One outdoor buried supply to two log cabins.

[Crazyf001]

My summer project is to build two large log cabins at the bottom of my garden. They are separated by a couple of metres. One is for use as a workshop / storage and the other for leisure / home office use.

I am planning on burying a 3 core 10mm2 SWA cable about 600mm under the mainly grass garden, with a metal junction box connecting to 10mm2 T&E from the RCD protected house CU with a 32A MCB. I am using 10mm2 cable throughout due to the relatively long run of over 40m to the workshop as I want to minimise volts drop.

Each cabin will have 4 or 5 double sockets and 1 or 2 internal lights plus an external PIR lantern. I don't want any visible cabling between the two cabins.

My question is this:

Can I have an RCD protected 2 way CU in EACH cabin (1 x 2.5mm2 spur and 1 x 1.5mm2 lighting circuits). The first workshop cabin would have the 10mm2 SWA cable IN and another SWA (unfused) 10mm2 SWA cable OUT to the other lighter load cabin CU. This is effectively a radial distribution.

Alternatively, would it be better to have a single 4 way CU in the workshop cabin with buried 2.5mm2 SWA and 1.5mm2 SWA cables to the second cabin.

Thoughts / opinions welcome.

Many Thanks.

 

[17thman]

Are you going to do the work yourself?

you dont really want to protect the distribution cable with and rcd at the supply end and then have rcd's within the cabins too. maybe poor discrimination.

Anyway. you are going about this backwards, if you are asking these questions you need to report what is going on in the work shop.

a 2kw heater and a desk lamp or a large hadron collider?

 

[Crazyf001]

I am going to do the work myself and yes - I am am going to notify the work.

The workshop will have 2kw heater, lighting domestic power tools, radio etc. Nothing industrial!

The existing house CU is a non-split type with RCD protection of all MCBs - there are 2 spare positions and I am planning on using 1 of them with a 32A breaker. The 10mm2 cable should give sufficient fault current to trip the MCB and 2 x 16A spur circuits and 2 x 6A lighting circuits should have sufficient discrimination from the 32A house breaker.

I understand what you say about duplicating RCD protection giving unreliable tripping in the event of an earth fault in the cabins. I'm not too worried about this as I'm hoping this will be a very irregular event.

My main concern is whether I am allowed to have two CUs on one radial supply.

 

[ban-all-sheds]

My summer project is to build two large log cabins at the bottom of my garden. They are separated by a couple of metres. One is for use as a workshop / storage and the other for leisure / home office use.

Why not put up one large one and build a partition to split it into two halves?

No need to bury a cable between the two sections, one CU in there will be fine, etc.

And I'd advise supplying it directly from the meter tails through a switchfuse, rather than from a way in the house CU.

What are you going to do about testing?

 

[Crazyf001]

My summer project is to build two large log cabins at the bottom of my garden. They are separated by a couple of metres. One is for use as a workshop / storage and the other for leisure / home office use.

Why not put up one large one and build a partition to split it into two halves?

No need to bury a cable between the two sections, one CU in there will be fine, etc.

And I'd advise supplying it directly from the meter tails through a switchfuse, rather than from a way in the house CU.

What are you going to do about testing?

Thanks for the reply.

I am building two cabins because the total internal area I need is over 30m2 which would take it out of the "permitted development" category. Building two permitted development allows me to get around this. It would also be more expensive for one (very) large kit with dividing wall than it is for two. I've ordered a 5.5m x 4.5m and a 3.5m x 4.5m matching cabins. The space between them will be partially covered and used for winter storage of garden furniture so no space is wasted.

I've gone off the idea of 1 CU in each cabin after talking to an E&P Engineer at work (I am an electrical engineer but not practiced at 17th edition wiring). I think I'm going to put one RCD 4 way CU in the large cabin and run a buried 3core 1.5mm SWA and a 3c 2.5mm SWA between them.

The E&P engineer suggested swapping the standard MK 80A RCD in my main CU for a Time delayed 80A 100mS RCD to ensure that the cabins trip first. This is probably a good idea but it significantly increases the scope of notifiable work (and inspection / testing). This brings me onto the suggestion of running tails from the meter to a switch fuse - I like the sound of this! What do you think about me installing a 2 pole Henley Block in the meter box to allow the tails to be split? Then run a 10mm2 cable from the Henley Blocks to an external wall mounted IP66 box with a switch/32A fuse. Then a 10mm2 SWA to the large cabin. Then two small SWA cables to the smaller cabin. This should confine the extent of notifiable work and testing of existing installation to the meter connection only as it will not affect the house CU installation.

Testing is a good one - and I could probably do with some advice here. I regard myself as meeting the official criteria for competency and have access to all the required testing equipment. Problem is I believe in the principle of independence of design / testing and would like someone else to inspect / test my work after I have tested it myself. I am happy to fill in all the design documents and electrical Installation Certificate and might get a sparky to conduct a Periodic Inspection Report to support the documentation. From what I have read, this should satisfy the Local Authority Building Control.

Does what I'm thinking sound ok?

 

[Chri5]

No they will want a eic.

The e & p guy is suggesting a time delay rcd that will adjust the trip for every circuit on it, this includes circuits in the main house?

Silly idea. Separate the mains by adding in a henly, from there resupply the main cu and use the henly as a tap in for the cabins via a fuse switch.

As you know this now means any fault conditions at the cabin will not trip rcds that have house circuit on them. Best design.

Surely one of your mates has a contact that is registered and will allow some side by side working. If not, phone around we aren't all evil. A chat, some inspections, test and paperwork, not much work just accept that we all have some costs before we even fart.

 

[Crazyf001]

Thanks Chris.

I think I'll take your advice and get a registered electrician involved from the outset so that he can see what I'm doing right the way through. One of the main reasons I want to do the work myself is that it means climbing under the downstairs floor voids (tight fit but I've done in several times) and burying the cables for a significant length. This is likely to take a while and an electrician is not the ideal person to do this sort of work.

If I do all the hard graft, including fitting cabin equipment and leave the connection to the meter and all the inspection and testing to the sparky, would this be acceptable - presuming I can get an electrician who is happy to take this on. I would make sure I had evidence of depth of burial and tape wrapping etc.

 

[1john]

If you can find someone will to do this then that sounds fine, I think you are over engineering things running two SWA's from the workshop to the summer house. I am all in favor of having a large supply to a workshop and distribution circuits there but for the summer house why not just run a 2.5mm SWA cable and install a 5A unswitched spur in the socket circuit for the lights. If you install a duct or two between the cabins then any future changes are gonna be achievable.


Is there going to be a water supply to either cabin??

 

[Crazyf001]

... why not just run a 2.5mm SWA cable and install a 5A unswitched spur in the socket circuit for the lights. If you install a duct or two between the cabins then any future changes are gonna be achievable.

Not a bad idea. I might just do that.

Is there going to be a water supply to either cabin??

No - but there might be a chilled beer supply!!!

 

[Crazyf001]

I have two more questions about this installation.

To separate the meter tails to supply the existing CU and new switch/fuse I need to add a pair of Henley blocks. I have it in mind that these should be ok in my flush meter box if I tuck them away in one corner leaving space in case the DNO ever want to put a bigger meter in. I had my doubts whether I am allowed to put any equipment in the meter box so did a quick Google search and found this:

http://www.eonenergy.com/FAQ/New-Connections/Meter-Box-Size.htm

e-on say that a space 500mm x 350mm should be provided for the meter with no consumer equipment in the meter box. This is a bit restrictive in terms of splitting the tails.

My questions are:


Q1. Is it allowable to put Henleys in the meter box?

Q2. If I'm allowed to put Henleys in there, can I also put the switch/fuse in there (leaving as much space as possible for a larger meter).

I am hoping that the space requirements that the DNOs ask for are just to make sure their meter fits and that there is no mandated restriction on fitting consumer equipment. After all, some internal meters are on a shared board with consumers equipment.

 

[ban-all-sheds]

I am building two cabins because the total internal area I need is over 30m2 which would take it out of the "permitted development" category. Building two permitted development allows me to get around this. ... The space between them will be partially covered and used for winter storage of garden furniture so no space is wasted.

IMO there's a strong case to be made that that is effectively one building. I say that because when I was planning a large-ish wooden outbuilding, which would have had an overhang on the roof, that the roof area, and distance from the house and boundary, would be used for size and distance criteria, not the walls.

I advise you to check your plans with the council. If they are OK, and there is nothing the council could do to stop you then you'll be fine. But if, OTOH, your evasion of regulations can be stopped, then relying on secrecy will be a bit risky, given that you'll be telling the council about the buidings....

 

[Crazyf001]

BAS - I know what you are getting at about the roof overhang. I've made sure that the nearest part of the cabin (the roof) will be over 2m from any boundary. I would probably get away with 1m but there is another "trap" as well that a jobs-worth planner might take exception to. The overall height of the cabins are just under 2.5m maximum height (deliberate design criteria), but the base will be elevated by about 200mm, which JUST takes it over 2.5m. To avoid problems, I have made sure that I apply the rules as if they are over 2.5m just in case.

I'm pretty sure that each building is treated separately for planning purposes as the Planning Portal's own guide shows a garden with several outbuildings with no restrictions between each other. The rules are fairly easy to interpret.

http://www.planningportal.gov.uk/permission/commonprojects/outbuildings/miniguide

The 30m2 limit is for the building regulations side of the development. I have already spoken the the Local Council Building Control office and they have confirmed that each building is taken in isolation. Both buildings are under the limit and they have no interest (apart from Part P of the wiring regs). I will be getting a part P certified electrician involved from the outset to inspect as I run the buried cables in before I build the cabins, and inspect / test afterwards.

Any advice on my Q1 and Q2 in my earlier post about equipment in the meter box?