Outdoor Power Shopping List!!

[Limbo]

Hi everyone. I'm just after some advice and knowledge about the current predicament that I'm in! I thought I'd found a good sparky who said he was certified etc and he turned up a few weeks ago to start the job. He has now disappeared from the planet as far as I can tell and he hasn't even really started the job yet.

I've now found someone else but he can't start for a few weeks so I thought I would carry on where John Wayne left me and get as many of the bit's and pieces together for the new chap (saving money at the same time) as I can.

Here's the deal:

I am having a workshop put up in my driveway in 2 weeks time and I want to have the power run from my house down to it for when it arrives. I want two double sockets in it and two strip lights. I also want to have a security light on the front of it and 7 garden lights up the path.

I am a competent DIY'er who has done a few electrical projects in my time but never outdoor work. So far I have dug a trench from the side of my house down to the driveway which is on avereage 18" deep. I'm using 2.5mm triple core armoured cable.

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• Limbo
• 23 May 2010
• 1

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• Limbo
• 23 May 2010
• 1

The only thing the previous sparky did was drill a hole through the wall and start to bury the cable down the side alley. He said he could use the empty space on the CU so drilled through the wall near that.

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• Limbo
• 23 May 2010
• 1

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• Limbo
• 23 May 2010
• 1

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• Limbo
• 23 May 2010
• 1

This unit is connected to this RCD

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• Limbo
• 23 May 2010
• 1

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• Limbo
• 23 May 2010
• 1

How the job is currently is that the cable is attached to the wall of the patio and is now ready to go into the trench that I have dug down the garden and to the drive.

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• Limbo
• 23 May 2010
• 1

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• Limbo
• 23 May 2010
• 1

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• Limbo
• 23 May 2010
• 1

Ideally I want to have a switch in the porch to turn the path lights on/off

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• Limbo
• 23 May 2010
• 1

Here are my questions finally!!

What is the best combination of rcb's/rcd's, as in should I have a CU down in the workshop/run everything from the existing CU etc.

Can I come off of the armoured cable as it goes past the porch (for my garden lights and switch) or will it have to come from the workshop back up the garden path to the porch (in which case I won't fill in the path trench yet)

Anything else I need to be aware of etc.

I have probably over simplified this post but I just want things to be clear. As I have said before I have a good knowledge of electrics but want to make sure I'm doing this right. You may be thinking "why doesn't he wait for the sparky to do it all" which is a fair point. I am however a person who likes to learn and know how things work. I also want to be prepared with as much information before the next sparky starts as I can. I would love to be able to go and buy everything I need before he starts and save myself some money.

A long post I know but hoping someone can be bothered to read it and any help greatly appreciated.

Regards

Craig

 

[ColJack]

hmm sure.. we all believe you..
it's a pandemic that all these sparkies mysteriously dissapear for no reason.. we ought to notify the authorities.. there may be a conspiracy here....

anyway..

your "original sparky" was a cowboy.
I think that SWA is too small, you might need 4mm for that length of run and it's too short at the CU, there's not enough length there to terminate it into the CU properly I don't think..

MCB at the house and a small CU at the shed with an RCD in it.

the garden lights will need it's own cable ( so that 2.5mm won't go to waste ) as there's no way to switch them from the porch if they are fed from the shed.. ( well there is but you're getting into RF switches and it gets complicated ).

 

[Limbo]

Wasn't sure if I'd get any kind of reply so much appreciated.

As for 'The Cowboy' amazingly it looks to be true. I've read plenty of posts on here where I've also thought...."mmmmmm sure" but I can only be honest with you all and tell you how it is. I hope that's enough. I may be wrong about him but we shall see.

I queried the cable when he arrived with it as I was expecting something a little more 'Heavy Duty'. The run is 38 metres so it's quite a length. Just out of interest regarding the garden lights would it be better just to come off of the house (there is a socket in the porch already) and just treat that as a seperate job to the workshop? My other thought is to link them with a PIR at the garden gate and run everything from the workshop. Can't have a switch in the porch I know but could be an option?

Anything else from anyone??

 

[plugwash]

Designing a power system involves the following.

First work out how you want things connected, what breakers you will want etc. Personally I'd suggest specing for at least 32A to any outbuilding, that way if you want to add a heater or similar later you will have the capacity to do it.
Secondly work out what cable sizes you will need (considering volt drop, disconnect times cable rating etc). Remember if you have connections feeding off each other then you need to control the TOTAL volt drop and the overall loop impedance (generally volt drop is a bigger issue than loop impedance though if you are using type B breakers). Going a bit larger than calculated is not a bad idea to allow for future changes.

I reccomend using 3 core SWA. You can use the armour as the only earth but this will give you higher loop impedances and can lead to a compromised earth if the cable is damaged.

 

[mikhailfaradayski]

Probably not the help you are looking for but,

im not so sure buying everything yourself in preparation will actually save you any money. Even if the next 'sparky' makes a mark up on his trade price to you, it'll probably still be cheaper than you could get it anyway.

I wouldn't bother installing that cable just yet, as it may or may not be suitable. Someone here might kindly offer to calculate EFLI and volt drop etc. If you give them a bit more info. Also, Lord Lucan from 'elusive electrical' may decide that the garage should be TT'd, but if 'lochness technical' does ever turn up, its him who will make the call, no-one else and you may find you have done al the leg work for nothing.

Apologies in advance if you are that '1', but i would estimate that only 1 in 10 people who post the old 'sparks gone on holiday/to hospital/the moon/scunthorpe' have actually spoken to a spark at all.

 

[nozspark]

If you've worked out how much capacity you need at the shed then you'll need to work out the voltage drop ---- see link

http://www.doncastercables.co.uk/cableCalculator.asp

as you will have lighting you'll need to work to the 3% allowance

 

[JohnnyG]

From what I can work out standard buried 2.5mm SWA will carry 21a over 38m whereas XLPE will carry 25a, so you really need to know what you've got and your expected load before you go with that cable, you're probably looking at a 20amp breaker by the way.
You could terminate the internal SWA in to an isolator/switch and run some hefty T&E from there to the CU to make it easier to connect too

 

[ban-all-sheds]

From what I can work out

Dunno how you're working it out, but it's not the right way.

standard buried 2.5mm SWA will carry 21a over 38m

No it won't - not even close.

whereas XLPE will carry 25a

1) No it won't - not even close

2) The only time XLPE can carry more than PVC is when everything it's connected to is also rated to 90°C, i.e. not very often IRL. The rest of the time it's got the same capacity as PVC cable.

 

[ban-all-sheds]

I've now found someone else but he can't start for a few weeks so I thought I would carry on where John Wayne left me

Please don't.

and get as many of the bit's and pieces together for the new chap (saving money at the same time) as I can.

You won't save money - he'll be paying less than you for anything you need.

I am a competent DIY'er who has done a few electrical projects in my time but never outdoor work.

The outdoors aspect makes no difference to the cable sizing - if you can cope with that for indoor circuits you can cope with it for outdoor ones.

So far I have dug a trench from the side of my house down to the driveway which is on avereage 18" deep.

You must leave it as is for the new electrician to look at, as it'll be his signature on the certificates to say it's OK. And you'll need the cable replaced anyway.

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• Limbo
• 23 May 2010
• 1

That's nasty.

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• Limbo
• 23 May 2010
• 1

So is that, and as ColJack says, probably too short.

This unit is connected to this RCD

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• Limbo
• 23 May 2010
• 1

So he was all set to put outside lights on the same single RCD that supplies your entire house.

Yee-hah!!

You're well shot of him - I hope you didn't pay him anything - he's a complete idiot.

What is the best combination of rcb'ss/rcd's, as in should I have a CU down in the workshop/run everything from the existing CU etc.

For your electrician to decide - he will be the one signing a certificate to say that he designed it.

Can I come off of the armoured cable as it goes past the porch (for my garden lights and switch) or will it have to come from the workshop back up the garden path to the porch (in which case I won't fill in the path trench yet)

For your electrician to decide - he will be the one signing a certificate to say that he designed it.

As I have said before I have a good knowledge of electrics

Not sure how many people here would agree with you, but feel free to show how with a 3-core cable running to the shed you could have a switch in the porch to control the garden lighting, taking account of the MCB rating for the sub-main, and the consequent cable size and accessory ratings.

but want to make sure I'm doing this right.

You don't need to do anything, you've got an electrician lined up.

You may be thinking "why doesn't he wait for the sparky to do it all" which is a fair point. I am however a person who likes to learn and know how things work.

• //www.diynot.com/wiki/electrics
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/electricity.html
  
  
• http://web.archive.org/web/20080213...inboone.com/domesticinstallations.html[/list]
  
  

  I would love to be able to go and buy everything I need before he starts and save myself some money.

  You can't - he'll be getting trade prices at wholesalers. You won't, so anything you buy will cost more.

 

[JohnnyG]

Dunno how you're working it out, but it's not the right way.

Oh, my apologies to the OP in that case.
I just used TLC's calculator, I guess I input something incorrectly although having another go gives me the same results, ah well.

 

[ban-all-sheds]

No matter what I pick as possible wrong inputs - non-lighting instead of lighting, 3-core instead of 2-core, 240V instead of 230V I cannot reproduce your figures.

Any chance of a screen-shot showing how you managed to get them?

 

[MrsBanal]

I get the same figures as JohnnyG if I pick 3 core PVC SWA.

 

[JohnnyG]

No matter what I pick as possible wrong inputs - non-lighting instead of lighting, 3-core instead of 2-core, 240V instead of 230V I cannot reproduce your figures.

Any chance of a screen-shot showing how you managed to get them?

By all means...

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• JohnnyG
• 24 May 2010
• 1

 

[bongos]

I think that when using the TLC website you must select 2 core for single phase installs even if you are using 3 core for a single phase install. Otherwise it thinks you are supplying the load via 3 phase...

Although for a 2KW load at 38m, the doncaster cables link nozspark links to shows the same.

 

[ban-all-sheds]

Err...

1) If you specify 3-core cable the calculator will do 3-phase calculations.


2) And anyway that's showing the cable carrying 8A, not 25....


3) Try again with your supposed figures of 21A and 25A loads, and see what answers you get.