supply to log cabin

[webby_58]

hello all

i have to get a supply to a log cabin and have the following arrangment :

TN-S supply in house
House CU MCB's no RCD
supply to garage CU via 16amp MCB (2.5 t/e) / garage CU has 63a 30ma RCD
approx 35mtr to garage and 50mtr to house in cabin approx 6 sockets maybe an electric heater and 1 light circuit

i propose to spilt the tails in the house into 100a henley block and take a new swa to the cabin. CU in cabin with 30ma RCD 6a MCB for lights 16a MCB for socket ring

my query is the cable size. do i have to use 10mm swa 3 core to provide the correct size earth ?

what would be the best way to do this also taking into account cost.

please help.......!!!!

thanks in advance guys.

 

[malcolmsanford]

I assume you have chosen the 10mm SWA for volt drop, have you done the calcs by any chance? becasue the load is quite small. Are you having any services ie water gas into this log cabin. If you not, and the cabin is not a metal frame design then that 10mm SWA should be fine, even if your using the SWA as the CPC, but you should do the Calcs.

You will need to fit a swich fuse or similar to protect the SWA after the henly blocks.

Why are you proposing a 16amp ring? If it is 6 sockets then why not either go for a convention A1 ring final of 32amp or you could go for a A3 Radial at 20amp. My personal choice would be a radial for this installation.

It might pay you to run a new SWA to the nearer garage, and then feed the log cabin from an improved garage installation. This way you can sell an upgraded, safer installation for both areas. As the garage already as a 63amp 30mA RCD protected CU, you could easily just come off this to feed the log cabin as a final circuit, this would be perhaps a A2 4mm^ Radial into the sockets, and then a local FCU in the cabin for the light. In this sort of circumstances I don't think by having both the garage and log cabin on a single RCD will fall foul of Reg 314 as they would not be classed as normally occupied areas.

There are many ways to do it, this may not be the best way for the situation, but it will give you another choice. In the end you are there and can only decide which is the best way to do it.

 

[Guest 004]

first off, you cannot export an earth to an outbuilding, you will need an earth rod to earth your cabin, and then just a main rcb for the switch will suffice

 

[Hawk81]

first off, you cannot export an earth to an outbuilding, you will need an earth rod to earth your cabin, and then just a main rcb for the switch will suffice

Please ignor this post as it is complete and utter rubbish. Please do not give replys if you do not know what you are talking about.

 

[malcolmsanford]

I just knew we would get the export of the earth, As it is a TN-S I would not even ask for the DNO permission. If there were services or the log cabin was of a metal frame, I might look at a TT for it, due to cost of running an extra bonding cable back to the MET, but that is another thread.

 

[i=p/u]

mr enigma, come on this abc boy stuff????? were you winding me up last night???

 

[High Tower]

first off, you cannot export an earth to an outbuilding, you will need an earth rod to earth your cabin, and then just a main rcb for the switch will suffice

I am with hawk, I'm afraid, it's a urban myth that earths cannot be exported, there is nothing in bs7671 that discounts the exporting of a earth, the only restrictions if I remember rightly is something to do with Pme's not being exported where the two buildings are a certain distance away, and share a common mass of concrete, it all comes under the escqr supply regs.

 

[Guest 004]

I am with hawk, I'm afraid, it's a urban myth that earths cannot be exported, there is nothing in bs7671 that discounts the exporting of a earth, the only restrictions if I remember rightly is something to do with Pme's not being exported where the two buildings are a certain distance away, and share a common mass of concrete, it all comes under the escqr supply regs.

Indeed there is a regulation, just trying to find it now

 

[Hawk81]

Indeed there is a regulation, just trying to find it now

I can not wait for this. Many others have tried to find this mystery reg and failed.

 

[Deleted member 9648]

I just knew we would get the export of the earth, As it is a TN-S I would not even ask for the DNO permission. If there were services or the log cabin was of a metal frame, I might look at a TT for it, due to cost of running an extra bonding cable back to the MET, but that is another thread.

Malcolm,that would only apply on a TNCS....as the supply is TNS the a main bonding conductor in the outbuilding can be sized according to the earthing conductor to the outbuilding,but not less than 6.0mm....544.1.1,and hence go to the CU in the outbuilding.

 

[Deleted member 9648]

Indeed there is a regulation, just trying to find it now

I'm not holding my breath.

 

[malcolmsanford]

Malcolm,that would only apply on a TNCS....as the supply is TNS the a main bonding conductor in the outbuilding can be sized according to the earthing conductor to the outbuilding,but not less than 6.0mm....544.1.1,and hence go to the CU in the outbuilding.

Yes agree, but if your exporting the earth to the cabin, and it as services or be metal framed then you have to bond them to the MET, so why run a bonding conductor of 6mm back 80 meters, or calculate a combined CPC/Bonding conductor, when you could TT the installation and use the CU earth bar for the MET of that installation, and create the equipotential zone within the cabin.

If he uses a 3 core SWA then the combined Conductor/SWA may be ample to provide that CPC/bonding conductor, but IMO if there are services then I would most likely TT the installation.

 

[johnboy6083]

just to throw a spanner in the works here.

If the summerhouse contained a hottub with extraneous parts ect, would there be a danger that there would be a potential difference between the ground that awet person may step on, and the earth from the TNS system that you have in the installation?
This in theory could cause a shock to somebody whp's resistance would be very low due to being wet.

In this circumstance, can you supplement the TNS earth with a rod, in order to bring the TNS earth to the same potential as the ground around your installation?

Ill be very interested to hear your replies

John

 

[i=p/u]

so say there was no services and using 32mcb from house would this be correct for bonnding

32x5=160x160x0.4 suare root of this divided by factors. i get 7mm csa for main bond. is this correct

 

[Guest 004]

just to throw a spanner in the works here.

If the summerhouse contained a hottub with extraneous parts ect, would there be a danger that there would be a potential difference between the ground that awet person may step on, and the earth from the TNS system that you have in the installation?
This in theory could cause a shock to somebody whp's resistance would be very low due to being wet.

In this circumstance, can you supplement the TNS earth with a rod, in order to bring the TNS earth to the same potential as the ground around your installation?

Ill be very interested to hear your replies

John

That is very near to the regulation that I am talking about