Shed Wiring - a few questions

[Tim84]

Hi, as it's just a shed I'm going to wire it up myself for the challenge and the cost saving then have someone check it for me when they connect the Garage Consumer Unit to my main one. I've put together a diagram of what I intend to do, here (light connection picture taken from this forum)

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• Tim84
• 18 Mar 2014
• 1

I have some questions I'd appreciate help on though please:
- can I run two seperate light circuits as shown but connect them to the same breaker in the Consumer Unit or is this a bad idea?
- does the PIR floodlight need a fuse added inline like it'd have if wired to a socket? If so how do I do this?
- 2.5mm twin core and earth for the sockets; can I use the same for the light circuits too or should I go 1.0mm or 1.5mm?
- the plastic back boxes of the light switches have no metal earth connection point, is this OK and therefore just no need to connect an earth wire to the box?
- any other comments?

Edit:
- also is it ok to run it through and behind Celotex PIR 50mm insulation boards?
- can I seal it into the insulation boards using expanding foam like a grommet when I poke the cable through for access?

 

[ericmark]

Start point is what power is required it's not a case of just stick in sockets you need to sit back and decide what you want to run. There is a huge jump in requirements when you exceed 13A so what is the shed for?

The British ring is to reduce cable required in a house and so reduces the loop impedance in a house today it is becoming questionable if correct method and radial circuits are being used more and more.

In a shed it is unlikely to need to be a ring. I know it seems a simple job but stage one has to be the supply. With a 20A supply with 2.5mm cable likely there will already be volt drop and could need a ring but with 6mm cable unlikely a problem distance will of course be important.

One expects at some time a PIR will fail so one will normally have a switch as well to isolate it should it become stuck on. Lamps outside normally twin pole switching so faults with not trip a RCD.

It is common to buy a role of 2.5 cable and use it for everything there is nothing wrong to use it for lights. Only problem is getting 2.5 wires in some of the terminals found in lights.

I have sat many hours in a shed like you show trying to keep warm over a paraffin heater while talking to the world total supply was 13A so not enough power for electric heating. The replacement was wired with 5 core SWA cable so the guy could switch on the electric heater in the house before going out to the shed two separate circuits for power and heating.

Although 5% may be the volt drop limit for his electric heater it did not matter that much.

In other words after the first shack was replaced he put some thought into the next one and got it as he wanted. This includes earthing methods which for a radio shack was TT but this may not be appropriate for other uses.

So sorry back to drawing board why do you want so many sockets what is the load and what is the supply? To supply a shed from a 13A socket in the house to run lights and plug in lawn mower many be OK but to run heater and kettle in shed then dedicated supply. No point in having a 32A ring from a 32A supply in house as house MCB would always trip first so at a 45A supply looking at a hefty cable.

Use 16A radial in shed and then a 20A supply from house likely all that is required so more info please.

 

[Aragorn84]

Eric makes some good points. You've drawn a lot of sockets, is this because you intend to run lots of items, or just because you would like lots of outlets available in different parts of the shed?

To answer some of your specific questions:

- can I run two seperate light circuits as shown but connect them to the same breaker in the Consumer Unit or is this a bad idea?

Thats fine, and is no different to what exists in your house. As eric mentioned, DP switching is probably a good idea for the outdoor light.

- does the PIR floodlight need a fuse added inline like it'd have if wired to a socket? If so how do I do this?

Probably not, but you'd need to check the MI's for the PIR.

- 2.5mm twin core and earth for the sockets; can I use the same for the light circuits too or should I go 1.0mm or 1.5mm?

1.0mm or 1.5mm is fine for the lights, but nothing stopping you using 2.5mm (bar the extra cost of the copper). It may be a little more difficult to terminate the bigger 2.5mm cable in the fittings.

- the plastic back boxes of the light switches have no metal earth connection point, is this OK and therefore just no need to connect an earth wire to the box?

Yes, just put the earth into a choc block, or if the faceplates are metal, connect the earth to them instead.

- also is it ok to run it through and behind Celotex PIR 50mm insulation boards?

Yes, but derating factors will apply to the cable. You need to be sure that the derated cable can carry the design current.

- can I seal it into the insulation boards using expanding foam like a grommet when I poke the cable through for access?

Not sure, some products will react with the PVC insulation, but i dont know about expanding foam.

I would also suggest that for your lighting circuits, dont use chocblocks randomly dangling round the place. Make all the terminations either at the light rose, or at the switch.

You've also not mentioned how you plan to supply the shed. This is likely more important and more tricky than the internal shed wiring.

 

[dhutch]

... wired with 5 core SWA cable so the guy could switch on the electric heater in the house before going out to the shed two separate circuits for power and heating....

That, is a bloody brill idea! Assuming the length, cable sizing, etc, works out.

...You've drawn a lot of sockets, is this because you intend to run lots of items, or just because you would like lots of outlets available in different parts of the shed? ....

Agree it looks a lot. My dad has a workshop in our garage, with a double socket every meter down three of the four walls, so that you can plug your mainsdrill/powersanders/planer/batterycharger in where you like, to make the most of the typically fairly short leads, without have wires and extensions all over the place with the associated risks. Its brill. But as you say, you cant run a 3kW fan heater off all of them!

The one really key thing missing here in my mind, is whats feeding the garage consumer unit!


Daniel

 

[JohnW2]

The replacement was wired with 5 core SWA cable so the guy could switch on the electric heater in the house before going out to the shed two separate circuits for power and heating.

As has been said, that's a clever idea. Indeed, if there is a single source supply, and you were happy about VD considerations, then that could be done with 4-core, or even 3 core (if no earth core was used).

Although 5% may be the volt drop limit for his electric heater it did not matter that much.

Quite - and if you are satisfied that it does not "impair safe functioning" of the loads, a VD greater than 5% (or >3% for lighting) is compliant with the regs.

Kind Regards, John

 

[Tim84]

Thanks for the replies everyone, some key points I need to update then!

- the supply to the shed will be SWA cable from a dedicated MCB in the main house Consumer Unit. This will be done by a qualified sparky so I don't fry myself. It will hook up to a Garage Consumer Unit in the shed with 2 x breaker (or 3, cannot remember)

- the use of the 'shed' is a small office (6' x 8') hence the proliferation of plug sockets. Running off these will be two computers, two monitors, printer, hard drive, network switch, phone charger and other fairly low power devices

- I intend to run the 2.5mm cable un-ducted in the 25mm air gap between the T&G shed wall and the Celotex insulation. To then get it out to the sockets I intend to basically poke it through the Celotex, through the OSB2 board that will be lining the shed and then into the relevant socket/light

- I did a ring design as I am a complete novice so just researched and came to the conclusion ring was best. Due to the small size of the shed the extra cable cost isn't an issue so if ring is best then I'll do it, if not then please shout

- the PIR will run off a 2-Gang light switch, one gang for the main internal light and one to the PIR; so cutting power is a flick of a switch. Not sure why the PIR would need a fuse but my other PIR runs off a flex with a fused plug on the end so thought I'd better check

- In all of the above I forgot to add a heater to the design. I want to add an electric or oil filled heater running either off one of the sockets in the shed or a dedicated supply form the Garage Consumer Unit


Hopefully the above is all clear? Thanks for the help so far!