Garden supply

[Observer]

First - I am aware that the work I am describing below is Part P notifiable. I haven't yet decided how to deal with that - for now, I'm just exploring the issues. I would appreciate advice on the following.

I want to provide an electricity supply for a shed I am intending to erect at the bottom of my garden. I have an external 13A socket fed from 30 mA RCD protected ring circuit with 32A MCB at the CU. I want to avoid new cable run from the CU so wish to take a feed from the external socket, if viable. In the new shed, I anticipate installing one or two lights, a power supply for garden tools and maybe a feed for a water pump for a fountain.

The external socket is on an external wall of the house and fed from an internal 13A switched fused spur. When the patio was laid, I had a SWA cable laid below the patio with one end terminating adjacent to the socket and the other end coiled with enough length to reach the shed location. Distance from the power socket to the shed will be max. 20-25 metres. As the socket is fused at 13A, I should be able to load the cable with ~3kW, which will be ample for my purposes. I believe I need 2.5mm cable size. However, I am not sure whether the SWA cable is large enough - it may be only 1.5mm. If not, would it be acceptable practice (short of taking up and relaying patio) to take existing cable to a waterproof JB which I can mount on patio wall (~2 metres) and then take correct cable size from there to the shed?

And once I have the supply to the shed, should I install another CU to distribute as described above?

Any other comments or advice?

 

[Observer]

Hmm. Is there something wrong with the way I asked for advice here?

 

[Roofer]

I think you've received no answers because the Sparks have all been stunned by your suggestions.

I'm just a simple Roofer but even I know that what you're considering and especially the way you want to go about it is totally unacceptable.

Get an electrician in - please!!

 

[Observer]

OK. This is all at the pre-planning stage at this point. I don't even have a shed so I'm not about to do anything at all - let alone anything stupid. All I'm trying to do is get an idea of the scope of a possible future project.

Let's break this down.

I understand that the best practice way of providing a supply to a garden shed would be a dedicated new circuit from the CU. That's a possibility but involves a long cable run along the side of the house. That would be unsightly and expensive. I don't anticipate a pressing need for power in a shed I have yet to erect - it's just on the 'nice to have' list.

When the SWA cable I mentioned was laid, the only purpose I had in mind was a small water pump for a garden feature which was and still is a no more than an idea. Perhaps taking the feed from the external socket was always a bad idea but I assumed it was OK because the sparks who wired the extension seemed OK with the principle of laying the cable to service the possible future need (although a different need, as I've explained).

Let's go back to the beginning. Is there any satisfactory way of providing power to an outbuilding other than a dedicated new circuit from the CU?

 

[Observer]

I know this thread (not very different to the question I asked) is pre-Part P. Is Part P the reason that the resident experts on here - including ban all sheds - seem to have become so pious?

 

[Steve]

OK. This is all at the pre-planning stage at this point. I don't even have a shed so I'm not about to do anything at all - let alone anything stupid. All I'm trying to do is get an idea of the scope of a possible future project.

Let's break this down.

I understand that the best practice way of providing a supply to a garden shed would be a dedicated new circuit from the CU. That's a possibility but involves a long cable run along the side of the house. That would be unsightly and expensive. I don't anticipate a pressing need for power in a shed I have yet to erect - it's just on the 'nice to have' list.

When the SWA cable I mentioned was laid, the only purpose I had in mind was a small water pump for a garden feature which was and still is a no more than an idea. Perhaps taking the feed from the external socket was always a bad idea but I assumed it was OK because the sparks who wired the extension seemed OK with the principle of laying the cable to service the possible future need (although a different need, as I've explained).just use this for the water feature then. Is it just connected to a socket? it need to be on a fused spur at 3A, unless you have a huge water pump.

Let's go back to the beginning. Is there any satisfactory way of providing power to an outbuilding other than a dedicated new circuit from the CU?yes, there is. Take a fused spur from a socket on a ring main. Run 2.5mm² SWA down the garden to the shed. There, fit ONE SOCKET (double or single) and another fused Spur for a light. dont fit any more than this, dont forget the total load is limited by the fuse at the house (13A).

 

[Observer]

Thanks crafty. That confirms what I worked out myself. The available external socket is already protected by a FCU which I imagine is 13A because it's switching the external socket.

I have a couple more questions, if you or anyone else can help.

1. From what I've seen, it's recommended to terminate the SWA into a metal enclosure. Is it possible to terminate in the existing external socket? If not, it should be possible to fix a metal enclosure in place of the existing socket and resite the existing socket next to it. Does that make sense? Does the answer to that depend on the existing earthing arrangement?

2. Is there a solution if the existing SWA is not 2.5mm? I can see the problem with a JB on the other side of the patio so if that is really out of the question, I could restrict the shed installation to lights only.

3. From what I've read, it's recommended to have an RCD protecting the external cable run (in addition to the RCD at the CU). Would that need to be different rating to the existing one? And which end of the cable run should it be?

 

[Steve]

I have a couple more questions, if you or anyone else can help.

1. From what I've seen, it's recommended to terminate the SWA into a metal enclosure. Is it possible to terminate in the existing external socket? If not, it should be possible to fix a metal enclosure in place of the existing socket and resite the existing socket next to it. Does that make sense? Does the answer to that depend on the existing earthing arrangement?is this an external socket fixed to a wall outside? is it currently connected to anything? The SWA can be terminated externally onto a plastic socket box, but you need to use an earth banjo to connect the sheath to earth. Certainly easier to terminate indoors to a metal box (metal boxes dont work outside usually - not waterproof)

2. Is there a solution if the existing SWA is not 2.5mm? I can see the problem with a JB on the other side of the patio so if that is really out of the question, I could restrict the shed installation to lights only. 1.5mm² SWA is OK for 13A at 20m, but you would have to watch voltage drop carefully. You have to remember that there will always be a limit to what you can have on this installation

3. From what I've read, it's recommended to have an RCD protecting the external cable run (in addition to the RCD at the CU). Would that need to be different rating to the existing one? And which end of the cable run should it be? If you have a 30mA RCD at the CU in the house protecting the ring main, there is absolutely no point in having an RCD at the exit point of this cable from the house. The 2 wont discriminate, and either could trip in a fault. Just use the CU RCD - it provides more than adequate protection

 

[Observer]

I have a couple more questions, if you or anyone else can help.

1. From what I've seen, it's recommended to terminate the SWA into a metal enclosure. Is it possible to terminate in the existing external socket? If not, it should be possible to fix a metal enclosure in place of the existing socket and resite the existing socket next to it. Does that make sense? Does the answer to that depend on the existing earthing arrangement?is this an external socket fixed to a wall outside? is it currently connected to anything? The SWA can be terminated externally onto a plastic socket box, but you need to use an earth banjo to connect the sheath to earth. Certainly easier to terminate indoors to a metal box (metal boxes dont work outside usually - not waterproof)

Thanks again crafty. Yes - the external socket (one of the grey MK waterproof ones) is fixed outside to external wall and fed from an internal, switched 13A FCU. I've been shown one of the earth banjo links you mentioned and understand how it's meant to work.

2. Is there a solution if the existing SWA is not 2.5mm? I can see the problem with a JB on the other side of the patio so if that is really out of the question, I could restrict the shed installation to lights only. 1.5mm² SWA is OK for 13A at 20m, but you would have to watch voltage drop carefully. You have to remember that there will always be a limit to what you can have on this installation

Understood. I will obviously be aware of the limitation but I can see the concern that a subsequent owner may try to hang more on the circuit than it will take. As discussed, it would be protected by the 13A fuse at the FCU. Is that good enough or would it be better to put a breaker in the circuit that will trip faster than a fuse will blow if the circuit is overloaded? (That was why I asked about installing a small CU at the shed end.)

3. From what I've read, it's recommended to have an RCD protecting the external cable run (in addition to the RCD at the CU). Would that need to be different rating to the existing one? And which end of the cable run should it be? If you have a 30mA RCD at the CU in the house protecting the ring main, there is absolutely no point in having an RCD at the exit point of this cable from the house. The 2 wont discriminate, and either could trip in a fault. Just use the CU RCD - it provides more than adequate protection

Understood again.

 

[Qedelec]

I know is pre-Part P. Is Part P the reason that the resident experts on here - including ban all sheds - seem to have become so pious?

If you want advice it does not help your cause by calling those who you are asking for help from, pious "don't kick over the beehive when your gathering honey"

The reason that no one replied was they suspected you were out of your depth and hoped you would seek professional help, this is what roofer suggested.

 

[Steve]

Understood. I will obviously be aware of the limitation but I can see the concern that a subsequent owner may try to hang more on the circuit than it will take. As discussed, it would be protected by the 13A fuse at the FCU. Is that good enough or would it be better to put a breaker in the circuit that will trip faster than a fuse will blow if the circuit is overloaded? (That was why I asked about installing a small CU at the shed end.)

The 13A fuse is fine. You dont need/want fast tripping when you're using your power tools add to that, you cant get 13A breakers (easily) so you would then be limited to only 10A.

 

[Observer]

I know is pre-Part P. Is Part P the reason that the resident experts on here - including ban all sheds - seem to have become so pious?

If you want advice it does not help your cause by calling those who you are asking for help from, pious "don't kick over the beehive when your gathering honey"

The reason that no one replied was they suspected you were out of your depth and hoped you would seek professional help, this is what roofer suggested.

I say it as I see it. I did some searching and read quite a number of old threads on similar issues going back as far as 2003. There is a noticeable change of attitude by a number of 'expert' posters. In many of the older threads, for example this one and this one and this one, the responses are helpful, even if advising caution where necessary. I have also read a lot of the recent threads and the feeling I get from those is that a number of the experts adopt a distinctly pious and patronising "don't try it sonny" tone. That's my perception and I find it unhelpful.

You may think I'm "out of my depth" but at least one person on here - crafty - has provided the guidance I was hoping for and confirmed that, in general, I am not. My chain of reasoning may not be as fluent as a professional's would be - but that's hardly surprising. As for seeking professional help - well the reason I came on here was to assess whether or not I need professional help. I'm still undecided on that. The bit I haven't done before is terminating an SWA cable - but then it appears that even BAS, who appears to be acknowledged as having (at least) reasonable expertise, needed some help with that not so long ago.

Thanks again to crafty.

 

[Steve]

No problem Observer, always glad to help.

Please be aware that many of the guys who give advice on here, including Ban-all-sheds and myself, are not in fact, in the trade.

However, I have learnt an awful lot from these forums and am seriously considering training to become a fully qualified spark.

Can I also add, sometimes when a DIYer first posts about a project they are contemplating doing themselves, they sound like they haven't a clue what they are doing, or suggest something completely dangerous - in these instances, we are reluctant to give advice other than "get an electrician" for their own safety. We dont want to be partially responsible for the death of that person . . . sometimes it is just the best thing to say to them. Others are so hell-bent on doing the work themselves, that we give them instructions on how to do what they want, but with a warning to get an electrician in. There is no room for error with electricity, and this is why sometimes, we seem a little harsh with people.

 

[Hightowermark]

I Always thought Crafty and BAS were qualified sparkys, or are you in another area?

 

[Steve]

I Always thought Crafty and BAS were qualified sparkys, or are you in another area?

I dont know what BAS does, but i have never been to college, just 6th form . . . Most of the things i know about electricity is what I have learnt on here! Though I have always had a fascination with electricity and all things technical. I must get out more . . .