Ask The Trades - Print Page

Ask The Trades
https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Wiring in timber frame outbuilding https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1253171542 Message started by CWatters on Sep 17^th, 2009, 8:12am

Title: Wiring in timber frame outbuilding
Post by CWatters on Sep 17^th, 2009, 8:12am

I'm building a timber frame outbuilding - basically 2"x4" frame with ply and oak cladding on the outside and OSB or ply inside. I'll obviously be using a Part P electrician to do the electrics but haven't got one lined up yet.

To cut a long story short...Does wiring in this sort of structure have to be done in conduit on the inside or can it be done with bare cable in the frame before the inside is lined with OSB? It makes a difference as to when I get an electrician out.

Title: Re: Wiring in timber frame outbuilding
Post by big_all on Sep 17^th, 2009, 11:40am

assuming your using 12mm boards if you drill a 22mm hole dead centre in the 4" you will be 2" away from the face so i believe in a safe zone

dont forget you need to downrate a cable that surrounded by insulation or pinned into position

Title: Re: Wiring in timber frame outbuilding
Post by wozzy on Sep 17^th, 2009, 7:54pm

obviously conduit would be lovely for rewiring, mechanical reasons, but are these walls or structure going to be any more onerous (is that the correct word?) than a normal stud wall?

Out of curiosity are the walls insulated?

Title: Re: Wiring in timber frame outbuilding
Post by CWatters on Sep 18^th, 2009, 7:58am

Thanks for the replies.

The building will mainly be used for storing mowers and the like. One half will be a potting shed. I guess the most likely loads are a float charger for the mower battery and possibly a low power heater for a seed propagator. Haven't decided if I will insulate the walls yet.

I'm hoping that the electrician will be able to connect into an existing armoured cable that was professionally installed three years ago to power a single garden socket. That currently has it's own 20A RCD in the house CU. If not then things will be a bit harder as my CU is physically full. Lots more to worry about. I think there is quite a long length of 2.5mm ? between the CU and outside wall where it changes to an armoured cable. Haven't yet worked out all the lengths but that could well be a factor I guess.

The building will be fine without the internal boarding so will leave that off until I can get the electrician in. Lots of other work to do on it until then - it still hasn't got a roof.

Title: Re: Wiring in timber frame outbuilding
Post by Y3 on Sep 18^th, 2009, 4:48pm

CWatters wrote on Sep 18^th, 2009, 7:58am:

20A RCD in the house CU.

Didnt realise they made em ;)

Title: Re: Wiring in timber frame outbuilding
Post by sparky415 on Sep 18^th, 2009, 6:59pm

CW,
A 20 amp supply is probably fine for a socket and a couple of lights but will you be turning it in to something else in the future (office/workshop)?
Can you access the outside of the SWA?
With any luck you will be able to see the size of the cable (3 x 4mm for example)
How long is the run?

If you do have to run a new cable you will have room in the fuse board when the old cable comes out :)

If I'm concerned about my cables in stud walls, although they are in the safe zones
(kitchen for example) I will sometimes nail on some 'safe plates' so the kitchen fitters don't get too carried away ;)

Title: Re: Wiring in timber frame outbuilding
Post by CWatters on Sep 19^th, 2009, 2:36pm

sparky415 wrote on Sep 18^th, 2009, 6:59pm:

CW,
A 20 amp supply is probably fine for a socket and a couple of lights but will you be turning it in to something else in the future (office/workshop)?;)

No it's just a garden store/potting shed.

Quote:

Can you access the outside of the SWA?
With any luck you will be able to see the size of the cable (3 x 4mm for example) How long is the run?

Just uncovered a section and unfortunately it looks like 2.5mm. It's marked 3 x 2.5. So existing circuit looks like this..

20A RCD (or do I mean an RCBO?) in house CU
approx 18 meters of 2.5mm to get to outside wall
Junction box inside
approx 20 meter armoured cable marked 3 x 2.5
Single 13A MK masterseal socket

If that were to be extended to the new building by the most obvious route the armoured cable would be approx 43 meters (gulp) making a total of around 61 meters each way in 2.5 mm.

Google says the resistance of 122 meters of 2.5mm is around 0.87 Ohms. At 20A that's a voltage drop of 17V which I think is more than you are allowed? Oh bother.

If the cable took a short cut across the lawn the total length would be a bit shorter at around 55-56 meters each way. Resistance would be 0.8 for a 16V drop so I don't think that works either.

Am I thinking on the right lines or have I missed something?

Quote:

If you do have to run a new cable you will have room in the fuse board when the old cable comes out :)

Ah yes hadn't thought of that. I guess I could disconnect the old one.

Title: Re: Wiring in timber frame outbuilding
Post by CWatters on Sep 19^th, 2009, 2:45pm

In an ideal world I'm looking to have two sockets and three lights out there. I know what we want to use the sockets for won't take anything like 20A total but what do I have to allow for?

Title: Re: Wiring in timber frame outbuilding
Post by sparky415 on Sep 19^th, 2009, 4:15pm

Its probably an RCBO,

Sounds like you will need a new SWA mate
You can please yourself with what you allow for
Personally I would put in a 6mm 3 core (4mm min)