Interesting summerhouse dilema

[LANDIE]

We recentley erected a summerhouse at the bottom of the garden which was fine until we realised how far the electricity supply would have to travel from the house. This adds up to about 160m! Now we have four other sheds which are all linked togeather with what appears to be 4mm SWA on a 40a MCB and 80A 30MA RCD. I have connected to the last shed which is 85m from the summerhouse with 6mm SWA I have fitted a garage CU with RCD. There are 4 double sockets and lighting, the largest load will be a 2KW heater. I have earths from the third core plus the cable armour. Now to the problem all appeared OK until a vacumn cleaner was plugged in which immeadiatley tripped the house RCD but not the SH RCD, the heater was then tried which had the same effect. What are your thoughts on the size of cable from the house to the sheds 4mm, the 6mm cable that I have used, is the house RCD suitable for this length of cable? The heater was tried in the furthest shed away from the house and worked fine. The sockets in the SH will power lamps, drills etc without problem.
Thanks

 

[londonboy]

At a quick glance this sounds like a large installation where an electrician would be required to calculate load, cable sizes etc.

I'd recommend employing an electrician for advice.
You maybe able to reduce costs by labouring for him.

 

[flameport]

Even with 6mm cable, 160m length will limit you to a tiny load due to volt drop.
For any sensible load, you will need significantly larger cable.

Tripping on high loads is probably due to the voltage drop being too great and causing an imbalance which the RCD detects.

All notifiable work, and not really something suitable for DIY.

 

[SUNRAY]

Assuming (always a silly thing to do) you have 90m of 4mm and 85m of 6mm, allowing for vertical rises etc, and a perfect 230V supply then on simple volt drop on the cable you are restricted to 17A at the furthest point as long as there's no load at the intermediate points and your house MCB should therefore be 16A or pushing it a bit 20A unless each shed is fused down from the previous, in any case your existing 40A is too big for 4mm cable.
Which ever way this is looked at this is a messy and complex circuit to calculate.

Your RCD problem reminds me of a N-E fault problem I had, I can think of no design problem that would cause the tripping.

 

[ban-all-sheds]

Landie - when you applied for Building Regulations approval, how did you say you planned to comply with Part P?

 

[jj4091]

Landie - when you applied for Building Regulations approval, how did you say you planned to comply with Part P?

I do not think building regs are required for a summer house, as it is classed as a temporary structure(presuming timber construction & no masonary involved)

 

[LANDIE]

Do you think I should replace the existing 4mm cable with 6 or 10mm? My original plan was to just run 10mm all the way to the house with a independent CU and RCD but a electrician looked at the job and told my other half that this was overkill and way to expensive for the purpose! Should I try a new RCD? If there was a earthing problem would this trip the RCD witheven load

 

[rebuke]

I do not think building regs are required for a summer house, as it is classed as a temporary structure(presuming timber construction & no masonary involved)

I don't think they are for the summer house (although I'm no expert so might be wrong), but the outdoor electrical work is most certainly notifiable under Part P (unless you're in Scotland), so unless you got an electrician who's a member of a self certification scheme to do the work (in which case you should be talking to him to put it right), you should have notified your LABC before doing the work...

 

[ban-all-sheds]

I have fitted a garage CU

That makes it notifiable - a Builidng Regulations completion certificate is required.

As for Scotland, the OP is in Cornwall. I know he said they didn't realise how far away the summerhouse was, but even so....

 

[LANDIE]

Would a earthing rod at the summerhouse make any difference?

 

[ColJack]

any difference to what?
volt drop? none whatsoever..
Ze at the last board.. yes it would, but you'd have to wire it as TT then....

 

[mikhailfaradayski]

As for Scotland, the OP is in Cornwall. I know he said they didn't realise how far away the summerhouse was, but even so....

LOL, main house in Cornwall, summer house in Arbroath. Would someone care to work out the volt drop for a 600 mile feed?
I think you may need something chunkier than 4mm swa

 

[LANDIE]

What I was getting at is would the independent earth have any effect on the house RCD tripping under load? Yeh it is an awfully long way to walk to reset the trip each time!

 

[ban-all-sheds]

It's the wrong place to have the RCD protection anyway.

 

[LANDIE]

Sorry whats the wrong place ? the house RCD or the summerhouse? What I need to clarify is it the voltdrop which is causing the trip or an earthing issue?