CU extension / 2nd CU

tonycoysh · Mar 5, 2012

Can someone tell me if I’m barking up the wrong tree or if what I want to do is acceptable, I can’t think of a reg that it would come under or go against but a lot of you know the reg’s a lot better than me.

I went to visit a mate yesterday, we were chatting away and he tells me that he’d like to sort out his outside wiring. He currently has a 13A extension lead going out to his shed and supplying a fridge, a freezer, lights in a summer house, and pond pump and uv light. Then from this, there is another 13A lead going to his car port which supplies a double socket and a security light, as well a strip light that plugs in.

So I look at the CU to find it’s a 10way with all of the ways in use.
So.. I either upgrade the CU to a larger one and incorporate the outside wiring or – and this is my question:-
Is it acceptable to install a secondary CU beside the main one and link into the main one. I.E Run tails from the supply side of the second RCD (in the main CU)to the secondary CU to then into an RCD protecting some outside circuits. This would then isolate everything from the Main switch in CU1 and save me splitting tails and getting DNO to kill the supply or fit an isolator. (I know involving the DNO and isolating and splitting the tails is the best option, but I thought of doing it this way and figured I would ask how some of you view it.)
The MET is a Henly block with CU Earth and Bonding going to it so I can use that to earth the 2[SUP]nd[/SUP] consumer unit.

Can I have you views and comments please.
- Covers his head waiting for potential slaps!!

malcolmsanford · Mar 5, 2012

Well for me I would go with Henly blocks spiting the tails into a new CU

1. Terminations you are going to have a snug fit fitting any size cable in to an already supplied RCD
2. By doing this there is a great chance your compromising the integrity of the existing CU by doing a Jerry rig procedure
3 You going to have 2 RCD of the same characteristics in series

Saying all of the above, it's going to be a darn lot safer than what he has now

tonycoysh · Mar 5, 2012

Thanks Malcolm.

I agree with all of what you have said, although point 3 - I would have wired it from the isolator side not the protected side so they would be in paralell. The exisiting two RCD's run from the main isolator into one then into the next, I was just thinking of extending this.

Ultimately I hate going to someone's house to find they got someone to replace/upgrade CU and find that they bought one just big enough and didn't think to by a bigger one with more ways to allow for future works. But hey ho.

I was thinking of checking to see if I could move things around in the existing CU and get a spare way but bearing the pond pump in mind I would rather it all on a seperate RCD or RCBO so that they don't get an RCD trip in the middle of the night or something ad loose all power.

I don't suppose there'll be a lot of price difference in getting an isolator switch fitted Vs. getting them to isolate the supply and come back to re-energise later in the day.

malcolmsanford · Mar 5, 2012

Nothing is ever easy is it Tony, before you call the DNO just make sure the fuse carrier is nice and snug, I lost count the amount of places I have been too, where the darn thing seems to just pop out on you when your checking it, after all best be safe than sorry, and make sure it is nice and snug for the nice DNO fuse puller man

telectrix · Mar 5, 2012

as above, i would split the tails feeding the existing CU, then your new CU is a separate installation. gives you more options . i generally find that giving the service head a mean look just makes those pesky seals roll over and fall off.

needasparks · Mar 5, 2012

For just a couple of extra low energy circuits why not just use one way of the existing board with a 40amp mcb and feed the new circuit plus the one moved circuit from said board in a new small 2-4 mod board. that way it keeps it with one main switch. And the seals stay fitted to the meter