power to the outbuilding?


Use a conduit with wire running through it? I have looked for outdoor conduit but haven't found any. The other alternative I have seen is armoured cable, but I want other cables to go there as well (such as phone) so the conduit method would be preferable.


Any ideas?


Thanks in advance icon_biggrin.gif
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alexnic

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PostPosted: Wed Sep 03, 2003 3:35 pm Reply with quote

SWA is the best option but this could be run thro' heavy duty plastic tubing (not conduit). very flexible usually in a roll. various diameters.That way you can run the phone cable in as well.
Possibly be able to get it in a builders yard or similar
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breezer

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PostPosted: Wed Sep 03, 2003 5:59 pm Reply with quote

steel wire armoured cable is best since it is already mechanically protected, but it should really be installed by an electrician, as it requires glands at both ends.

You can not run low voltage cables (telephone) in the same conduit as mains cable, as you may get interfearance or worse if one some how gets in touch with the other.

best get an electrican, but there is nothing stopping you from digging the trench, which saves on labour costs
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paulfromswindon

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Joined: 01 Sep 2003
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PostPosted: Fri Sep 05, 2003 3:36 pm Reply with quote

Hi wjm,

I agree with the other guys, SWA is always the cable of choice. You can run cables in a conduit, but is is impossible to guarantee that they are (or will stay) waterproof. A DIYer could terminate SWA but it's important to get it right as the armour acts as an earth, either just to protect the cable or the whole installation (see below).

brezzer is right, you can't run phone wire with electric cable. You can buy external grade phone cable (it's usually grey or black), and you could put this in a conduit, or even clip out down the fence. Only use internal grade (the white stuff) if you can be sure it won't get wet.

The size of the SWA needs to be judged based on what you are putting out there, but also has to take into account the distance (from the supply within the house to the outbuilding, not just the distance between buildings).

I would add a few things, though - I would at least get some advice from a sparky, becuase there are issues with this type of work. The external installation should be protected by a 30mA RCD as well as the appropriate fuse or circuit breaker.

In addition (and please be careful how you read this), it's sometimes the case that you do not use the earth from the house, but instead use a local Earth spike outside the garage, coupled with an RCD inside the garage. If you don't understand what this means, please don't guess! This scenario tends to apply when the outbuilding is more than a few metres from the house, or if your house has what's known as a PME supply (it should say on a yellow label by the meter if you have). It's because the earth connection in your house may be different from the earth you are standing on in your outbuilding (ie there could be a voltage difference, so you could actually get a mild shock by touching an earthed socket). I can explain in more depth if you want.

You could consider a compromise - get a sparky to wire the cable out, install the RCD(s) / Earth rod and put a consumer unit in the garage, then you wire the garage.

If you need more advice on this, please feel free to ask.

Cheers

Paul
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wjm

from United Kingdom

Joined: 23 Jan 2003
Posts: 26
Location: United Kingdom

PostPosted: Sun Sep 07, 2003 5:13 pm Reply with quote

Cheers for the help guys,

SWA is the way forward then.

I've been looking around on the net for the plastic tubing to run it in and have had no luck. I will try a builders yard when it gets closer to build time. What about running it in ABS waste pipe using solvent bends? Any thoughts?

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breezer

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PostPosted: Sun Sep 07, 2003 5:26 pm Reply with quote

what you are looking for is called "conduit" you want the plastic version, but pastic 40mm "push fit pipe" is also good, but use 2 x 45 degree bends not 90 degree bends as you will never get the cable through.

Incidentally, you don't have to use pipe at all, swa cable can be buried directly in the ground, but it would be a good idea in case you ever wanted a bigger cable, you could atatch the new one to the old one and pull one out and the new in at the same time, but i doubt if you will ever need to (squirt of washing up liquid does wonders for reducing friction as you pull it through, that said you could put the pipe on the cable, assemble the joints then bury it)
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wjm

from United Kingdom

Joined: 23 Jan 2003
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Location: United Kingdom

PostPosted: Sun Sep 14, 2003 10:51 pm Reply with quote

Cheers folks,

This seems an obvious question and I think I know the answer but I thought I'd make sure.

When choosing the SWA cable it should be 2 core with the shield acting as the earth, right? I don't need 3 core with one of the cores being earthed as well as the shield, do I?

Also, paulfromswindon, the power supply is not a PME supply, however would it be prudent to use an earth spike as well? Would it be ok to connect this to the earth coming from the house? or would it be overkill? I don't think it would be dangerous or am I wrong?

Thanks for all your continued help and advice icon_lol.gif
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breezer

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PostPosted: Mon Sep 15, 2003 5:14 am Reply with quote

some say that you should have an extra core and use this as the earth, but when i was taught (lots of moons ago) you use the wire armour as the earth, so long as it has a gland that is connected properly at both ends (and a good connection to earth) it will be fine
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wjm

from United Kingdom

Joined: 23 Jan 2003
Posts: 26
Location: United Kingdom

PostPosted: Mon Sep 15, 2003 7:14 am Reply with quote

Thanks breezer, that's what I'd hoped icon_smile.gif

Anyone any thoughts on the earth rod issue? I think paulfromswindon has gone on holiday icon_cry.gif .
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alexnic

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PostPosted: Mon Sep 15, 2003 7:38 am Reply with quote

As your house is not a PME installation, I personally would take your earth from the connection at the board. There will probably be an earth block beside the CU
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paulfromswindon

from United Kingdom

Joined: 01 Sep 2003
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PostPosted: Tue Sep 23, 2003 10:34 am Reply with quote

Hi Guys,

Just to put my bit in.

Breezer is correct, you don't have to use SWA with an internal earth conductor, but I would always do so (thereby using both the armour and internal conductor). Copper is always the best conductor, and is easier to test (you have to test steel earths with a tester that pumps 25 amps down the steel earth). For the extra few quid that is would cost to get 3 core SWA it'll make for a safer job. Still use use the armour as well though cos it has to be earthed.

If you do decide you are going to use an earth spike, it's irrelavent cos you will only earth the armour at the house end, not the shed end, cos you'll be using a local earth.

Have fun

P
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spark

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Joined: 19 Sep 2003
Posts: 189
Location: United Kingdom

PostPosted: Tue Sep 23, 2003 8:51 pm Reply with quote

I agree that 3 core is probably the best bet for you but, as breezer said, there's nothing wrong with just using the SWA as the protective conductor provided you've done the calculations. Often the cross sectional area of the SWA is significantly larger than a copper core (though copper is better conductor than steel). For example the SWA of a 6mm 3 core cable has the cross sectional area of 23mm icon_exclaim.gif . Another point worth noting is that the test Paul refers to is known as a 'high current test'. I have never carried out a high current test on an SWA gland and nor have I known any electrician to do so. Only once have I used such a test and that was on a 1000amp rising busbar to ensure the integrity of the connections all the way up the bars. Using the three core would mean there are no calculations to be done, your earth loop reading is more likely to comply and if you're not familiar with terminating the glands, you're making absolutely sure you have a high integrity earth.
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the-shack

from United Kingdom

Joined: 29 Sep 2003
Posts: 11
Location: Nottingham,
United Kingdom

PostPosted: Mon Sep 29, 2003 10:08 pm Reply with quote

wjm wrote:
Use a conduit with wire running through it? I have looked for outdoor conduit but haven't found any. The other alternative I have seen is armoured cable, but I want other cables to go there as well (such as phone) so the conduit method would be preferable.


Any ideas?



Hi

I have done this kind of thing before.

Depending on the number/size of cables needed I would use PVC waste water pipe.

I would also run 2 lengths of pipe. 1 for the AC/Mains and one for the low voltage stuff. The large bore pipes makes it easy to pull cables through. Use 2 45deg connectors and not 90deg connectors.

Finally put some rope through the pipe to allow attaching on the cables to pull through. When you do pull a cable through also pull through another rope for use in the future.

Another good method of protecting buried cables is garden hose.

Hope this is of use icon_biggrin.gif

Ray
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Doward

from United Kingdom

Joined: 03 Oct 2003
Posts: 1
Location: United Kingdom

PostPosted: Fri Oct 03, 2003 9:38 am Reply with quote

In a similar vain to wjm's shed - what would the differences be if the supply to the house IS PME (TN-C-S).
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