Garden cabin wiring variation

[dextrous]

Am having services installed for my log cabin (approx 20m from main house and thus 30m from consumer unit).

The original specifications were to run 4.0mm armoured cable to the garage with 4 way RCD consumer unit within the cabin to supply rings and lights for this and the adjacent garage, and a 65A isolation switch (with new tails to it) adjacent to the main house CU.

As the water pipes being run to the cabin (hot and cold) have been insulated for their entire length, and run within 110mm drainage pipe, the contractor has discussed the option of using 6mm (non-armoured) t&e cable through this ducting instead. Obviously, any variation in the cost of materials used will be accounted for, so this isn't a way of him increasing his profit, and I'm not worried about any extra cost for using SWA should that be more appropriate in your opinion..

BCO will be inspecting the building work (and hence the cabling run) and one of his electricians will make the final connection and thus certify.

So, finally, and at last (I hear you say) are there any problems with doing this or any advantages/disadvantages either way?

 

[Adam_151]

T&E in plastic drainpipe does not comply with the requirement for mechanical protection!

Also worried about the local of overcurrent protection at source as well!, oh and 4mm² seems a little on the light side

The 'contractor' I presume is the main contractor... if he is designing and installing and the electrical contractor is doing the final connections and testing (not neccessarly a problem if its being done by building notice)... except for...is he signing the design and contruction sections of the EIC, and is he competant to do so?

 

[ColJack]

32A fused switch in the house, can't just be an isolation switch.. you're stepping down to 4mm.. ( 37A max buried..)

needs to be SWA whether it's in a duct or not... the duct does nothing to stop you putting a spade through it.. earthed metalic sheath required..

thought about heating?

electric?

need something bigger than a 4mm then..

 

[dextrous]

Hmm! Questions to be asked then!

 

[dextrous]

What would your best guess of SWA cable size be then, if we were to allow for 2-3 or 4 kW heater?

 

[ColJack]

what else will be in there power wise ( estimated usage? ) ?
calced it based on 6Kw, 35m and 2 core PVC SWA ( doesn't matter if you use 3 core.. but the calculator asumes it's 3 phase if you try it )..

it sugests 6mm..

 

[dextrous]

what else will be in there power wise ( estimated usage? ) ?
calced it based on 6Kw, 35m and 2 core PVC SWA ( doesn't matter if you use 3 core.. but the calculator asumes it's 3 phase if you try it )..

it sugests 6mm..

Lighting circuit, TV, PC, table lights - nothing heavy other than a fan heater (or two?).

Not that I understand it, but is there a need to use 3 core then? Haven't really got a clue about what 3 phase means. If you can be bothered to explain, it'd be nice, but don't lose any sleep over it if you think life's too short at times.

If I don't here from you, I'll ask the contractor to upgrade to 6mm (three core) SWA then, which I assume is OK to run through the trunking, now that it's buried (with draw strings!)

PS - thanks for doing the calcs for me.

 

[Lectrician]

I would also go for 6mm and 32amp - on your run this is ok.

Is it really a good idea running hot water over that length!?

 

[Adam_151]

For a fag packet design:

If we are assuming 4kw of heating load (not sure what heating you having, but I've just taken the figure you have given me), now taking the fact that its a log cabin as well, you might want to boil a kettle as well in there, and you might have some small applicances running, then I'd suggest you certainly dont want to be looking at less than a 32A submain (7kw), and it might even be worth going to 40A (9kw) as you only want to dig the trench once

Calculating for volt drop (based on 35n cable length(, and working to a max of a 3% drop (you are planning to have lighting in this cabin I presume!), for 32A it comes out at 10mm² which drops 2.2%... recalcuing for 40A, gives 10mm again which now drops 2.7% (which leaves a little for your final lighting circuits, and quite a bit more for final heat and power circuits)

So if it were me, I'd be putting 10mm² 3c armoured in, unless of course you choose to loose some of the heating?

Oh and you haven't told us what the suply type is, whether there are any extraneus conductive parts in the outbuildings, etc yet

 

[dextrous]

I would also go for 6mm and 32amp - on your run this is ok.

Is it really a good idea running hot water over that length!?

Well, it's only 25m from the combi. Probably won't use it that often to be honest, but it's there if needed. Installing it now seemed as good a time as any.

While I'm in such a polite mood, thank you too for your confirmation of the specs. (Am so glad noone has said 16mm or something indecently expensive!)

 

[dextrous]

So if it were me, I'd be putting 10mm² 3c armoured in, unless of course you choose to loose some of the heating?

OK - still reasonable

Oh and you haven't told us what the suply type is, whether there are any extraneus conductive parts in the outbuildings, etc yet

Pardon?

 

[ColJack]

without going into college level detail,

3 phase is what electric is made in and distributed as down the street..

3 live wires and a neutral.

each house then gets one live and the neutral from the main wire.. your next door neighbours will be on the other 2 wires, one one side, one the other..
so every 3rd house is on the same live wire.. ( so if your power goes off, check 3 doors up and down, not next door, theirs may still be on.. )

some places get all 3 live wires ( such as factories and some big houses. )

 

[ColJack]

extraneous conducticve parts.. bits of metal that some part of is buried or in the concrete floor..

metal framework, water pipes ( so yes you have ), gas pipes ( might be a better idea.. run gas in then you can have a gas heater in there and a small stove.. )

put in a spare duct while you've got the trench open, you might want telephone, cable / satelite tv, internet etc down there too... ( cctv might be a good idea as well.. 2 way, to see the office from the house, and the front door etc from the office? )

 

[dextrous]

without going into college level detail.....

3 phase is what electric is made in and distributed as down the street..3 live wires and a neutral.

each house then gets one live and the neutral from the main wire.. your next door neighbours will be on the other 2 wires, one one side, one the other..
so every 3rd house is on the same live wire.. ( so if your power goes off, check 3 doors up and down, not next door, theirs may still be on.. )

some places get all 3 live wires ( such as factories and some big houses. )

I see. Dare one ask, for what reason this is done, other than possible installation costs of main cabling by the suppliers and (total guess) a way of providing backup should one of the "wires" fail?

 

[dextrous]

extraneous conducticve parts.. bits of metal that some part of is buried or in the concrete floor..

metal framework, water pipes ( so yes you have ), gas pipes ( might be a better idea.. run gas in then you can have a gas heater in there and a small stove.. )

put in a spare duct while you've got the trench open, you might want telephone, cable / satelite tv, internet etc down there too... ( cctv might be a good idea as well.. 2 way, to see the office from the house, and the front door etc from the office? )

Water pipes are plastic, running a gas pipe is a whole can of worms in itself (although an LPG heater is curiously becoming attractive to me!).

The pipes and cable are running through a 110 drainage pipe, with extra draw strings within, so should be enough room to run other cables along (although best laid plans. ..).

So now I'm between minds - 6mm with 32A as you say, or 10mm with 40A as Adam is saying. Just dunno - only about £30 between them I'd guess, but er, well, you know