Electrical Question

Getting power to a shed

I plan to build a large shed/workshop at the top of my garden in a year and will be wanting power to it. The shed is approximately 20m from the back door and the CU is a further 12m from the back door as it's next to the front door.

I currently have the kitchen ceilings down for other work that is being done so thought now would be a good time to drill through the exterior wall, leave a 25cm of cable poking outside and then run a cable up a channel in the kitchen wall through the kitchen void and will leave it under the upstairs floor (about 3m from CU) for now until i am ready build the shed.

The shed will just have lights and a few power sockets, nothing that will need a lot of power.

My questions are;
a) would 4mm cable inside and outside be ok?
b) I was going to put the 25cm in a junction box out of the rain for now, which IP do I need?
c) I read that the junction box needs to be at least 2.5m from my back door, is this correct?
d) Are there any other rules/regulations I need to take into consideration?

PLEASE NOTE. I am not planning connecting the cable myself. I just want take advantage of the kitchen ceilings currently not being there by putting the wire through this void. One end would be left curled up in hallway ceiling void ready to be connected to the CU at a later date and the plan for the other end was to curl it up in a junction box on the outside of the kitchen wall.

3 Answers

Best Answer

Mark, as previously stated, first and foremost Part P of the building regulations applies to this work and as such if your planning on installing just a section of cable above the kitchen ceiling you need to register the planned works in advance with your local authority building control! This cable will need to be SWA cable so as to be protected from nails or screws etc, as per BS7671:2008.

With regard to 4mm cable, chances are it will be ok, but at 37m the voltdrop needs to be considered, and as such 6mm or 10mm steel wire armoured cable would be better. If your house has a PME supply, then the SWA to the shed will also need an earth rod installing to make the sub-main TT at the shed end (this is so that the PME regulations regarding exporting of the suppliers earth are fully complied with).

I cant stress strongly enough against using an IP67 junction box as this will always be a potential weakness and is very poor electrical design! Why cant you get 40m of SWA and leave the excess coiled up outside the kitchen?

The SWA cable should be run direct (unbroken) from your consumer unit straight to the shed. At the shed end an insulated 2 way RCD consumer unit should be used with the SWA armour wires bonded down to earth via a copper plated earth rod (if your incoming supply is PME). If you have a TNS supply the the SWA armouring can be bonded to the Main Earth Terminal in the house. The SWA should be ideally buried at 450mm deep in the garden and laid on a bed of sand or ducted along its length.

On completion of the work you will need to get LABC (building control) to inspect and test the work (approx £200 -£300) so that a part p building regulation compliance certificate can be issued.

To be honest with you you would be better of getting an NICEIC registered electrician to install the cable etc for you as they can self certify their work and log the completed job with building control.

Hope this helps please click on "like" if it does.

Answered 1st May 2012

Electrical Safety Services

Member since 17 Oct 2011

An electrician will answer all of your questions and the rules and regs are,if you are not qualified,you cannot do it!

Answered 1st May 2012

Roc builders

Member since 25 Aug 2011

For 37m lenght will be enough 4sqmm SWA cable if load will not be more than 3kw.

Answered 2nd May 2012

V.A.Electrical services

Member since 30 Oct 2010

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