Electric from my house to my sheds

[pricer07]

Hi,
My sheds are 80 meters away from my house.
Ive got an outside plug fitted so could I just dig a trench (450mm deep) plug the one end into the outside socket and then get a socket for the other end? What size armoured cable would be best? Im thinking 6mm 3 core as I would only run like a 2hp compressor and few 5w lights.
Thanks

 

[PrenticeBoyofDerry]

you won't be able to terminate a plug to SWA!
your topic heading mentions sheds, please expand on this.

 

[pricer07]

how would I get the electric to the cable without going to the trip box? I cant get it to the trip box. I just want power to my sheds but don't want a generator running all the time. thought I would just install 6mm cable unground from my house to the shed. What would I need to have at either end of the cable? Thanks

 

[PrenticeBoyofDerry]

What size cable are jointing to?
It's not as simple as spurring of a house socket and routing 80m.
There are a lot of things to condsider, what type of earthing arrangement do you have, any idea of the impedance on the existing circuit, the size of device protecting this circuit and is it RCD protected?

 

[pricer07]

What size cable are jointing to?
It's not as simple as spurring of a house socket and routing 80m.
There are a lot of things to condsider, what type of earthing arrangement do you have, any idea of the impedance on the existing circuit, the size of device protecting this circuit and is it RCD protected?

I guess it would be 2.5mm?? would it not run off the house earth? Sorry but didn't realise all this.
If you were doing this from scratch, what would you do?
I cant get the cable to the consumer unit so what way round it is there? could you connect it through a socket in the house? I wouldn't be doing the connecting.

 

[ban-all-sheds]

What size armoured cable would be best?

This will tell you:

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

Select 2-core SWA, even if you'd be using 3-core, and PVC even if you'd be using XLPE.

 

[ban-all-sheds]

I guess it would be 2.5mm??

You guess? You can't do electrical design by guesswork.

would it not run off the house earth?

Not necessarily - depends on what type of supply you have, and on certain factors in the shed.

Sorry but didn't realise all this.

I suspect there's quite a lot you don't realise.

If you were doing this from scratch, what would you do?

Split the tails and go through a switchfuse.

I cant get the cable to the consumer unit so what way round it is there?

Lose the "can't", and find a route.

I wouldn't be doing the connecting.

You need an electrician to do all of it. I expect he'll be happy to let you dig the trench though.

 

[ericmark]

The two things I notice are 80 meters and 2 HP motor so run amps around 6 with start amps around 20 with a single phase motor so volt drop really needs to be worked out at 20 amp. Looking at tables 6mm² drops 7.3 mV/A/m so simple maths around 11.5 volt permitted volt drop 5% so 11.5 volt so right on the boarder line with volt drop that is clearly with 6mm² all the way back to the consumer unit.

At that distance earth loop impedance would also be right on the boarder line and likely one would need to install as a TT supply. There are papers on how to decide if to export a house earth or to TT and I have no intention of repeating what is a free down load from IET web site but clearly this will use jargon and unless you under stand phrases like TT and TN-C-S it will not help you much.

The way you talk about generators likely you already have earth electrodes and the like installed but it's not just a case of what you have it's also about what is around you and the meters required are very expensive and as BAS says not really a DIY job.

 

[syl]

I am literally in the middle of doing the same over a very similar distance. The 3 core 10mm2 SWA went in a couple of weeks ago (I got a mini JCB to dig the trench and put the cable through 63mm twinwall conduit) and the sheds are set up with sockets/switches where I want them.

The electrician is coming back (he came out before I did the work, to go through the design) to connect the SWA, run the internal cabling and do the testing.

My power requirements are minimal, but I only want to do this once (conduit just in case) and the third core of 10mm2 cable can be used as earth, even if the armour ever degrades in future. As the trench makes a mess going in I put 2 spare conduits in to take a water pipe and data cables, if they are ever needed in future.

Untitled

• syl
• 19 Nov 2013
• 1

Untitled

• syl
• 19 Nov 2013
• 1

 

[JohnW2]

At that distance earth loop impedance would also be right on the boarder line and likely one would need to install as a TT supply.

That's an interesting statement. IF EFLI is the only issue, it would seem a bit odd to TT an outhouse solely to achieve the required disconnection times for L-E faults. The TT'd outhouse installation would obviously not provide the required disconnection times (probably no disconnection at all in most cases) with an OPD - one would have to rely upon an RCD. That being the case, wouldn't it make (electrical) sense to simply rely on an RCD anyway, and not bother about the TTing (but retain the, much lower impedance, TN earth)?

I can see at least one reason why people might take your approach, but it would probably be in an attempt to achieve strict ('blind') compliance with regs, rather than for any electrical reason. With a TT'd outhouse, even the regs could not complain about one relying in an RCD for protection against L-E faults, since there is no alternative. However, in the absence of TTing, some people might find regs which suggested that it was unacceptable to 'rely upon' an RCD for protection (arguing that an RCD could only be used as 'additional protection' in TN installations). However, IMO, that would be (electrically) daft

Kind Regards, John

 

[mfarrow]

At that distance earth loop impedance would also be right on the boarder line and likely one would need to install as a TT supply.

That's an interesting statement.

Even more so because you still have a requirement to protect the armoured cable.

80m of 3c 6mm² will give loop of 0.584 ohms, ignoring armour. Assuming you've a good Ze then you should be OK, as even on a 50A BS88 you have a good 1 ohm to play with.

 

[JohnW2]

At that distance earth loop impedance would also be right on the boarder line and likely one would need to install as a TT supply.

That's an interesting statement.

Even more so because you still have a requirement to protect the armoured cable. ... 80m of 3c 6mm² will give loop of 0.584 ohms, ignoring armour. Assuming you've a good Ze then you should be OK, as even on a 50A BS88 you have a good 1 ohm to play with.

Indeed. Another point about 3-core is that (again ignoring armour) the R1+Rn is the same as the R1+Rn - so that inadequacy of the EFLI (disconnection time-wise) would quite probably also mean inadequacy in relation to protection from L-N faults (something which RCDs obviously cannot help).

Kind Regards, John.