Installing consumer unit in Granny flat with sub meter

mightyworthit · 17 Sep 2008

I have built an extension which incorperates a seperate'granny' flat. I have wired the lighting and mains circuit and now wish to connect it all up at the consumer unit.
My intention is to connect this consumer unit to the original house consumer unit using armoured cable (twin cored) It will be travelling 40 m or so in loft space to my Granny flat CU. What size cable will I have to use. I have been loosely advisewd to use 25mm is this correct?

The armoured cable on arriving to the Granny flat CU will go via a mains on/off switch (any special size or style?) Then via a sub meter, then into a consumer unit. Is this correct? I intend to use a standard Cu from screwfix or similar.

I will NOT be connecting the armoured cable to the original house as I guess this raises other complications

Thanks!

mightyworthit · 17 Sep 2008

I should add that the original house CU is next to the meter.
The flat has gas heating and water, but will use an electric cooker.

davy_owen_88 · 17 Sep 2008

Would I be right in assuming that because you have carried out major building work that you have informed building control of your actions and that they will be carrying out the inspecting and testing? If not, tut tut tut... Get on the phone and hope you've done everything right or expect some serious problems.

Firstly, I would recommend that you go with 3-core SWA because that will give you a dedicated protective conductor in parallel with the armour.

To determine the cable size required you need to give us maximum demand and the installation method for the cable along the entire run.

What is your earthing arrangement, and what is the type and rating of your main service fuse?

I would not recommend you wire this submain to your existing CU, it would be better to wire up a switchfuse using henley blocks and feed the submain from that.

mightyworthit · 17 Sep 2008

Would I be right in assuming that because you have carried out major building work that you have informed building control of your actions and that they will be carrying out the inspecting and testing? If not, tut tut tut... Get on the phone and hope you've done everything right or expect some serious problems.

The BC love me and on board.

To determine the cable size required you need to give us maximum demand and the installation method for the cable along the entire run.

Ok cooker circuit 25a
two lighting circuits on 5amp circuits total 1000 w
two ring mains on 32 amp circuits with living room and kitchen, i.e TV video , Washing machine, kettle, heating pump, shower fan, dishwasher and tumble dryer.
Gas heating and HW.

Is this what you mean?

To determine the cable size required you need to give us maximum demand and the installation method for the cable along the entire run.

I guess you mean how it is mechanically fixed within loft space and roof space then down through stud wall to CU. Not encassed yet coverd in insulation for 3m or so.

What is your earthing arrangement, and what is the type and rating of your main service fuse?

The original CU is earthed with rod in ground , to water mains and I could if needed earth to gas at Gas meter.

I cannot find the fuse rating of my main service fuse but the meter says 80a on it!

I hope I am helping myself here!

THanks

frostybadger · 17 Sep 2008

The armoured cable on arriving to the Granny flat CU will go via a mains on/off switch (any special size or style?) Then via a sub meter, then into a consumer unit. Is this correct? I intend to use a standard Cu from screwfix or similar.

Personally i would have a switchfuse fed straight from the mains via a henly block, armoured from this to sub meter, then onto the isolator, then into the Consumer Unit.

Stops anyone stealing the leccy

frostybadger · 17 Sep 2008

Get rid of the insulation run and you can use 16mm 3 core on a 63A switchfuse with a volt drop of 6.59volts. If you cant avoid the insulation though then its going to have to be 25mm.

plugwash · 18 Sep 2008

frostybadger said:
Personally i would have a switchfuse fed straight from the mains via a henly block, armoured from this to sub meter, then onto the isolator, then into the Consumer Unit.

I would put the isolator before the meter for practical reasons, you need something to terminate that big armoured onto. An isolator in a metal box is suitable for terminating armoured onto, most meters are only designed to take meter tails.

Stops anyone stealing the leccy

Who is this granny flat for? If it's a familiy member or someone else you have high trust in then sub metering is fine but if you are planning to rent it out on the open market I would advise getting a seperate supply. That way if the tennants steal leccy it's not your problem.

Lectrician · 18 Sep 2008

63 amp Wylex moulded switch fuse at origin.

16mm or 25mm split concentric run internally to the building but not burried in a wall(far easier and cheaper than SWA). SWA would be required if you wanted to bury the cable in a wall or stud at a depth of less than 50mm from the surface, or you wanted to run outside the zones. A 30mA RCD protected the submain is obviously a no no.

63 to 100amp isolator moulded isolator >> tails to meter >> tails to CU.

Chris.J · 18 Sep 2008

I have a similar installation, put in quite a few years ago, TT earthing.
The Flat supply comes from a Henley block straight after the meter, but via a 100mA slow RCD & fuse, to protect the cable.
At the Flat, a standard TT installation with it's own earth rod. My house earth is not exported to the Flat. The SWA armour is only connected at the Flat end.
Is this not a better arrangement?

mightyworthit · 18 Sep 2008

Great, getting the picture now.

So the way I see it,

Henley blocks from after meter to a 63 or 100 amp slow rcd and fuse. (to be done by electrician)

25 mm split concentric cable- Will this need an earth if the Flat CU has its own seperate earth?

into

100 amp moulded isolator
25mm tails to
sub meter
25mmm tails to
Dual RCD split load CU

One further question, can I extend the run of the split concentric cable or more to the point put a connector on it to extend it? OR am I better of allowing for slack in the cable to allow a possible change of position of the house consumer unit. (All part of major house building/renovation work).

This is not work I am planning on doing myself

You guys are great.

Spark123 · 18 Sep 2008

mightyworthit said:
The original CU is earthed with rod in ground , to water mains and I could if needed earth to gas at Gas meter.

Yes, you need all your Main Protective Bonding in place before you carry out any additions or alterations.

Spark123 · 18 Sep 2008

mightyworthit said:
One further question, can I extend the run of the split concentric cable or more to the point put a connector on it to extend it? OR am I better of allowing for slack in the cable to allow a possible change of position of the house consumer unit. (All part of major house building/renovation work).

Plan where you want it now or leave enough cable to locate it later.

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Installing consumer unit in Granny flat with sub meter

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