getting power to an outbuilding - will this work?

Postby spartax » Sat Apr 05, 2008 3:12 pm

Hi all.

I want to build an outbuilding at the end of my garden. it will be 28 metres from the consumer unit in the house and the building will be 5.2M x 2.8M (approx 15 square metres). I currently have a spare cooker radial run from the consumer unit (as we have just switched over to gas cooker), and this is very close to the rear of the house, which would save the large effort involved in running another 'spur' from the consumer unit .

The out building will be used as a studio, running a computer, a mixing desk, 4 or 5 sound modules (synthesizers) and 1 power amplifier (100 watts). I will probably also want a small heater unit and might occasionally run a guitar amplifier ( I understand that amplifiers can 'demand' quite a heavy load at times, albeit in very short bursts). I will also need a lighting circuit (flourescent or low voltage halogen).

I intend to get a small consumer unit installed with 2 circuits (power ring main and lighting). This is not something i will be doing myself, but I just want to know if this means of getting power to it would be , before i start building the studio. If this won't work, and i have to run more 'spurs out from the consumer unit, it might not be worth building.

This spare cooker 'spur' is essentially a radial circuit. if this is used to supply the new consumer unit in the studio, can a ring main then be run from this around the studio?? and if so, is this just too great a distance to run from the household consumer unit. Or, would this require a separate ring to be run from the consumer unit in the house, out to the studio and back?

Also, would it be acceptable to use a junction box to extend the spare cooker spur out to the studio, or would a complete new cable have to be run from the house consumer unit right through to the studio?

If this won't work, or would not be safe due to lengths etc, how would i get the power out to this building?

Thanks in advance to anyone who can offer any advice on this before i embark on the building work - if this is not going to be feasible, then obviously it won't be worth building it!

Posts: 3
Joined: Sat Apr 05, 2008 2:32 pm


Simply Build It

Postby ericmark » Sun Apr 06, 2008 7:45 am

As I am sure you realize it is not straight forward but in general terms what you are talking of doing seems OK. Taking from cooker switch would most likely work.

But you will need to work out all loadings and cable runs including earthing methods. We don't know what your existing earthing arrangements are and you may need earth rods and/or heavy earth cable plus I would think steel wire armored cable which may not be easy to gland into a cooker supply.

I would suggest you get someone on site to advise. Although we can guess on what you need we could get it completely wrong when we are not there.

You also do need to work out demand. If we assume the cooker unit is supplied in 6mm cable then we must continue with same size. Thats 7.3 mV/A/m volt drop and max allowed for lights is 3% volt drop so 28 meters at 32 amp is 6.5 volt drop and allowed 6.9 so very close to limit. This may mean you would need to go from consumer unit with 10mm cable so you really should get someone on site to work it all out.

All best Eric

Postby spartax » Sun Apr 06, 2008 2:20 pm

Thanks for that advice Eric. Very useful. I wouldn't know where to begin calculating loads etc, but as you say - probably best to get someone in. i sort of assume most elecricians wouldn't want to do a site vist and quote on a building that isn't even started yet, esoecially if i don't really know if it will even get built!

The main thing is knowing that it is feasible to do, and so that is a useful starting point.


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Joined: Sat Apr 05, 2008 2:32 pm

Postby ericmark » Sat Apr 12, 2008 6:38 pm

Try the following link:- ... tdoors.pdf
I think this should help.

Postby spartax » Sun Apr 13, 2008 8:27 pm

Thanks for that info,

I had already seen this, but it does not make it clear whether i can run a ring main off the second consumer unit within the outbuilding. I assume this is possible, but any advice regarding this would be useful. at least i know it is possible to get electricity out there, even if i have to get a new line run from the consumer unit with 10mm cable.


Posts: 3
Joined: Sat Apr 05, 2008 2:32 pm

Postby ericmark » Sun Apr 13, 2008 10:17 pm

Yes you can run a ring main. Although in a shed I would not think it really necessary. Remember 300mm between computer LAN cable and mains and mains cables surrounded in metal i.e. steel wire armored are lest likely to cause interference on your LAN cable.

Postby apprentice08 » Mon Apr 14, 2008 6:52 pm

hi guys i am new to this but i have a very important question if you can help me with.

I have an existing consumer unit near the meter, i have installed a second consumer unit to to power the extension to the house.

i need to power lights sockets and shower only 9.5 kw shower

I have run 2 6mm twin and earth wires to power the new consumer unit, Each 6mm wire is on a 50amp mcb.

Can somebody advise if that is ok, or do i need to put 6mm twin and earth both on one mcb to power the second consumer unit?.

plzzzzzzzzzzz help
Posts: 6
Joined: Mon Apr 14, 2008 6:39 pm

Postby ericmark » Tue Apr 15, 2008 9:51 am

6mm twin and earth what? 6242Y or 6242X the more normal 6242Y PVC even on ref 102 is only 35 amp. So if we assume PVC then you are well over the top.
1. Cables run in parallel have their own special rules see post Wiring for a 11Kw Electric Boiler where a lot is explained.
2. Routes are very important unlike flex which you must be able to see throughout its route cables buried in walls, ceilings and other voids have current carrying capacity which varies and there are tables which allow one to look up these values. Then there are also tables to correct these valves according to the temperature routing through a loft space it could reach 40 deg C with summer sun on the tiles.
3. Amps = Watts/Volts so 9.5Kw = 41.3 amps so 2 x 50 amp is double what you need.
4. Incoming fuse is normally between 80 and 100 amp so 2 x 50 amp MCB's would possible allow main incoming fuse to blow before the trips activated.

I am very surprised building control did not pick this up when you applied under Part P. I would assume your not a registered person. There are other considerations as well such as the length of cable and how this will affect volt drop, prospective short circuit current, and earth loop impedance remember you will need to submit your paper work to building controls who if they have any sense will have a spread sheet in excel or similar into which they enter your reading if anything like the one I use it will high light any reading that don't match so will alert them of any chances of cheating before they ever visit the site.

As an apprentice I will assume you know all about Part P and the tests and forms that need completing but if thats not the case you will find links in projects section. Well worth a read if you have not already done so.

Also hijacking someone else's post is not the way to get answers you should post as new post.

Postby apprentice08 » Tue Apr 15, 2008 5:03 pm

thanks eric, i didint mean to hijack the forum, aplogies in advance.

However can you give me a solution to the problem, any advice be well appreciated.

Shall i put both 6mm cables on single 50 amp breaker. I have no means of taking saw cable otherwise i would have done that.
Posts: 6
Joined: Mon Apr 14, 2008 6:39 pm

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