10mm Steel Wire Armour cable to man cave

[Deleted member 325510]

Hello, I hope some knowledgeable person can help me.
I want to run a Steel Wire Armoured cable to my man cave at the bottom of the garden.
I will dig the trench and lay the cable, and a friend will terminate it, connect it and test it.
He has suggested that I need 20 metres of 3 core, 10mm cable. But I have been reading that it is possible to use two core, with the armouring as the earth.
As the three core is more than twice the price of the two core, can I use two core, and use the armouring as the earth wire?

Thank you.

 

[Murdochcat]

Not sure about twice the price but I always run 3 core

 

[davelx]

Using the armour alone as the CPC requires some calculations to see if it is adequate. Steel has a higher resistance than copper and in some cases a third core can be needed in addition to the armour.

This is something for your electrician friend to work out as only he has all the information needed. If he says you need 3 core then you need 3 core..it's his design decision.

 

[Murdochcat]

Also why 10mm swa - what are you planning to use this for?

 

[Deleted member 325510]

Also why 10mm swa - what are you planning to use this for?

it's only a man cave for now, but who knows for its future use.
And I understand the volt drop on such a run can be excessive.
To replace it in the future would be prohibitive

 

[Taylortwocities]

As above. Your experienced, registered and competent electrician will have made the cable calculations. Frankly, he is the electrician who will be certifying that he has designed installed and tested the whole thing, so why are you asking hung-over people on an internet forum rather than him?

There may be many reasons for 3-core including distance, the supply type, and much more. Although the armour must be earthed, sometimes the armour isn't suitable to provide an adequately low impedance to meet the EFLI figures at the man cave end.
There are ways of not exporting the earth (making the man cave a TT environment, for instance) but that has cost implications and design issues that may outweigh the additional cost of 3-core SWA.
On that point....

the three core is more than twice the price of the two core,

No it isn't. Look at different suppliers.

 

[SUNRAY]

As the three core is more than twice the price of the two core, can I use two core, and use the armouring as the earth wire?

No it isn't. Look at different suppliers.

Generally around 50% extra.

Anyway back to the main question; as has been said already it is your electrician who surveyed and designed this circuit and will be certifying it so follow his design. For £30 or so is there any real question?

 

[endecotp]

as has been said already it is your electrician who surveyed and designed this circuit and will be certifying it so follow his design.

Sunray, in another thread you’ve just posted a long story highlighting the incompetence of some “professional” electricians; I think it’s legitimate to ask “is this right?” in this case. Sure there may be reasons for not using the armour as the earth but it’s also possible that it’s being done because “that’s how I always do it”.

For £30 or so is there any real question?

£30 pays for a night in the pub. Or it’s 3 hours working in a minimum-wage job. Don’t imagine that everyone else doesn’t notice +/- £30 in their bank account.

 

[pete01]

£30 pays for a night in the pub

In 1990

 

[endecotp]

In 1990

In 1990, it would pay for a week in a pub. £1.02/pint. Today at £5.40 it buys me 5½ pints.

 

[pete01]

In 1990, it would pay for a week in a pub. £1.02/pint. Today at £5.40 it buys me 5½ pints.

Oh good point, im always buying the mrs a double gin an tonic answel £30 disappears in no time

 

[ebee]

You need to work out the minimum sizes and number of cores that you need for now, then work out what difference there will be for future additions, then the price/inconvenience factor and see which is the most worthwhile particular answer once everything is considered.

I once came across a farm type property and the main house had been completely rewired then a series of 4.0 SWA to a barn then another barn then another barn interlinked between so the very end was quite a long length total of 4.0 SWA with loads upstream of it at various places.
At this last building (dog kennels) they wanted lighting points and a few sockets (not too much of a problem so far but then Electric to this whole building on open moorland exposed to the elements.

It took me some persuading that electric heating was not viable without running a new SWA from the origin to this new building and I got their plumbing/heating bod to agree to gas bottle heating just like they had in the existing house but with separate bottles and boiler etc.
That way a few straightforward changes so that each barn was on its own circuit and the kennels would be on its own circuit, sufficient to run all of the lighting a number of sockets and capable of running 2 x 2kw fan heaters and a kettle.

I could not believe all the 4mm SWAs were daisy chained and they were expecting to run the additional load too "as advised by their original electrician" of course people sometimes misquote who told them what too, so you never know

 

[flameport]

20 metres of 3 core, 10mm cable.

The difference between that and the 2 core version is less than £50.

If you really must save that £50 then get 3 core 6mm² instead.
6mm² is good for at least a 10kW load. You will not be needing anywhere near that - and if you did, an entire new supply would be required at a cost of £1000s.

 

[SUNRAY]

Sunray, in another thread you’ve just posted a long story highlighting the incompetence of some “professional” electricians; I think it’s legitimate to ask “is this right?” in this case. Sure there may be reasons for not using the armour as the earth but it’s also possible that it’s being done because “that’s how I always do it”.

Quite right, however OP has explained it will be someone else doing the 'electrical' work and what he supplies will have to be what the someone else has requested. It is not unreasonable to ask questions, in fact that's precisely what one does when getting multiple estimates for a job.

£30 pays for a night in the pub. Or it’s 3 hours working in a minimum-wage job. Don’t imagine that everyone else doesn’t notice +/- £30 in their bank account.

I'm under no missaprehentions about the value of money, having been made redundant 4 times and one of those was just after buying a property based on the income, I understand that very much.
However by the time all the other costs of the glands, CU (presumably) sockets, switches, backboxes, lights etc are totted up, £30 (or £50 as JW has mentioned - I'll happily admit I haven't had reason to buy anything bigger than 1.5mm² for several years) will form a fairly part. Quite frankly I'd be inclined to think that if money was in such short supply as you hint at, OP would possibly not be looking at the cost of the rest of it as a viable expenditure.

EDIT: TLC tends to be an easy place to purchase SWA for me, 2C @£4.10 Vs 3C @ 5.99 makes a difference of £37.80, straight down the middle.