Electrics in a shed - does it need to be a ring main?

[PrenticeBoyofDerry]

Or on the purchase invoice, but checking the cable would be the preferred method.
Another question (not sure if been asked or answered)
How is the SWA routed to the shed, what method is used for this route?

 

[Ahtyrrell]

If the installation is all in conduit why the twin and earth, surly singles would be much easier to work with?

 

[JohnW2]

For 20m run in SWA, a minimum 4mm2 csa cable would only really be suitable for a 20A socket radial and a 6A lighting circuit.

That would indeed be true if the designer could not satisfy themselves that the voltage drop at full load using 2.5 mm² cable would not "impair the safe functioning" of the lighting fittings, such they had resorted to applying the 'deemed-to-satisfy' limits suggested in Appendix 4 of the regs.

There will inevitably be variations between designers as regards their opinion as to what VD would 'impair the safe functioning' of simple lighting fittings likely to be found in a shed. Some, of course, will not bother (and/or feel confident) to think or formulate an opinion, and will simply go with the App. 4 figures.

Kind Regards, John

 

[191]

OK the bad news: the SWA is only 2.5 mm.

The worse news: I've paced it out and it's more like 25 metres to the CU in the house.

I think the SWA buried underground was intended only to get some lights and maybe one socket in the old shed. Am I being too ambitious to put three sockets in, bearing in mind the power will only be used for lighting and charging a scooter, plus one other tool/appliance.

Thanks again in advance.
1-9-1

 

[191]

OK the bad news: the SWA is only 2.5 mm.

The worse news: I've paced it out and it's more like 25 metres to the CU in the house.

I think the SWA buried underground was intended only to get some lights and maybe one socket in the old shed. Am I being too ambitious to put three sockets in, bearing in mind the power will only be used for lighting and charging a scooter, plus one other tool/appliance.

Thanks again in advance.
1-9-1

 

[AndyPRK]

2.5mm will be fine for what you intend.

Just use a 16 or 20 MCB for sockets

 

[191]

If the installation is all in conduit why the twin and earth, surly singles would be much easier to work with?

Why all the SWA indeed! I'm guessing it's bad form to strip twin and earth into its constituent strands? The earth, of course, would need resheathing.

Curiously, when I went to buy cable it was far cheaper to buy twin and earth (in small quantities) than it was to buy separate conduit wire. Why is that??

1-9-1

 

[JohnW2]

OK the bad news: the SWA is only 2.5 mm. The worse news: I've paced it out and it's more like 25 metres to the CU in the house. ... I think the SWA buried underground was intended only to get some lights and maybe one socket in the old shed. Am I being too ambitious to put three sockets in, bearing in mind the power will only be used for lighting and charging a scooter, plus one other tool/appliance.

Fair enough. It really depends upon how pragmatic you want (or are prepared) to be - and all we can do is tell you about the facts and regulations. In truly pragmatic terms, I have no doubt that the cable would be fine for the purpose you describe. Potential problems would only arise if you (or a future occupier) decided to put much greater loads on the shed sockets than you currently envisage.

As I implied in my previous post, in terms of your envisaged usage, voltage drop (VD) in the cable is probably the only 'issue' as far as regs are concerned. The regs say that the voltage drop for a lighting circuit should not be such as to 'impair safe functioning' of the lighting equipment - or, if one can't (or can't be bothered to) make a decision about that, the VD should not exceed 3% of the nominal supply voltage (i.e. 6.9V).

If one ignores voltage drop in the wiring within the shed (probably fairly negligible), 25m of 2.5 mm² cable will result in a VD of about 0.45V per amp of load. Hence, if one works to the 'default' maximum VD of 6.9V, that would be OK up to a total current draw of about 15.3A - seemingly far more than your stated total requirements at the present time. In reality, you would undoubtedly have to look very hard to find lighting whose 'safe functioning would be impaired' by VDs considerably in excess of 6.9V, and nor is any of the other equipment you might plug into sockets be likely to be very fussed about the actual voltage.

Kind Regards, John

 

[191]

Thanks for that, John

Being pragmatic (and bloody minded) I don't care about future occupiers! So long as the MCBs are rated low enough, it should stop anyone running welding equipment in the shed.

FYI, I have three shiny new IP65 fluorescent fittings, each with an 18W T8 tube. Not much of a current draw there.

Treating this as a "glorified 13A extension lead" I suppose 16A or 20A MCBs are the order of the day.

By the way, not having much fun forcing this twin and earth down the conduit, even on a radial circuit! I might bite the bullet and just get some conduit wire to finish the job!!!!

1-9-1

 

[EFLImpudence]

Plus, of course, a greater than the 'allowed' maximum voltage drop on the actual voltage will result in a voltage significantly higher than that allowed by calculating using the nominal voltage.

 

[JohnW2]

Thanks for that, John ... Being pragmatic (and bloody minded) I don't care about future occupiers! So long as the MCBs are rated low enough, it should stop anyone running welding equipment in the shed.

Indeed - but don't put too much reliance on MCBs in that sense. For example a standard 20A MCB (B20) will allow 29A to flow for about an hour, and appreciably higher currents than that for shorter periods of time, without tripping.

FYI, I have three shiny new IP65 fluorescent fittings, each with an 18W T8 tube. Not much of a current draw there. Treating this as a "glorified 13A extension lead" I suppose 16A or 20A MCBs are the order of the day.

Quite so.

By the way, not having much fun forcing this twin and earth down the conduit, even on a radial circuit! I might bite the bullet and just get some conduit wire to finish the job!!!!

Is this conduit straight, or fairly straight? You shouldn't really have problems feeding just one 2.5 mm² T&E through 20mm conduit. Is it 'old' (kinked) cable, perhaps? If you're having a problem, why not feed something thinner through first, and then use that to pull the T&E through?

Kind Regards, John

 

[JohnW2]

Plus, of course, a greater than the 'allowed' maximum voltage drop on the actual voltage will result in a voltage significantly higher than that allowed by calculating using the nominal voltage.

Indeed - at least, usually.

To be fair to the regs, I suppose they have to consider the (very unlikely) possibility that the actual supply voltage could theoretically be as low as 216.2V - which maybe influences the default VD maxima suggested ('for lazy people'?) in Appendix 4!

Kind Regards, John

 

[191]

Is this conduit straight, or fairly straight? You shouldn't really have problems feeding just one 2.5 mm² T&E through 20mm conduit. Is it 'old' (kinked) cable, perhaps? If you're having a problem, why not feed something thinner through first, and then use that to pull the T&E through?

The straight bits of conduit are straight, until you get to the 90 degree bends! I have the conduit around the top of the shed above door level, and have junctions with vertical conduit leading to the sockets and switches.

Past experience of forcing T&E down oval conduit in block walls has taught me to put the smaller cable through first (i.e. the 1.0 mm for the lighting) leaving a "larger" gap remaining for the fatter cable. I've tried feeding through a "mouse" to pull the 2.5mm T&E, but it's becoming too much effort.

The 2.5 mm was nice and straight before I started, but it's looking a bit second-hand now!

I think I'll be resorting to individual conduit wire, and saving this T&E for another project. At least it is all good practice before I rewire my garage later in the year!!

1-9-1

 

[SimonH2]

OK, so change the 32A MCB to a 20A one and then you can 'daisy chain' the sockets (ie a radial circuit) instead of a ring.

Am I right in thinking that a 25A MCB is also OK if the manufacturer offers that option ?

If the installation is all in conduit why the twin and earth, surly singles would be much easier to work with?

Well there is a practical issue. My local wholesaler will sell T&E by cut lengths, but singles only in full rolls - so unless you intend doing a lot of work, or can blag the materials from someone else who does, it's not really cost effective to use singles for a small job.

 

[EFLImpudence]

OK, so change the 32A MCB to a 20A one and then you can 'daisy chain' the sockets (ie a radial circuit) instead of a ring.

Am I right in thinking that a 25A MCB is also OK if the manufacturer offers that option ?

Not in conduit - but reference method C you could.