Advice on additional cu

Bendy · Sep 10, 2011

I need some help to clear up a couple of issues I have when aadding additional consumer units for garAge or pv installation.

1. Can smaller tails than the incoming ones be daisy chained from the existing cu to the new one as long as they are rated for the new cu. (16a and 6a) eg 4mm. I'm assuming that the mcbs in the new cu protect the cable from overload and that the main fuse will protect the cable from short circuit.

2. If I cannot fit additional tails into the existing cu and split the existing tails( assuming an isolator is fitted because obviously I would never pull the main fuse to do this:smile5

on many jobs there is not enough length to do this and as I wouldn't want to open the meter so I would need to change the tails into the existing cu. My question is if I am only changing the tails what testing would I have to do other than earths on the existing cu? One spark I know is telling me I would have to test every circuit in the existing cu and claim responsibility for it.

3. Obviously i need to check the service bonding on these installs and I know that they both need to be bonded in 10mm back to the MET. My query on this is why could I not earth a service on the other side of the house with a rod? What is the reasoning behind a single earth?

Thanks for you help

Ben

GLENNSPARK · Sep 10, 2011

If you wanted to put a sub board in the garage then firstly you would assertain the type of earthing arrangements you had for the house: TT....TNS...TN-C-S.
The next port of call would be to work out a run fot the supply cable from board 1 to the garage. You would not run this cable from the RCD. just a fuse/breaker at board 1. You would install an
RCD at the garage board (this will give you discrimination)
You need to know what the max supply needs to be for the garage. You can work this out by the amount of current using equipment in the garage.. so you would typically have a 6A for the lighting and a 16A or 20A for points....so you now know the size of cable you will need (should be a SWA if underground or clipped direct)...If you have any extranious conductive parts within the garage and its a TN-C-S earthing then can "export" the earth using a 10mm. Theres nothing to say that the earth you export cant be the earth for the garage install as well........

barnzi · Sep 10, 2011

If it's less than 30A, can you not run it from a MCB in the main consumer unit, assuming there are no problems exporting the PE as well? I've seen this done a few times.

GLENNSPARK · Sep 10, 2011

Bendy said:
I need some help to clear up a couple of issues I have when aadding additional consumer units for garAge or pv installation.

1. Can smaller tails than the incoming ones be daisy chained from the existing cu to the new one as long as they are rated for the new cu. (16a and 6a) eg 4mm. I'm assuming that the mcbs in the new cu protect the cable from overload and that the main fuse will protect the cable from short circuit.

2. If I cannot fit additional tails into the existing cu and split the existing tails( assuming an isolator is fitted because obviously I would never pull the main fuse to do this:smile5 on many jobs there is not enough length to do this and as I wouldn't want to open the meter so I would need to change the tails into the existing cu. My question is if I am only changing the tails what testing would I have to do other than earths on the existing cu? One spark I know is telling me I would have to test every circuit in the existing cu and claim responsibility for it.

3. Obviously i need to check the service bonding on these installs and I know that they both need to be bonded in 10mm back to the MET. My query on this is why could I not earth a service on the other side of the house with a rod? What is the reasoning behind a single earth?

Thanks for you help

Ben

You would only need to test work you have done (and anything that may be affected by work you do)......

Bendy · Sep 10, 2011

barnzi said:
If it's less than 30A, can you not run it from a MCB in the main consumer unit, assuming there are no problems exporting the PE as well? I've seen this done a few times.

If there was a spare way in the existing cu that would be my first choice but I've come up against a few situations where the board is already full or is far too old for me to want to touch. Eg fuse wire replaced with push in mcbs
I'm doing a lot of pv installs so adding the additional cu also reduces the likelihood of the pv tripping the rest of the house.

GLENNSPARK · Sep 10, 2011

Take the supply cable for the garage board from an MCB in the house board (not the RCD) you can then use an RCD at the garage end...this you will give you the discrimination you want (the garage board RCD will trip without affecting the RCD at the house board)...........Dont forget that a fuse feeding a sub board is 5s dissconection time.....Table 44.4 (P49) BS7671 and for breakers to BS EN 60898 Table 41.3 (P49) BS7671.........