Electrics To Lean-To 'Outhouse'

[ManicInfinity]

Hello

Firstly I know that the work I am proposing here is Part P notifiable. For now I just want to come up with an acceptable plan.

I live in a maisonette and have built a small lean-to (brick, block and slate roof) on a concrete slab, with DPM/DPC etc (so in that respect it is like a small shed).

Electrically it will contain
- a couple of power sockets, on a 2.5mm T&E radial circuit - supplying small power tools (as well as low voltage decking lights). A future owner might stick in a fridge but that is about all that would fit in it.
- a single florescent light and a couple of external security lights (all switched internally) on a 1mm T&E radial circuit.

The house CU is fully populated, MCB only, has no RCD protection and is TN-S earthed. As such I am proposing to run a spur from the existing 32A MCB protected ring (which itself only powers a few sockets at the back part of my maisonette) using a FCU into a new garage-style CU located in the lean-to. The garage-style CU comes with a 40A 30mA RCD as well as a 6A and 32A MCB (and an extra space that will be left blank that accepts single pole RCBOs).

Unfortunately the fused spur (submain?) needs to run externally for approx 6m to get to the lean to, under some raised decking (which is being installed at the same time). I was hoping I could just use 2.5mm T&E cable for this run (as I have loads spare), directly clipped to the decking sub-frame joists where no rodents will be able to chew the wire / no sunlight will be able to degrade the wire. Another advantage is that I don’t have to use external adapter boxes outside to go from 2.5mm T&E to SWA and vice-versa.

I was wondering if anyone could help me answer some questions:
- Is my plan sound? I need to work out that the loading is ok on all the components, but I don’t mind if there is no discrimination in the lean-to itself (if the CU as a whole flips) so long as it doesn’t end up activating the MCB in the house?
- Should I buy a 16A MCB for the new CU (and chuck out the bundled 32A MCB) as the power sockets are being installed radially not ring, or is this not a concern?
- Do the regulations allow for 2.5mm T&E cable externally or must I go SWA?
- What rating FCU should I be looking at using?
- Should I up the lighting wire to 1.5mm?

Thanks everyone for taking time to read this 

 

[PrenticeBoyofDerry]

I very much doubt you can rule out the activity of rodents, there are plenty about and the the pulse of an electrical cable is something they are attracted to, also there is frost and the overgrowth of weeds and plant live to consider. So using unprotected 2.5mm2 T&E would not be recommended
An unfused spur should ideally only run for a distance no greater than 3 metres.
So I would reconsider your plans and run 4mm SWA cable directly from the CU. If it is lean to, why the need to go externally to the shed?

 

[ManicInfinity]

Thanks PrenticeBoyOfDerry.

Unfortunately the position of the lean-to against the property means I cannot run cable into it internally. It abuts a tiled bathroom which itself only has a lighting final radial, hence me need to spur it off elsewhere and route externally.

Note the spur will be fused (you stated unfused) and I hoped that would be okay given the small load required off it. I am trying to avoid running it back to the main house CU as that will require me to get it changed (as it's fully populated) and also ripping up all the new carpets and floorboards that were laid down last year during renovations.

Could running the T&E externally through trunking be an alternative to SWA? I take it rodents won't be attracted to the wiring in the decking kit as it is low voltage?

Thanks

 

[Aragorn84]

Theres no point putting a CU in the shed if its spurred off a ring main. Just stick the sockets directly on the circuit (with a DP isolator if you fancy) and use an FCU for the light.

The new sockets in the shed require RCD protection, you could use an RCD FCU to achieve this, or put a RCD in a small din rail enclosure in the shed before the sockets.

I wouldnt run unprotected T&E externally. If you can put it inside earthed steel conduit, then i think that would be ok.

 

[ManicInfinity]

Thanks Aragorn84.

I appreciate your comments.

My thoughts behind using a CU were that I didn't want to spur of a spur of a spur (for the power sockets) and because I wanted the lighting on a separate 6A circuit.

I understand a CU is probably overkill, but for £25 it does give me the ability to isolate the power and lighting circuits and protect everything behind an RCD (especially the external security lighting). As I said I am not worried about discrimination, I just want it to be safe.

Am I right in thinking that with your method, each light (1 inside, 2 outside) would need a FCU, and if I put an RCD in a small din rail enclosure before the sockets I am kind of making my own CU (but without the lighting protected)? I can see it works, just not convinced it is the most logical option and probably costs about the same?

As for the unprotected T&E - I can see this is a bad idea and will ensure it is protected as you suggest or SWA cable. I am aware the shielding in both cases also needs to be TN-S earthed.

 

[Aragorn84]

Each light would need an FCU, or you could have one FCU and then switches hung off the load side for each light, much the same as you'd have with an MCB.

A CU doesnt give you the ability to fully isolate the seperate circuits, as the MCB's only switch the live. You'd still want a DP isolator for your outside lights for instance, incase of water ingress or similar, and those DP isolators could easily have fuses inside as well so in reality the CU gains you nothing.

 

[ManicInfinity]

Thanks Aragorn84

I see what you are getting at, and where you are coming from and will head back to the drawing board this evening. But apart from needing a DP isolator and ensuring the feed is armoured, does my current plan of using a CU in the way I've specified actually fall foul of any regs?

I'm hoping to get a sparky around soon to talk it over as I need to get the work certified, and perhaps the rest of the electrics in the house for good measure.

Cheers

 

[ManicInfinity]

Aragorn84

Just another note, the garage CU unit I bought (and can return if I take your advice and don't use it), comes with a BG Electrical CUR4030 'RCD Double Pole 40A 30mA ', so both circuits can be isolated in one go.

I accept that the 6A and 32A MCBs themselves only switch live though. Are you saying that each external light must have a DP isolator in series with it, or is the ability to isolate both circuits sufficient?

Honestly I am not stuck on using a CU (even though I have one), I am just trying to better understand the various options available to me.

Cheers

 

[Aragorn84]

Aragorn84

Just another note, the garage CU unit I bought (and can return if I take your advice and don't use it), comes with a BG Electrical CUR4030 'RCD Double Pole 40A 30mA ', so both circuits can be isolated in one go.

I accept that the 6A and 32A MCBs themselves only switch live though. Are you saying that each external light must have a DP isolator in series with it, or is the ability to isolate both circuits sufficient?

Honestly I am not stuck on using a CU (even though I have one), I am just trying to better understand the various options available to me.

Cheers

Its not a requirement per-se, its just a good idea to have one for any outdoor equipment.

Example Scenario: You go out to your shed to fetch something from the freezer and find the powers off as the RCD has tripped. Try to reset and it trips again. Some investigation uncovers the security light is full of water as its seals have failed. Turning the lighting MCB off wont allow the RCD to reset, because theres still a neutral/earth fault present. If you had a DP isolator for the security light, you could switch that off isolating the failed light, and you'd get your power back on.

Given most lights generally need a switch anyway, it makes sense that you use a DP switch.

 

[ManicInfinity]

Thanks Aragorn84n

I'm going to sketch out some diagrams later. Instead of a CU, I will think about having a metal clad RCD DP 13A isolator in the shed, which feeds both the power circuit and lighting circuit radials. My challenge is mixing usable space with the various switches.

I will have one internal and two external lights, and had planned to have them run off a 2G switch (1st switch for int, 2nd switch of both ext) to save space.

Either I change this to two sets of 1G (both fused DP) or just run a single DP isolator between the RCD DP 13A isolator and the lighting circuit. I realise with the latter option I also lose internal lighting if there is a neutral/earth fault present in the either of the external lights. But at least I still have power to the sockets.

As for the spur, do I need to have a FCU at the point I take it off the ring main (before the cable exits the property), or can I just spur it via a junction box?

 

[ManicInfinity]

So...

- I've decided to ditch the CU (I agree it makes absolutely no sense having one off a spur) and
- I will be using SWA cable for the external run, clipped to the subframe joists of my decking, 2.5mm (not 4mm as advised). The run is only 8m.

But I have one more quandary - where do I put my RCD... in my property or in the shed. For example:
a) directly connect a 13A RCD DP FCU to the ring before the cable exits my property to the lean-to, and in the lean-to have it run into a master DP isolation switch, from which a power socket radial and 6A FCU lighting radial will run.
b) directly connect a 13A FCU to the ring before the cable exits my property to the lean-to, and in the lean-to have it run into a master 13A RCD DP isolation switch, from which a power socket radial and 6A FCU lighting radial will run.

My heart wants the RCD in the shed, so it is relatively easy for me to get to if activated (but then I have two 13A fuses to confused things). My head says it should be in the property in case the armoured cable is ever compromised (which also means only 1 fuse). What would you recommend / the regs say?

 

[JohnW2]

My heart wants the RCD in the shed, so it is relatively easy for me to get to if activated (but then I have two 13A fuses to confused things). My head says it should be in the property in case the armoured cable is ever compromised (which also means only 1 fuse). What would you recommend / the regs say?

Go with your heart.

Kind Regards, John

 

[Aragorn84]

Agreed.

If your worried about two fuses, use something like this:

http://www.ebay.co.uk/itm/40A-30mA-...UK_BOI_CircuitBreakers_RL&hash=item3a8d71863e

and this:

http://www.ebay.co.uk/itm/Contactum...al_Components_Supplies_ET&hash=item257e4f8ad3

 

[ManicInfinity]

I see where you are going...

...that's a great idea

I still get my 13A fused spur, which is great, but I also get RCD protection as well and don't have two fuses that could blow to worry about.

(I know it was eluded to earlier but I had my mind on other things)

Thanks guys