Workshop supply reinstatement advice please...

[AAAMUSEMENTS]

I rent a metal clad, mostly timber lined workshop building just outside the boundary of this property. The supply used to be connected into the neighbours garden workshop but I dismantled this supply a while back. I now need to connect the workshop to the supply from this property.

There is an existing piece of 4x6 SWA coming from the workshop. I can make this reach a small wooden shed at the end of my garden. I have purchased another piece of 4x6 SWA to go from this wooden shed to the meter box. In here I plan to connect it into a dedicated CU in the meter cupboard, supplied from a Henley on my side of the isolator switch.

I am planning to use two cores of the SWA as "conductors in parallel" to increase the capacity of the cable.

So, thinking as I go along:

Main Isolator

25mm tails to Henley, 25mm tails to house CU (already all installed)

NEW 16mm tails to garage type CU

Fitted with 60amp(?) mcb

Adaptable box with SWA connected, earthed to MET

SWA armour as earth conductor

2x live conductors, 2x neutral conductors (Colours to use?)

Along walls and catenary wire to wooden shed

In wooden shed, into metal trouser/Y- box to act as junction with old cable. Supply also taken off to supply wooden shed lights/ power.

Would the wooden shed then need it's own CU? (Presumably as sockets would need RCD protection).

Further existing cable run to outside property metal workshop. Existing Wylex metal consumer unit (Hmmm...)

New RCD installed (not one at the moment in there, the previous occupant was relying on the neighbour's house RCD to provide protection... )

Existing mcbs to supply ring, 110v transformer (hard wired), spurs and flu lighting/ outside flood. All wired in SWA to metal-clad fittings.

I know that this raises the issue of an earth exported a long way, and I would prefer not to make this into an earth-export-will-somebody-please-think-of-the-children!!! type thread but I would very much welcome any comments and criticism of the plan. I really don't want to have to TT a workshop that is not mine - the previous occupant was someone who took a long-term view of these things, and last year he sadly died, well before his time - and having expended a lot of time and money building a workshop that he barely had the chance to use...

Many thanks in advance.

 

[Chr!s]

What type of flooring is there?

Regards chris

 

[AAAMUSEMENTS]

What type of flooring is there?Regards chris

In the workshop it's concrete of probably varying construction and pedigree! But if you mean for a spike, then one could be installed outside if necessary.

 

[Doc Hudson]

Probably need some more info as I am sure someone will want to know distances of cable run, expected load, earth type at house etc..?

Doc H.

 

[AAAMUSEMENTS]

Probably need some more info as I am sure someone will want to know distances of cable run, expected load, earth type at house etc..?Doc H.

In terms of expected load, I don't have a definitive answer. The old set up was on a 60 amp MCB and supplied 2 workshops back then, with no tripping issues. The cable run was almost identical length, and the total CSA of the new install will be greater than the old one. I can get specific figures for cable lengths if needed though.

The earth type in the house appears to be TNCS.

 

[AAAMUSEMENTS]

Other issues you may have are if there is any extrauous conductive parts1 the long run(true earth and exported earth potential) and a concrete floor.

2 the armouring cpc will also have to act as a boding conductor, two issues, it wont be big enough and they dont allow it.

Yep, lots of issues I know.

Can't find the info on the CSA of the metal sheath of an SWA cable, so I couldn't comment on it being undersized. If I remember correctly you also have to divide the CSA of steel by a certain number to find it's equivalent in copper?

?:|

One of the reasons that I am unwilling to TT, as well as the fact that the building is not mine and perhaps never will be, is that the CU installed in the workshop is a metal one... and so it would have to be changed for TT?

:|

 

[AAAMUSEMENTS]

Its about 8 times the equivalent(steel to copper)

Right thanks, divide by eight does ring a bell.

Just need to find the CSA now!

 

[Doc Hudson]

In terms of expected load, I don't have a definitive answer. The old set up was on a 60 amp MCB and supplied 2 workshops back then, with no tripping issues. The cable run was almost identical length, and the total CSA of the new install will be greater than the old one. I can get specific figures for cable lengths if needed though.The earth type in the house appears to be TNCS.

You can't verify if volt drop limits are met without knowing the lengths of the cables, conductor sizes and the expected load. Whatever worked or didn't work before is not normally an acceptable calculation to sign for on a certificate. This work sounds as though it will require Part P, building regulation compliance as well.

Doc H.

 

[AAAMUSEMENTS]

You can't verify if volt drop limits are met without knowing the lengths of the cables, conductor sizes and the expected load. Whatever worked or didn't work before is not normally an acceptable calculation to sign for on a certificate. This work sounds as though it will require Part P, building regulation compliance as well.Doc H.

Indeed, I will verify cable lengths asap.

 

[AAAMUSEMENTS]

How about distribution circuit to shed(divorce earth at shed), Place earth electrode at shed

Fit 100 mA td rcd at shed feeding 30 mA cu in shed

Also feed a distribution circuit to workshop 30 ma Cu

Regards chris

That's an interesting possibility that I hadn't considered, thanks for that.

 

[AAAMUSEMENTS]

Right thanks, divide by eight does ring a bell.Just need to find the CSA now

Its time by 8

So 10mm copper requires around 80mm steel equivalent

Regards chris

So without even doing any calculations I can tell that the armour will not be sufficient!

TT it must be then, of some variety or other.

 

[spinlondon]

Here is a link to some information regarding the CSA of steel armour.

http://www.gadsolutions.biz/regulations/swa-as-the-cpc.

Your options are to use a core of the SWA as a bonding conductor, assuming it's at least 10mm

 

[steptoe]

FYI AFAIAA

up to 25mm SWA of a limited length or 16mm SWA limited by voltage drop doesnt require any further earth along with it, the SWA is adequate .

dont take the cable size as being the armour size, its generally much larger.

as for exporting a TNCS earth,

well I think deep down you know the answer, and in all honesty how much will it really cost you for a rod and some 16mm earth cable.?

 

[AAAMUSEMENTS]

FYI AFAIAAup to 25mm SWA of a limited length or 16mm SWA limited by voltage drop doesnt require any further earth along with it, the SWA is adequate .

dont take the cable size as being the armour size, its generally much larger.

as for exporting a TNCS earth,

well I think deep down you know the answer, and in all honesty how much will it really cost you for a rod and some 16mm earth cable.?

It's not really the cost at issue, it's more the fact that even as early as the new year they can say now sling your hook, and anything I have put in I will have to go and pull out again...

Anyhoo moving on, do the metal walls, doors and roof count as extraneous, and should they in theory be bonded?

 

[Apache]

It's not really the cost at issue, it's more the fact that even as early as the new year they can say now sling your hook, and anything I have put in I will have to go and pull out again...

An earth rod is cheap and relatively easily pulled back up!

There's this thread in the useful links section that may help with armour sizes being suitable as earth.

Anyhoo moving on, do the metal walls, doors and roof count as extraneous, and should they in theory be bonded?

Is this not a time to get your test meter out and test for continuity?

As a lay person I would assume that anything going into the ground has the possibility of being extraneous so structural steel work, but bonding a door attached to a wooden frame could be more dangerous than not bonding it. We are back to running fly leads to metal window frames.

Exporting the earth to the shed and then bonding it to the metal door that was insulated from the rest of the structure might give you quite a tingle on a wet day. Bonding is not a benign thing that can do no harm - you can give people a shock with it.

Can I possibly direct you to the exporting earth threads. There will be a test at the end.

 

[AAAMUSEMENTS]

I have read just about all of the earth export threads, and they always end in the same way - with an argument and me with a headache!

 

[PC Electrics]

Why not use a new length of 6 or 10mm swa and just lay it across the garden in an unobtrusive route (bottom of fence or wall, through the flower beds etc etc). TT the workshop if you feel you must (headache coming). Mount your accessories on a board. That way if you get "kicked out" all you have to do is lift out your board with its stuff on in one piece and pull back your cable - all of which can then be reused for something else in the future.

 

[AAAMUSEMENTS]

Fair points, but;

The workshop is already all fitted out with sockets, striplights etc. all SWA back to the CU in there, and it's all been done very nicely with a few notable exceptions that I can easily remedy.

The existing cable that already comes from it is 4x6, and the cable that I have to do the new run is 4x6.

I haven't done any calculations, but in the workshop is a compressor, welders, pillar drills, various 110 trannies, all the usual power tools and a very heavy duty planer that is evidently a fuse blower as the previous owner had terminated the cable into a JB from which ran 2 (yes really) 13 amp plugs (a nice death trap for the unwary!).

Anyway I reckon 60amp at the supply end to be "sufficient". If I had wanted a quick fix I could have run a 2.5 x 3 core from the detached garage, which is 16 amp max, and had endless fun and exercise walking back to reset the breaker!

The point I am trying to make is that I want to do a safe job, but not an expensive, complicated and time consuming one. It just doesn't warrant it.

Will now don tin hat, and retire to bunker...