Shed Wiring Question

[ilprincipe86]

Hi all,
I am looking to build a shed with power, using an existing power source - but have a couple of questions over the current wiring of the shed power feed as I am fairly certain its not correct (I moved into the house about 6 months ago and this was the wiring in the house).
N.B I dont use the power at the moment - its switched off at the consumer unit as the current shed isnt water tight. Part of this plan is to move the shed location as part of wider garden landscaping work.

Current setup

- 32A Feed from House consumer unit (2.5mm wire) goes to an external wall (about 10 metres away)
- From the external wall it was been housed into a plastic casing which is clipped to bottom of the fence
- This runs about 15 metres to where the current shed is and feeds both lights and 1 double socket

Proposed setup

- Install a WP junction box where the current cable goes outside
- Connect to SWA cable to existing feed and run in a trench to new shed (Could someone confirm recommended trench depth?)
- Run SWA Cable into Shed (exact distance yet to be confirmed but say 15 metres as the current set up)
-Connect SWA to new Garage CU (Like the following: http://www.screwfix.com/p/bg-garage-kit-enclosure-5-module-ip55-40a-rcd-dual-6a-32a-mcb/68849)

I would use the Shed unit to run deck lights, maybe have 1 or two double sockets and a shed light.
As a proposal does that sound viable? In my previous house the electrician took the power feed directly from the outside electrical box and ran the power feed from that to a out house I had but wondered whether this would be required in this instance given that power goes to the external wall at the moment anyway?
Be keen to hear your thoughts all.

Thanks in advance.

 

[ban-all-sheds]

32A Feed from House consumer unit (2.5mm wire)

The first is too big for the second.

- From the external wall it was been housed into a plastic casing which is clipped to bottom of the fence

Not necessarily robust enough - fences can move about, blow down etc.

Connect SWA to new Garage CU (Like the following: http://www.screwfix.com/p/bg-garage-kit-enclosure-5-module-ip55-40a-rcd-dual-6a-32a-mcb/68849)

Why do you want a CU?

Are you aware you'll need Building Regulations approval for that?

Who do you plan on doing the work?

Be keen to hear your thoughts all.

• 
  
  
  
• [wiki]Electrics%3Abooks[/wiki]
  
  
• [wiki]Electrics%3AMCBtoCable[/wiki]
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.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.
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• [wiki]Electrics%3AMCB2[/wiki]
  
  
• http://www.tlc-direct.co.uk/Book/7.13.3.htm
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• [wiki]Electrics%3ASWA_gland[/wiki]
  
  
• http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf
  
  
• [wiki]Electrics%3APart-P[/wiki]
  
  The references above which are in italics are out of date. The principles are still valid, and you can absolutely learn valid things from them, but you will need to refer to the latest edition of the Wiring Regulations to get the correct data.

 

[Taylortwocities]

First off. What do you plan to power in the shed?
Second off. Existing feed. Does this come from an RCD-protected circuit?
Third off. Just turning off the MCB does not completely isolate a circuit. The MCB only turns off the line conductor, You would need to disconnect the neutral too.
Fourth off. If the cable is only 2.5mm², that MCB should be 20amp (25A if you can find one). 32A is too big for the cable. So if you stay with the existing internal wiring, 20amps is all that you'll be able to load up (see first off). So you may need bigger cable all the way (inside and outside) depending on loading, voilt drop etc.
Fifth off. Yes SWA. SIze needs determining (see above).
Sixth off. Depth of trench. This is a guide https://www.tlc-direct.co.uk/Book/7.13.3.htm
Seventh off. An RCD CU in the shed is not necessary if the circuit is already RCD protected (see second off). You'll need to make the sockets into a ring, if you retain the 32A MCB, or wire the shed in 4mm² cable.
Last off. You are intending to provide a new circuit. This is notificable work, so you'll need to raise a Building Notice with the Local Authority (and poay their fee) BEFORE you start. Or use a registered electrician.

 

[ilprincipe86]

Thanks for the quick reply Taylor - points 3,5, 6 and 8 noted. Further responses below;

First off. What do you plan to power in the shed? Deck lights, Shed light, occasional power tool usage, drill/lawnmower etc.
Second off. Existing feed. Does this come from an RCD-protected circuit? Yes, it is RCD protected.
Fourth off. If the cable is only 2.5mm², that MCB should be 20amp (25A if you can find one). 32A is too big for the cable. So if you stay with the existing internal wiring, 20ampos is all that you'll beable to load up (see first off). So you may need bigger cable all the way (inside and outside) depending on loading, voilt drop etc.
Seventh off. An RCD CU in the shed is not necessary if the circuit is already RCD protected (see second off). You'll need to make the sockets into a ring, if you retain the 32A MCB, or wire the shed in 4mm² cable. If a 20amp fuse is used, the existing 2.5mm cable retained (connecting to SWA outside), and a RCD CU in the shed is not used, is it a case of wiring this straight into the shed sockets, light?

I appreciate your help on the above, If anything just so I have a view of the potential upheaval (possible cable removal/installation) and whether its worth the time/hassle/labour cost!

 

[Iggifer]

why have you linked to this thread? I assume thats a mistake?

 

[riveralt]

. If a 20amp fuse is used, the existing 2.5mm cable retained (connecting to SWA outside), and a RCD CU in the shed is not used, is it a case of wiring this straight into the shed sockets, light?

Two added questions:
Is the shed going to be wood, plastic or metal?

20Amp MCB is the correct size for this size cable - you don't need the additional RCD.
SWA needs some skill and patience to work with?
Your incoming cable if possible is better connected inside the shed.
The shed circuit will be a radial so straight into a metal socket and add sockets as you need.
Your lighting circuit should be 2.5mm² T&E from one of the sockets up to a Switched FCU (fused down to 3Amps) then 1mm² T&E onto your light.

 

[JohnW2]

How sure are you that the 2.5mm² T&E cable is safe from the elements and humans?

I took the OP to be saying that the proposal was to replace the current outdoor 2.5mm² T+E ('in a plastic casing and clipped to a fence') with buried SWA, leaving T+E only inside the house? Did I misunderstand?

Kind Regards, John

 

[riveralt]

How sure are you that the 2.5mm² T&E cable is safe from the elements and humans?

I took the OP to be saying that the proposal was to replace the current outdoor 2.5mm² T+E ('in a plastic casing and clipped to a fence') with buried SWA, leaving T+E only inside the house? Did I misunderstand?

Kind Regards, John

Check... amended.

 

[JohnW2]

Last off. You are intending to provide a new circuit.

Is that true? If a 'garage CU' were installed, there would presumably be the usual debate about whether or not the wiring downstream of that CU represented 'new circuits', but if there were no garage CU, I'm not sure what the 'new circuit' would be.

Kind Regards, John

 

[ilprincipe86]

Hi John, yes thats what I would have thought would have needed doing as reading around the forum the common view is SWA should be used outside which it currently is not.
Either way it sounds like one way or another the current configuration could be amended without a massive overhaul of existing wiring etc (which was a concern).
Ill update once I have had the work completed as to what method was used.

Thanks all for the inputs.

 

[JohnW2]

Check... amended.

Sorry, I don't understand - could you clarify? If the 'amendment' you're referring to is "...the existing 2.5mm cable retained (connecting to SWA outside)...", that still seems to imply that 2.5mm² T+E would only be retained inside the house.

Kind Regards, John

 

[riveralt]

Check... amended.

Sorry, I don't understand - could you clarify? If the 'amendment' you're referring to is "...the existing 2.5mm cable retained (connecting to SWA outside)...", that still seems to imply that 2.5mm² T+E would only be retained inside the house.

Kind Regards, John

I think the OP talked about the existing cable inside the house not me - I have removed any reference to T&E outside the house that I asked about re elements etc.
Still don't know if the new shed is metal or not though.

 

[JohnW2]

I think the OP talked about the existing cable inside the house not me - I have removed any reference to T&E outside the house that I asked about re elements etc.

It was the OP - but I assumed your questions about 'safe from the elements' referred to the external T+E - which, as I understand it, is to be replaced by SWA. Anyway, as you say, you have now removed that.

Still don't know if the new shed is metal or not though.

We don't. (and nor do we know whether there are any other extraneous-c-ps, like a metal-pipe water supply, since the OP has not been asked that yet)

Kind Regards, John

 

[ban-all-sheds]

If a 'garage CU' were installed, there would presumably be the usual debate about whether or not the wiring downstream of that CU represented 'new circuits',

Only that initiated by those who are hard of thinking or pathologically perverse.

 

[mikeyd]

Only that initiated by those who are hard of thinking or pathologically perverse.

It's just a simple question about 'Wiring A Shed'.

I understand that you must have a dislike of sheds, why don't you just avoid posts mentioning them?

I would offer advice but, (like others I'm sure) I keep silent, in case you try to drag me down/up to your level..