Not just another 'Shed Thread'.

[conny]

After reading a number of shed threads it got me thinking of how I might tackle such a project, (assuming I had a shed worth electrifying), and I came up with the following 'Method Statement'.

Please feel free to constructively criticise or advise.


To Install Electric Supply to a Shed.

(All work to comply to BS7671: 17th Edition)

1/ To use 3 core, 6mm^2 SWA as the supply cable, buried in trench at a minimum depth of 460mm, encased in underground ducting and marked with a 'Warning' tape directly above.

2/ Mount an IP65 rated 'Garage CU' externally directly behind a 13A socket which is mounted inside the lounge and which has been confirmed as being part of the 32A ring main and not a spur.

3/ Cable to be run from internal socket box through 20mm conduit into external CU, (this cable is NOT to be connected to socket until all other connections have been made and tested accordingly). On exiting CU cable is to be correctly glanded using 20mm cw gland before entering trench.

4/ Cable to be routed to shed as per item 1 and terminated at a 30A double pole isolating switch inside the shed.

5/ From the double pole switch a 20A Radial circuit to be run to 2 double gang sockets using 2.5mm T&E and a single, switched, lighting circuit using 1.5mm T&E to be installed.

6/ All work to be carried out in a professional manner and tested to current IEE regulations and all relevant certificates completed.

7/ Trench to be back-filled with gravel to discourage any future planting taking place in area of cables.

 

[cjard]

And to think I thought this was yet another thread about a particular forum member...

Is this for a planning/regs application?

 

[ban-all-sheds]

If your idea is to supply a CU from a ring final socket circuit I suggest you stop after #1 and get an electrician.

 

[winston1]

After reading a number of shed threads it got me thinking of how I might tackle such a project, (assuming I had a shed worth electrifying), and I came up with the following 'Method Statement'.

Please feel free to constructively criticise or advise.


To Install Electric Supply to a Shed.

(All work to comply to BS7671: 17th Edition)

1/ To use 3 core, 6mm^2 SWA as the supply cable, buried in trench at a minimum depth of 460mm, encased in underground ducting and marked with a 'Warning' tape directly above.

2/ Mount an IP65 rated 'Garage CU' externally directly behind a 13A socket which is mounted inside the lounge and which has been confirmed as being part of the 32A ring main and not a spur.

3/ Cable to be run from internal socket box through 20mm conduit into external CU, (this cable is NOT to be connected to socket until all other connections have been made and tested accordingly). On exiting CU cable is to be correctly glanded using 20mm cw gland before entering trench.

4/ Cable to be routed to shed as per item 1 and terminated at a 30A double pole isolating switch inside the shed.

5/ From the double pole switch a 20A Radial circuit to be run to 2 double gang sockets using 2.5mm T&E and a single, switched, lighting circuit using 1.5mm T&E to be installed.

6/ All work to be carried out in a professional manner and tested to current IEE regulations and all relevant certificates completed.

7/ Trench to be back-filled with gravel to discourage any future planting taking place in area of cables.

1. Why 6mm?
2. Why off the ring and not a separate circuit? And why is this unnecessary garage CU outside?
3.
4. Why an isolator?
5. So your socket and lighting circuits are both connected to the isolator with no protection? Apart from that why 1.5 mm cable for lights when 1.0 mm is all that is required?

 

[ericmark]

1) Start again.

2) Decide load greater than 13A or less than 13A.

3) If less than 13A then feed can come from ring, if over 13A feed direct from consumer unit.

A ring final circuit has to follow a special set of rules. It was designed during the end of the second world war in preparation of a massive house building program in the UK where it was realized it's adoption would reduce the amount of cable required and dispense with the two sizes of plug (5 and 15 amp) allowing all items to use same plug and also reduce the size of the fuse box. Oddly is seems now, it was designed for electric heating. 7/0.029 cable was used which works out at about 3 mm² the pins were brass for whole length (no plastic sleeve) so today's ring final is not as good as the original design.

There are some faults with the system it is banned in some countries so we have a set of rules, which if followed will in the main ensure the cables are not overloaded. This includes if an item is over 2kW and fixed it is not supplied from the ring. So for example the 3kW immersion heater will have its own dedicated supply.

As to if a shed should be considered as a fixed appliance is open for debate. Can't get an 8.7A fuse (2kW) maybe a shed should be fused to 10A? but I would regard it as any other fused spur and limit to 13A.

Figure of 8 circuits are not permitted for ring finals, you can use the lollipop where the first part of the run is in a heavier cable then it splits to a ring, but once it is a ring you can't take a heavier cable to another point then re-split into a ring. The problems are many including the current draw on unequal length conductors.

I am not to be frank sure about the earth. So using 2.5 mm² 4 or 5 core cable may be a possibility, but one would need to consider volt drop. So before considering that approach step one is to measure the loop impedance or prospective short circuit current at the mid point of the existing ring. In real terms the ring would need testing at the consumer unit as it should be with an EICR with the wires removed and linked and all sockets tested for resistance and notes made if ring or radial for each socket and volt drop worked out from the R1 + R2 figures. This I would say is not a DIY job. The test kit used by electricians includes a low ohm meter which must use at least 200 mA when testing, an insulation tester which uses 500 volt for testing, a RCD tester able to measure tripping time as well as mA drain, and often a loop impedance tester which often has two displays either in ohms or amps. Without these meters any thoughts of special wiring like extending a ring final to a shed is just not an option.

For a ring final using 2.5 mm² cable we have a maximum amount of cable 106 meters. Problem is we don't know the route of the cable in a house, and the 1/3 rule for drilling beams was not in when the ring final was designed. So it's not uncommon to find even a small house is getting close to limit. That 5 core SWA if only 10 meters long will count as 20 meters and that does not include cable used in the shed. So in real terms I personally would never consider extending a ring final into the shed. For me main problem is you can't isolate the shed alone it's whole ring or nothing.

So in conclusion I hope you can see how your plan is simply never going to be an option even using 5 core SWA cable.

 

[winston1]

There are some faults with the system it is banned in some countries so we have a set of rules, which if followed will in the main ensure the cables are not overloaded. This includes if an item is over 2kW and fixed it is not supplied from the ring.

Not this again. There is no such rule, it is vague guidance only, and completely ignored by manufacturers who routinely fit moulded plugs to fixed appliances such as washing machines, dish washers, tumble dryers, and even some ovens.

 

[conny]

Thank you guys for the replies. As I stated in my post I welcome constructive criticism etc.

BUT, I also said, " how I might tackle such a project, (assuming I had a shed worth electrifying)," which I wrongly assumed you would be able to see I have no plans to electrify my shed. 1/ Its a rented house that I am in. 2/ The shed is quite small and neglected.

As I am not an electrician I certainly woul;dn't be tackling it myself because if I had a shed worth doing I would want it done correctly and contain probably more than just 2 sockets and a light. Lol

 

[EFLImpudence]

There are some faults with the system it is banned in some countries so we have a set of rules, which if followed will in the main ensure the cables are not overloaded. This includes if an item is over 2kW and fixed it is not supplied from the ring.

Not this again. There is no such rule, it is vague guidance only, and completely ignored by manufacturers who routinely fit moulded plugs to fixed appliances such as washing machines, dish washers, tumble dryers, and even some ovens.

Indeed - and, once again, that vague guidance only mentions cooking appliances exceeding 2kW - plus immersion heaters, comprehensive electric space heating and loads of similar profile, i.e. may be on for hours.

 

[ericmark]

There are some faults with the system it is banned in some countries so we have a set of rules, which if followed will in the main ensure the cables are not overloaded. This includes if an item is over 2kW and fixed it is not supplied from the ring.

Not this again. There is no such rule, it is vague guidance only, and completely ignored by manufacturers who routinely fit moulded plugs to fixed appliances such as washing machines, dish washers, tumble dryers, and even some ovens.

Where does fitting a 13A plug and being plugged into a ring final connect together? There is nothing in the appendix or main text of the regulations which says you can't have a dedicated spur terminating with a 13A socket so that fixed appliance can be easy removed for maintenance. Years ago it was common to have a 15A radial and 15A socket to supply the immersion heater so the plumber did not need to terminate any wires when swapping.

Having a 13A plug and being powered from a ring final are two completely different things.

As to shed wiring I see the idea of having a standard wiring method. The IET wiring matters does have a description of how to wire an out building.

electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

 

[winston1]

There are some faults with the system it is banned in some countries so we have a set of rules, which if followed will in the main ensure the cables are not overloaded. This includes if an item is over 2kW and fixed it is not supplied from the ring.

Not this again. There is no such rule, it is vague guidance only, and completely ignored by manufacturers who routinely fit moulded plugs to fixed appliances such as washing machines, dish washers, tumble dryers, and even some ovens.

Where does fitting a 13A plug and being plugged into a ring final connect together? There is nothing in the appendix or main text of the regulations which says you can't have a dedicated spur terminating with a 13A socket so that fixed appliance can be easy removed for maintenance. Years ago it was common to have a 15A radial and 15A socket to supply the immersion heater so the plumber did not need to terminate any wires when swapping.

Having a 13A plug and being powered from a ring final are two completely different things.

As to shed wiring I see the idea of having a standard wiring method. The IET wiring matters does have a description of how to wire an out building.

electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

The fact is most houses have ring finals. Very few have dedicated spurs terminating with a 13A socket. No manufacturers instructions say, "Don't plug this into a ring final, use a dedicated spur."

 

[EFLImpudence]

Where does fitting a 13A plug and being plugged into a ring final connect together?

I don't really understand the question but - at the socket?

There is nothing in the appendix or main text of the regulations which says you can't have a dedicated spur terminating with a 13A socket so that fixed appliance can be easy removed for maintenance.

There wouldn't be, would there? Aren't you saying it is all but compulsory?

Years ago it was common to have a 15A radial and 15A socket to supply the immersion heater so the plumber did not need to terminate any wires when swapping.

Did immersions used to come with flex and 15A plugs?

Having a 13A plug and being powered from a ring final are two completely different things.

I suppose they are - unless...

As to shed wiring I see the idea of having a standard wiring method.

How can there be such a thing?

The IET wiring matters does have a description of how to wire an out building.
electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

Not much help for a novice, is it?

 

[ericmark]

Non portable appliances are installed it is up to the installer to ensure the supply is suitable. In the main only when installed very near the end of a ring final will there be a problem and only then when there are multiply items plugged in at the end of a ring.

So consumer unit in the kitchen and oven, tumble drier, washer, and dish washer all plugged in one end of the ring final and expect problems. Consumer unit other end of house and all these heavy users in the centre no problem other than tripping the MCB every round and again.

In real terms just two items which can cause problems in the house which could be plugged in.
1) The immersion heater.
2) The dryer.
Nothing else runs for long enough drawing maximum amps to really cause problems.
We would not dream of running an immersion heater from the ring final so why try running a dryer from ring final?

And yes the immersion heaters were pre-wired in the counties workshops with cable and plug so the plumber could fit them without the electrician going to the house. The 15A socket for immersion heater was the only 15A socket in the house, it was used mainly as no fuse in a 15A plug so it did not get hot should cloths be pushed against it in the airing cupboard.

I do realise the linked IET report is not easy DIY reading. However neither is wiring an out building. I also realise this was pre 2008 so there are some things missed, it is now more complex not less. For a set job ignoring the complex issues is often OK. If you know 5 meters from house which has a TN supply why cloud it with all that is required for TT? However this guy seems to be looking for a one size fits all. Sorry that will not work.

 

[EFLImpudence]

We would not dream of running an immersion heater from the ring final so why try running a dryer from ring final?

We would not - but they used to and I have been to places where they are (and with plug in timer) and have been no problem at all.

Other places, all wired 'properly' and terminals burnt.

 

[JohnW2]

We would not dream of running an immersion heater from the ring final so why try running a dryer from ring final?

That 'trying' is probably done millions of times every day. I may be wrong, but I would suspect that the majority, quite possibly the 'great majority', of dryers in the UK are run from ring final circuits.

Kind Regards, John

 

[ban-all-sheds]

The IET wiring matters does have a description of how to wire an out building.
electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

Not much help for a novice, is it?

And also it is focussed on whether or not to export the supply earth to the outbuilding.