Power to Shed project

Some pics from my power to the shed project. Wiring up the shed first before the SWA is run to the shed from the house (20m).

Earthing T-N-C-S (PME).
Shed CU = 40A 30mA RCD.
1x 20A MCB for 2x 2g sockets. Not anticipating any more load than this.
1x 6A MCB for a 80W light.

Cabling is all done with singles ran in 20mm conduit. 2.5mm singles ran as radials for the sockets and 1.5mm single ran for the lights. As part of the set up, a connection will be available for decking lights plugged into a socket via a junction box with a 1A in-line fuse where the circuit shall leave the shed via ducting.

Got the consumer unit in place but not got it wired up yet as still to do the various tests, such as insulation and continuity.

You don't need 1.5mm for lightning. Use 1.0 mm.
How is it fed at the house end? If it is through another RCD you don't need a RCD in the shed. In fact if fed from a 20A MCB in the house you don't even need a CU in the shed. Feed the sockets direct and the light via a switched FCU.

Selecting conduit and singles or twin and earth or even ali-tube is part to do with ease to install, part to do with looks, and part to do with protection but in the main conduit is used where multi supplies are going the same way and to install plastic conduit with so few cables is not really cost effective.

To have one odd twin and earth kind of spoils the look. As does see through box those cables do not really look good enough to show off with a see through lid. With industrial premises having boxes where you can see routes may help but not really in a shed.

I will assume the materials came at the right price and that's why used but not really a show piece. I do question the use of consumer units in a shed with so little power keep it simple a switch FCU to reduce power for lights and a RCD FCU to supply the whole shed is normally all that is required.

Assuming a SWA supply then question one is how to terminate in a plastic box? Yes I know you can use an earth nut but SWA is a stiff cable and will likely warp a plastic box so I would fit a metal box of some type and conduit into the consumer unit to get a box not likely to be warped by the SWA. With Ref method D using needs 4 mm for 32A MCB and 6 mm for 40A MCB these are stiff cables.

As to TN-C-S what makes you think that is best supply? It may be but it's not a case of house is TN-C-S so shed is TN-C-S distance shed to house and surrounding buildings have to be taken into consideration.

But what I asked after reading the post was what is the question? Bulkhead lamp is unlikely to take 80W without melting. Other than that is will work but what is the post asking?

The junction box isn't see through. Just not got the cover on yet. As for earthing, you can't get a better earth than PME. If I use a 3 core 4mm SWA and one of the cores is the CPC, I've worked out prospective fault current at the shed end is 418A. More than enough to trip the MCB back in the house (using a spare way). There are no conductive extraneous parts so shouldn't need a 10mm CPC.

I did think about using a FCU but just preferred to use a garage type CU. Power tools and what not will be powered from here with a kettle too. The singles cable I got from a colleague. Didn't fancy spending £80 on a 100m reel when I only needed 5m. That's why I am using 1.5mm for the light circuit as he couldn't get me 1.0mm. But hey, saved me a fortune.

Got some prices on SWA. Best so far has been £1.65 per metre.

2005_16_autumn_wiring_matters_electrical_installations_outdoors may be out dated but the information is still valid.

What we look for is the voltage gradient under fault conditions and for a garden shed 5 meters from house where the gardens of an estate all back onto each other then yes a TN-C-S supply is ample.

However on the edge of an estate where garden is backed onto by fields and the shed is 100 meters from the house likely the TN-C-S under fault conditions could cause danger where any extraneous-conductive-part is earthed to the DNO earth which will be bonded to an earth rod so considerable distance away. You now state no extraneous-conductive-part but that was not made clear with original post.

One needs to make a judgement as to if in the location TT or TN is used there is no magic formula it is for the skilled person to use his judgement as to which is appropriate.

Since you state your ELI at shed is 0.55 it would be likely in your case TN-C-S is OK.

I did say "I will assume the materials came at the right price" by which I mean you had got them without paying for them which clearly you did. I was careful not to suggest they were obtained from dubious sources.

But still may comment holds as what is the question?