Two Core Armoured Cable but Plastic Consumer Unit!

I hope that someone can help. The first garage to my house has power, with its own consumer unit, with a 100amp main switch, and spare MCB slots. I can completely isolate this consumer unit at the main board, where's there's an RCD for it.

The second garage is a few metres away from the first one, and I wish to provide this too with power (six double sockets on a ring main), and light (a couple of strip lights on one switch; and a small strip light in a telephone box outside the second garage on a separate outdoor switch).

I already have a lot of 10mm2 twin core armored cable, so would like to make use of this, partly to save cost, and partly as my house is built on solid rock, so trenching is next to impossible. I've thus bought a garage consumer unit, with a 32amp MCB for the sockets, a 6amp MCB for the lighting (I plan on wiring both lighting circuits to the one MCB), and a 63amp RCCB.

My thinking is that I can wire the new circuits to the second garage consumer unit, then connect this with the armoured cable to the larger consumer unit (once isolated) in the first garage, with a suitable (40 or 63amp?) MCB. By doing so, I can avoid having to meddle with the main board.

But I have a problem as to earth. The armoured cable earth is in the form of its steel sheath, so I've bought a suitable gland. But the second garage consumer box is plastic! How do I connect the earth of the armoured cable to the earth block in the second garage consumer unit?!

Additional questions:

a) Am I right in thinking that the line and neutral of the armoured cable from the consumer unit of the first garage should be connected to the 'Line' terminals of the RCCB, numbered 1 and 3? If so, which to which?

b) On both the neutral block and the earth block of the second garage consumer unit, terminal 2 in each case has no screw head. Why is this?

c) There is a busbar from the terminal marked 4 on the load side of the RCCB to the bottom of both the MCBs. This bar has a protective plastic strip over it. Should I remove this?

d) Am I right in thinking that I should connect the line phase of each of the circuits to the top terminals of the MCBs, and the neutral phase of each to the neutral block?

e) On the neutral block, one of the terminals (4) has connected to it a thick cable that runs to the 2 terminal on the load side of the RCCB. That leaves terminals 3, 1 and N free on the neutral block (2 has no screw head). Does it matter which of these I use for the neutral phases of the circuits?

f) Finally, the telephone box is a cast iron K6 red box. Should I separately earth this direct to ground with a stake, or in some other way? It will have in it the light connected to the new garage consumer unit, and a working phone.

Sorry for all the questions, but am very keen to learn. Many thanks.

A P-Nut can be used but I have found the plastic on most consumer units is too weak to take a 10mm SWA cable and it may be better to use an adaptable box and a tube nut and two brass ends to connect both together.

However reading rest of questions I question you have the skill required?

Since the supple comes from the house you will need to notify the LABC likely you have already done this to build garage.

Most consumer units come with a warning that you need to check all screws for being tight so you will need to remove covers to do this and then replace.

Last time I have seen an earth connected to a phone line was with a party line with button to press to be able to make a call.

Most of your questions I am sure will have easy answer once we look at what you are doing but I am not prepared to guess.

Hi EricMark - thanks for the advice. A P-nut sounds like a possible solution, with a cable from that to the earth block. The plastic of the new consumer box seems strong, so I'm sure this will work. As to my other questions, I think I can see what's what, but one learns through repetition and stretch, so I want to check I have all clear in my mind. The earthing of the telephone: it's the cast iron telephone box itself that I'm wondering about.

Your cable and CPD sizes are dependant on what precedes the new installation.

IE, if your existing garage is fed with 2.5 via a 16A breaker, there is no point trying to install a 32A ring final in the new garage.

Hi SecureSpark. The existing supply from the main board at the house to the first garage is via the same 10mm2 armoured cable, with a 63amp breaker.

Post up pictures inside and outside of the existing and the new consumer unit please, this will help no end!

The original garage how much load is on that ??. Will the original 10mm for the first garage now support a 2nd?

How big is your DNO protective device it can be 60a, 80a, or 100amp??

Are you going to overlaod the exisitng installation but adding this new building ??

What earething system have you got. Some DNO do not like to export the earth if it is PME.??

Will there be any other services in the garage ie water or gas, becasue then you will have to run a 10mm bonding cable back to the main earth terminal.

You do know that all this work is certifiable by either you contacting your Local Building Control or getting a scheme registered Electrician.

How far is this new garage, from the house? You will have to take into consideration distance for the size of cable, though 10mm sould be ok but may not be. Also you will have to work out all the Zs for the circuit to make sure your trip times are good.

If the DNO don't let you take the earth out, do you know about TT systems??

There is normally no earth on a telephone system. It is considered as extra low voltage.

But read what MalcofArabia has said. The testing of any electrical system needs the use of expensive meters even to hire they cost £60 and add to that the LABC charges to DIY costs more if done correct than to get it done for you.

I am retired and I don't do any domestic work as just not worth all the hassle going through LABC who to me charge well OTT for what they do.

To say to anyone yes that will be £50 for me and £120 for the LABC seems a little unfair I do all the work and they get Lions share of money.

ericmark said:

Last time I have seen an earth connected to a phone line was with a party line with button to press to be able to make a call.

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ericmark said:

There is normally no earth on a telephone system. It is considered as extra low voltage.

Click to expand...

YoungManGoneWest said:

Finally, the telephone box is a cast iron K6 red box. Should I separately earth this direct to ground with a stake, or in some other way? It will have in it the light connected to the new garage consumer unit, and a working phone.

Click to expand...

Eric - this is a K6 phone box.

1john said:

Post up pictures inside and outside of the existing and the new consumer unit please, this will help no end!

Click to expand...

Thanks for that 1john. I shall aim to do that tomorrow.

MalcofArabia said:

Will there be any other services in the garage ie water or gas, becasue then you will have to run a 10mm bonding cable back to the main earth terminal.

Click to expand...

Yes, there will be a sink. I shall earth bond that back to the new consumer unit.

My thinking re. earthing of the telephone box is that this is made of cast iron. Intention therefore is to earth bond this back to the new consumer unit too.