Consumer unit connections

[paul1972]

Hello again.
Am thinking about having some power run to a large brick built stable in the autumn (no animals in it just tools) and am costing it out for now.
The run will be in 10mm 3 core swa and the lenth of run will be 75 mtrs. to new CU.
Total loading will be no more than 3kw. Cable size if calculated correctly should take into account the 4% VD. and cover the loop impedance okay.
The cable will be buried in part 1/2 mtr deep and clipped direct for the rest and will terminated in a 4 way rcd protected board and housed for extra safety in a box on the wall. Dont think earth spike will be needed as no water etc in stables. The largest load will be a 2kw shredder and taking diversity onboard logically would not be using it in dark so 1kw lighting will not be on. Think have done home work akay.
however at CU end can it terminate directly in RCD proctected side of split load.

 

[Adam_151]

10mm² maxes out for volt drop at 27A for that distance, however that load will be no where near the max load for the cable, so the cable will be running cooler than the 70C max, so the volt drop will be less, its actually difficult to quantify*, its just normal to just use the 70C rateings, however I wouldn't feel too guilty making it a 32A submain and 'winging it' slightly , of course you could just stick to a 20A submain (but that'd be too easy )

Whats the earthing arrangements for the house? TT? TNCS? TNS? and does the stable have any ECP s? (structural steel work in the ground, etc)

You would want to take it from the non RCD side of the split board as you have an RCD at the sub board (which I'd actually make split and have the lights on non rcd), Though wondering if it still counts as agricultural? if it is still going to be barn or is it being converted into workshop? if the former, then a time delay rcd at the orgin of the submain may be appropiate (would have to read up on what rateing is required)

Where in lincolnshire are you anyway? (not that I'm qualified or anything yet, just wondering )

*its something that some designer kind of person pointed out on the IEE forum that I keep meaning to read into further

 

[paul1972]

Bourne, Nr stamford in far south.
Take it from the question your from lincs too.
Believe supply to be pme.
Incoming supply via wooden poles in our village. runs down wall as 2 thickish cables assumed live and neutral/earth.

 

[ban-all-sheds]

1) 3kW might be today's load, but if you ever need more in the future, a 75m (or getting on for that) trench is not something you want to do twice. Give serious consideration to 16mm² or even bigger.

2) Don't put it on the RCD side of the CU - use an RCD CU in the outbuilding, and have the cable on the non-RCD side or via split tails and a switchfuse.

3) You will struggle mightily to terminate large SWA directly into the CU. If you don't go the switchfuse route, the best thing, if there is space, is to put a metal adaptable box right next to the CU, terminate the armour there, but leave the cores long and take them into the CU.

Rough sketch:

 

[Adam_151]

Yeah, I'm just outside lincoln itself on the south side.

Anyway, over that distance, and with it possibly/possibly not being agricultural, and the supply being PME, I'd be *very* tempted to make it TT in the barn

 

[paul1972]

will do, many thanks

 

[ban-all-sheds]

Anyway, over that distance, and with it possibly/possibly not being agricultural, and the supply being PME, I'd be *very* tempted to make it TT in the barn

Why?

There's no way an earth rod will give you a better EFLI than a PME supply - why deliberately choose an inferior solution?

 

[paul1972]

why would the new cable installed be placed on the non rcd side of existing board.
Is it due to tripping out the ring mains in house in event of fault?
is this why it has been suggested to place on rcd side of CU in Stable.
Also there is no steel work within the stable so no extre..(however it is spelt) bonding requirements to take into account.
The stable will serve as a tool/garden workshop running at most a 2kw shredder.
To meet half way can an earth spike be install AS WELL.
Am I correct in remembering that the steel sheath of SWA must obviously be earthed but a seperate earth must be supplied (i.e my 3rd core)

 

[Adam_151]

There's no way an earth rod will give you a better EFLI than a PME supply

Yes, that undoubtedly the case, but you have to ask yourself if it matters in the grand scheme of things, if the OP is leaving the building as a stable and just using it as a workshop, then its still very much a stable IMHO, becuase its perfectly feasible that another owner might actually use it for its intended use... agricultural buildings require RCD protection due to increased fire risk from combustable straw anyway

- why deliberately choose an inferior solution?

PME is inferior in this circumstance , over that distance, I expect theres the possibility that there might be potentials between terra firma and an exported PME, couple that with the fact that it could be classed as an agricultural building (which I beleive PME is discouraged from anyway... at least without a metal grid in the floor)

[and if you had a grid in the floor, you now have an ECP, and your earth core now is a bonding conductor as well, which I beleive should have a resistance of less than 0.05 ohms and 75m of 10mm² comes out at about 0.13 ]

I think exported PME is very much a judgement call, I would be happu doing it for the standard detached garage, but not for something like this

 

[scousespark]

Paul. There are a number of regs to follow for a agricultural premises, and the work you describe will not change it's use.

As for your description of your supply - 'Incoming supply via wooden poles in our village. runs down wall as 2 thickish cables assumed live and neutral/earth.' I believe this would indicate a TT supply. You need to check your supply cut-out to confirm your earthing system. If it has no Earth conductor attached, it is a TT supply and the exporting PME discussion is redundant. If there is an Earth conductor you have overhead PME and need to decide how you want your barn supply earthed.

As you are working on an agricultural installation, I would suggest you get a spark in who works in this particular field (no pun intended). You are describing a big install for a diyer.

 

[Steve]

'Incoming supply via wooden poles in our village. runs down wall as 2 thickish cables assumed live and neutral/earth.'

I believe this would indicate a TT supply. You need to check your supply cut-out to confirm your earthing system. If it has no Earth conductor attached, it is a TT supply and the exporting PME discussion is redundant.

not nessecarily. Our supply runs around a few houses in different cables, before it gets to ours - twincore cable, seperate cores, and concentric. All the supplies are PME. PME supply doesn't involve an earth conductor anyway, does it??? Since the earth is carried as neutral, being tied down to earth potential.

 

[ban-all-sheds]

Lots of TTs have been converted to TN-C-S.

 

[Lectrician]

Adam has it sewn in my eyes. Agree completely.

PME can appear in many forms these days.

 

[scousespark]

All I was trying to point out was that the OP was describing a typical TT installation. A farm, fed from poles with cables fed down the wall.
It is entirely feasible that it is PME, as quoted by the OP, but you need to check this at the cut-out not by looking at the supply cables. The quickest way to determine your erthing system is to test the Ze, but not sure this is something the OP is equipped to do. If there is an earth conductor between the cut-out and the MET then it is a PME system.

BAS you are correct in that lots of TT installations have been updated to PME, but this is by no means standard as this will put the responsibility on the REC to minimise the Ze.

 

[Chris.J]

A remote building full of tools is a prime target for a Burglar; so suggest you might want an Alarm in there. If so, consider running an 8 core cable in the trench (suitably protected) for a remote keypad at the house; also a phone cable?
You might also want to be able to switch on (security) lights from the house, so another run of low voltage cable (to switch relays) could be usefull??
Cable is cheap, digging trenches is not!