New Supply or radial?

[Empin]

Hello all.

This is my first (probably of many) question so please be gentle.

I have been trying to get an answer to this but seems it is not as straight forward as it looks so i am hoping you wise souls can give me a clear concensus (or just a majority) view.

We have a large 3ph supply to our farm which is metered as it enters the property with the boards fuses and an RCD on our boundary.

It runs U/G 300m  to a barn and into a distribution board all certified and proper for using in the building, the cable was sized to minimise voltage drop at huge cost!!!

We are now putting up a dwelling about 150m from the barn. I have run a cable from one leg of the supply in the barn U/G to the new dwelling using oversize 2 core SWA to minimise voltage drop. In the dwelling it is then resin joined to an SWA of 25mm and I have run this through a cold roof to the C/U 5m away. The earth is TT as there is no suppliers earth so the barn is TT and it seemed pointless running a cable from there.

Now the supply from the suppliers meter is fused and RCD protected but 450m away, this covers the requirement to protect within 3m that seems to crop up so is my supply in the house ok from this point of view  (there is no meter in the house )or would you people consider it a new supply in the house and requiring protection as it enters?

Thanks in advance.

Empin

 

[phil d]

Firstly, who is doing the work? Because you say an oversized cable to minimize volt drop, in a properly designed system the cable size is worked out correctly for the length of run, and loading, and therefore would not be undersized, it would be the correct size. In your case it sounds to me as though someone has just taken a wild guess at the size of cable required, also to joint that cable into a smaller cable a short distance from point of use is a bit bizarre to say the least.

I would suggest you need to consult an electrician regarding this, and if you have, then I'd recommend you consult another one. Sorry to be blunt, but contrary to popular belief, electrical work isn't as simple as just, " throwing a few wires in".

 

[Sidney]

So the whole installation is covered by one RCD?

What is the earthing arrangement for the first DB?

What size cables are currently there and what size have you run in?

What is the expected demand of the new dwelling?

Have you calculated the volt drop from source to the new dwelling taking into account the 3% maximum for lighting?

On a very quick calculation for volt drop on a 40 amp load you have run in a 150mm cable???

 

[Murdoch]

Yes, and yes

Plus the new house will need its own supply meter

What does the spark say who's going to sign it off?

 

[ProDave]

Plus the new house will need its own supply meter

That depends who will occupy the house?

If part of "the farm" and lived in by a familly member that may not be a big concern.

 

[Evans Electric]

I don't see why the house needs it's own meter TBH.

 

[Empin]

Hi

All the cables were sized using the cable suppliers sizing calculations for 100kva and the use of the term oversize was in relation to its cost about £4000 ten years ago. the supply to the dwelling has been sized correctly for a potential supply of 30kva but only utilising a 23kva box at present as the demand for this dwelling is designed to be low and a joint was used for two reasons. Firstly it was a lot cheaper than another 8m of cable and secondly the supply cable is too thick to radius for the run in the house (also I would not like to try to fit that to a C/U). There is no requirement for the house to have a meter and there is no plan, at present, to fit one. The earths as i said is TT both at the dwelling and the barn as, being in a rural area, we have no board earth supply. I wired the barn ten years ago and paid a spark to fully check it and sign it off no favours asked or given.

I am doing the work but under the non competant persons scheme with building control. I am not ticketed but sensible and was taught by a great guy who spat nails if you ever said neutral It always had to be 'return'. I can understand the bias against non certified people doing eletrical work as i imagine that you have come across some pretty dangerous and stupid things in your careers. The spark has not been appointed yet but hopefully we will meet him next week and i was just asking opinions as I thought it is all a matter of interpretaion.

The strict interpretation of the regs would mean, to me, is that there is protection within 3m of the meter so the rest of the supply is conforming but it is open to interpretation as i say.

 

[UNG]

Hi

All the cables were sized using the cable suppliers sizing calculations for 100kva and the use of the term oversize was in relation to its cost about £4000 ten years ago. the supply to the dwelling has been sized correctly for a potential supply of 30kva but only utilising a 23kva box at present as the demand for this dwelling is designed to be low and a joint was used for two reasons. Firstly it was a lot cheaper than another 8m of cable and secondly the supply cable is too thick to radius for the run in the house (also I would not like to try to fit that to a C/U). There is no requirement for the house to have a meter and there is no plan, at present, to fit one. The earths as i said is TT both at the dwelling and the barn as, being in a rural area, we have no board earth supply. I wired the barn ten years ago and paid a spark to fully check it and sign it off no favours asked or given.

I am doing the work but under the non competant persons scheme with building control. I am not ticketed but sensible and was taught by a great guy who spat nails if you ever said neutral It always had to be 'return'. I can understand the bias against non certified people doing eletrical work as i imagine that you have come across some pretty dangerous and stupid things in your careers. The spark has not been appointed yet but hopefully we will meet him next week and i was just asking opinions as I thought it is all a matter of interpretaion.

The strict interpretation of the regs would mean, to me, is that there is protection within 3m of the meter so the rest of the supply is conforming but it is open to interpretation as i say.

Your figures and story don't add up as being a viable installation.

The cost you have quoted appears to be short of a zero or two if you are to meet the volt drop limitations at a cable length of nearly half a kilometre at the dwelling let alone 300M at the barn and then you do need some left for your final circuits

As for the guy teaching you he should have stuck to joinery as he obviously didn't know electrics

 

[Empin]

The barn ran big 3ph motors without problems and always read at 230/40 on all legs. It also ran a couple of 3ph scroll compressors that were wired with low voltage protection and phase protection (by myself)  and never gave a flicker. One motor of 8kw we never got around to fitting soft start to and never flickered when started DOL so I think that the supply is fine and well within spec. You are welcome to come and test it if you dont believe me. As for pricing then it was in the days of 2.5T/E for twenty quid or so just before the rise we did all the work ourselves and even at todays prices the cable would only be£10- £12000 so I cant see your £40,000 figure stands up. The cable to the house was two core SWA and 75mm.

I dont know if you have ever dealt with remote supplies as they are boosted regularly and we have a better voltage than a lot of town houses that i know although a bit variable we have even seen close to 250 at times when demand is low locally.

House reads 230 and steady as a rock so no problems there.

So back to the question.

 

[Evans Electric]

Whatever size cable you are using would need ...in my opinion ... a means of isolation at the barn end  . Suggest a  DP  sw/fuse   at say , 40A or  60A  to suit your calcs . 

 

[Empin]

Whatever size cable you are using would need ...in my opinion ... a means of isolation at the barn end  . Suggest a  DP  sw/fuse   at say , 40A or  60A  to suit your calcs . 

Thanks for that.

It may not be strictly necessary but it makes compliance easier to prove. I will also add another RCD whilst I'm at it as i want to put a time delay RCD on the meter board to replace the 30ms one,  its a long walk!!!!

 

[Evans Electric]

The isolator , as far as I know,  is necessary  as its a reg .       RCD  would be at the house end  as per the TT system  

 

[Empin]

The isolator , as far as I know,  is necessary  as its a reg .       RCD  would be at the house end  as per the TT system  

There is an isolator in the meter hut but bringing duplicates into the house is probably the best way forward.

 

[Evans Electric]

I don't understand "duplicates" in that context.

 

[UNG]

Whatever size cable you are using would need ...in my opinion ... a means of isolation at the barn end  . Suggest a  DP  sw/fuse   at say , 40A or  60A  to suit your calcs . 

As he is set up for a 23KVA supply with the possibility of 30KVA not sure a 40 or 60A switch fuse would be much use

 

[Evans Electric]

Well that,s up to Empin to sort  ..I'm just offering advice .  I have no idea what his requirements are for his house ,  most small dwellings  rarely stray over 40A . 

 

[Empin]

I don't understand "duplicates" in that context.

The supply is on an RCD and isolator already just at the meter so the new ones would be really be dupliates on the supply circuit.

 

[phil d]

The supply is on an RCD and isolator already just at the meter so the new ones would be really be dupliates on the supply circuit.

Maybe, but you need local isolation and also if the rcd is remote from the property you won't want to be going outside in the dark, or wet to reset it, think about it logically and you'll begin to understand where we're coming from.

 

[Evans Electric]

As Phil says ,   OK  you have an isolator  at the meter  ...wouldn't that kill the whole place  ?  

Outgoing sub-mains from a central point,  each should have the facility to be isolated  and fused accordingly . 

As its a TT  system I guess the overall RCD is  100  mA   .  The house would have 30mA  ,s  in the board.  

 

[phil d]

As Phil says ,   OK  you have an isolator  at the meter  ...wouldn't that kill the whole place  ?  

Outgoing sub-mains from a central point,  each should have the facility to be isolated  and fused accordingly . 

As its a TT  system I guess the overall RCD is  100  mA   .  The house would have 30mA  ,s  in the board.  

By the way this is going I can see it starting to get dangerous! From what I can tell the OP has 1 supply coming in which if TT is going to have a 100mA rcd on it, if he's going to rely on that for everything then not only is he going to have the inconvenience of the whole supply going off if one part trips, there's also potential for accidents. Imagine if the house tripped it and someone was using something electrical on the farm, the RCD gets reset by the householder, a machine on the farm restarts unexpectedly and someone gets hurt! Personally I wouldn't want to be the one stood in court explaining that one.

theres a number of good reasons why we design systems the way we do, achieving  discrimination etc, and thinking about it, the way he's going the section on external detrimental influences pops into my head, because in my opinion, a fault in the house causing a loss of power to the rest of site would have to be classed as an external detrimental influence.I wonder how this is going to be signed off, or more importantly by who, because one thing is certain, I wouldn't put my name anywhere near the paperwork.