Armoured Cable connection to CU

[mk2phil]

Hi guys,

I've moved into a house that has had armoured cable laid from the outside wall of the house down to the garage. I need to get it into the house so I can make the necessary connections in order to get power down to the garage. I have 3 questions about this though:

1. I have the gland/cover for the cable, but am unsure of whether it needs to be terminated into a plastic external junction box on the outside wall, or a round type galvanised 'socket' that I've also seen whilst out and about. Or should I just continue the AC all the way through the wall cavity and up to the CU without switching to internal style grey cabling?

2. The internal copper cores of the AC are i think 6mm. What size cable do I need to therefore run from the point at which it enters the internal wall of the house to the CU..... 6mm also preseumably? can you get 6mm T+E?

3. At the garage end i'll be adding a small garage style CU which I'm assuming will have a RCB on board that end. Should I also fit a mini circuit breaker RCBO type thing on the house CU feed also? 32 amp?

Many thanks,
Phil

 

[Spark123]

You'd be better getting an electrician to do the job for you, if you live in England / Wales this is notifiable works under part P.

 

[ban-all-sheds]

1. I have the gland/cover for the cable, but am unsure of whether it needs to be terminated into a plastic external junction box on the outside wall, or a round type galvanised 'socket' that I've also seen whilst out and about. Or should I just continue the AC all the way through the wall cavity and up to the CU without switching to internal style grey cabling?

Is it long enough to reach the CU?

Is the CU sturdy enough to gland SWA into?

Is there enough physical space in it to accept a SWA gland?

What did you consider when deciding that the CU was the best source for power to the garage?

2. The internal copper cores of the AC are i think 6mm. What size cable do I need to therefore run from the point at which it enters the internal wall of the house to the CU..... 6mm also preseumably? can you get 6mm T+E?

You want to design and install sub-mains, and design and install CUs and final circuits, but you have so little familiarity with electrical installation work that you don't even know what sizes T&E comes in?

That doesn't add up, really.

3. At the garage end i'll be adding a small garage style CU which I'm assuming will have a RCB on board that end. Should I also fit a mini circuit breaker RCBO type thing on the house CU feed also?

Why do you think that having RCD protection at both ends is a good idea?

32 amp?

What is your design load for the cable?

Are you sure it's 6mm²?

Have you done your voltage drop calculations?

Also, how do you plan to test the circuits in the garage? What tests, when, and with what equipment? Do you know what each test you should do is for, and what a good result looks like?

Do you have a TN-C-S supply and extraneous-conductive-parts in the garage?


The thing is, installing new CUs, outside supplies, submains etc is not a trivial job, and I can assure you that it involves knowing far more than you think it does.

Asking questions here can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.

You can't carry out a job of this magnitude by asking whatever random questions happen to occur to you. You've already shown that you have some dodgy misconceptions - what if you get something wrong because you have no idea your knowledge is wrong? What if you miss something because you simply have no idea it even exists, and just don't realise you don't know it?

• 
  
  
• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• //www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/7.13.3.htm
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• //www.diynot.com/wiki/electrics:swa-gland
  
  
• //www.diynot.com/wiki/electrics:part-p
  
  
• //www.diynot.com/wiki/electrics
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/electricity.html
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
  
  
• http://www.tlc-direct.co.uk/Book/1.1.htm[/list]I suggest you get stuck into the last link right away - it won't give you design ideas, and unfortunately it doesn't refer to the current edition of the Wiring Regulations, but it's free, and will still give you a good grounding which you can augment with more up to date publications.

 

[SimonH2]

Taking a charitable view, it's is possible that this chap is moving into a house, and the previous owner had laid this cable but not got round to connecting it.

In answer to the OP :

1 There are many ways of doing it - it depends heavily on how much cable there is*, what the wall is made of, how much space there is, what tools you have, etc, etc.
My preference, if there is space and since I have some f**k-off big drills available, would be to keep the outer sheath intact all the way into the house - that avoids any question of having to make outside joints which could be liable to water ingress. If the CU is of a suitable strength, then gland the cable off directly into it, otherwise use an intermediate box (of which there are many types to choose from) to anchor the gland. You'd just take the inner cable (sheath and all) right through while terminating the wire armouring with a gland at the box - with the right earthing arrangements for the armour of course.

Exactly the same decisions will need to be made at the garage end.

2 The correct answer is to use the right size cable taking into account the conditions, fixing method, and circuit protection. Yes, 6mm T&E is available.

3 I believe the regs allow this cable to be non-RCD protected as it will have an earthed metallic armour. From a simple practicality POV, you want the RCD at the garage end. Think about it - when it trips, do you want to have to fumble your way in the dark back to the house and trample dirt to the house CU to reset it ... or would you prefer to have it to hand in the garage to reset ? Not only that, but in the garage you can have two RCDs (or RCBOs) so that tripping the one feeding the sockets won't (normally) trip the lights as well.


But as BAS has pointed out - this is notifiable work. You either employ a professional to do it (he should be able to do it safely without any difficulty) and notify through his scheme, or you can pay your LABC whatever their charges are (typically several hundred quid) to notify. Given your clear lack of knowledge, I'd suggest paying someone to do it would be the best course of action.


* A while ago I was helping a friend. He'd hired a mini digger and dug the drive up, we'd dropped in a water pipe, power cable, and phone cable (armoured) down to the drive. I planned to take the phone cable all the way into the house (sans armour once it was clear of the ground), but the daft twit (not the words I used at the time) went and cut the "excess" cable off. Hence an external JB was needed

 

[ban-all-sheds]

Taking a charitable view, it's is possible that this chap is moving into a house, and the previous owner had laid this cable but not got round to connecting it.

Absolutely.

But even so, every single question I asked remains apposite.

 

[bernardgreen]

Is the cable 3 core plus armour or two core plus armour.

If 3 core then exporting the earth to the garage on the third core will mean the armour is only protecting the cable and not providing the earth for the garage. Still needs to be earthed.

If only 2 core then relying on the armouring for the earth to the garage does require that glands to the armour are properly done to ensure the earth exported to the garage remains sound.

It may be worth considering a TT supply to the garage where the earth is provided by an earth rod in or adjacent to the garage.

 

[SimonH2]

If only 2 core then relying on the armouring for the earth to the garage does require that glands to the armour are properly done to ensure the earth exported to the garage remains sound.

I was under the impression that the armour wasn't acceptable (insufficient equivalent CSA) to use as the earth.

 

[Aragorn84]

The armour on a lot of typical domestic sized SWA cables is fairly significantly larger than the CSA of the main conductors, even when taking the steel to copper equivalent factors into play.

Table here:

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[SimonH2]

Well, well, that's completely the opposite of what a (now retired) lecky told me some years ago

 

[bernardgreen]

The reason for using a core rather than the armour to provide the earth is the poor reliabilty of joints where glands terminate the ends of the armouring. The steel to brass interface can create electrolytic corrosion making the joint high impedance. A second reason is that any damage to the outer sheath can expose the steel to moisture leading to rusting of the armour.

 

[RF Lighting]

What a load of nonesense.

Generally the reason for using a third core is a lot of electricians don't really understand earthing, and think they have to use a copper conductor.

 

[flameport]

I was under the impression that the armour wasn't acceptable (insufficient equivalent CSA) to use as the earth.

It is almost always acceptable to use as an earth (cpc). That is why 2 core armoured cable exists - it would be pretty useless if the armour couldn't be used.
It is also why 3 core armoured generally has cores coloured black/brown/grey, as it is intended for 3 phase installations without a neutral.
And following on - 4 core is usually brown/black/grey/blue, which is for 3 phase and a neutral.

The problems occur when people also want to use the armour as equipotential bonding, as is usually far too small for that.

 

[SimonH2]

Well you learn something every day

 

[bernardgreen]

What a load of nonesense.

Generally the reason for using a third core is a lot of electricians don't really understand earthing, and think they have to use a copper conductor.

I will agree with that but do not agree that what I said was a load of nonsense.

I have seen SWA cable that had lost all it's armouring to rust / electrolytic corrosion for several feet on an industrial site. First indication was loss of earth to the building supplied by the cable. When the cable was drawn into the not water proof ducting the outer sheath was torn but this damage was not visible until the cable was removed.

Perhaps the failed glands I have seen were installed by "electricians" not well practised in fitting glands to SWA cable.

 

[seasickstevie]

... That is why 2 core armoured cable exists - it would be pretty useless if the armour couldn't be used.

I've often wondered about that.