Would appreciate help with a CU layout diagram

20 Jan 2024
Reaction score
Hello all,

(Dual Row CU, All RCBOs, Wiring Diagram?)

We're fitting a new bathroom with electric shower etc and our existing CU is already full.
It's a 13 slot hager, 63 amp main fuse, with one RCD for socket circuits and pretty much everything
else direct on MCBs. Fairly ancient. (Based in Ireland).

Originally I talked to our electrician about installing a 2nd CU, but we've since agreed to install
a bigger unit and combine all circuits into one. It's a bit tricky and we might have to run two CUs
temporarily on split tails. The electrician will be testing all the existing circuits initially, but I suspect
we might be into some rewiring as part of the process. See at the time. He's due to start in
the next few weeks.

I said I'd get enough hardware to sort the bathroom as a starter and he advised tell the distributor what
circuits we want supported initially and they'll spec out the kit. That's done and we've received a Hager Gamma Dual Row - 26 Modules,
4 RCBOs, and 1 x 63Amp MCB (which apparently acts as the main fuse). I like the idea of RCBOs for all circuits,
and subject to the electricians thoughts I'm inclined to go that route for the rest.

My own background is in IT, comms networking, and I'm a sucker for network diagrams
so thought I'd try a layout for the CU ! They help me understand how things connect etc, and also might make
it easier to have a discussion with the electrician about module distribution when work starts.

But I'm scratching my head about a few things:

1 - The MCB that acts as the main fuse - does that sit before or after the main switch.
2 - How is the Line In to the busbar on row 1 extended to the busbar on row 2.

I've done a couple of diagrams exploring the MCB (main fuse) position, and have assumed that
connecting the busbars is just done with some appropriate flex, but on that latter point
it seems there might be different ways of doing that too. Would the flex share a connection to
the busbar with an RCBO, would it originate from the main switch, or are there some sort of
terminals that could sit on both busbars and provide a dedicated connection between them.
This part of the setup has me stumped.

I know the electrician will just come in, do what he does, and I could just wait until it's done to find out,
but my curiosity is getting the better of me! Basically where does that 63Amp MCB go, how is it connected,
and how is the 2nd busbar integrated into the system.

I'm anticipating there's probably a number of ways to skin the cat, but would love to hear if
I'm wildly off the mark or heading somewhere in the right direction.

My two (stab at it) diagrams attached! Thanks for any pointers.


  • Consumer Unit Plan - MCB to Switch - 2024.jpg
    Consumer Unit Plan - MCB to Switch - 2024.jpg
    115.1 KB · Views: 40
  • Consumer Unit Plan - Switch to MCB - 2024.jpg
    Consumer Unit Plan - Switch to MCB - 2024.jpg
    108.2 KB · Views: 42
Last edited:
Sponsored Links
The op is over thinking this.

The lengths of the cables for the existing circuits could very likely determine the order of the circuits

Changing such a large unit seems OTT , so why not add a 2nd unit?
Sponsored Links
Many countries commonly use much larger CUs than the UK and often have far more circuits in domestic properties. I believe that’s the case in Ireland. 20-amp radials for sockets and possibly dedicated radials for various loads throughout the house.

I‘m surprised to see a main MCB though, rather than a D02 bottle fuse. In case of a short circuit the main MCB could trip, plunging the whole house into darkness (lack of discrimination).
Not sure what Ireland do about SPD's but I suspect the op is mistaken about the 63A MCB before the main switch
thanks for the various replies guys,

@Murdochat - re the large CU, I did ask the distributor if they had the 18 module Hager, but the 26 was the only one available (next size up). I'm not even sure if the 18 is available in Ireland? I guess I'm also trying to cater for possible future growth in the house. I can't see us installing a heat pump, solar, or getting an EV, but a circuit out to the shed might be handy at some time. Putting in the 26 module should avoid us, or any future owner, having to readdress it.

Our existing box is very old, cracks around the casing where the knockouts were, and generally looks well past it's 'sell by' date! One of the reasons for just swapping it out completely. And yep, the length of the existing cables will be a thing. I had a chat with the electrician about it and he has various thoughts on how to work around this. I know new cabling will be required for some circuits, but fortunately it's a small house and running wires won't be a big task. There's also a load of redundant cabling from an old 12V windmill setup. This was decommissioned before we bought the house and I want to use this opportunity for a general clean up anyway.

Re SPDs and MCB. From what I can see SPDs are not regulation in Ireland (yet), but I appreciate the benefit of putting them in. We did have a lightning strike some years ago which took out a laptop. I'm wondering if I might ask the electrician to leave a couple of slots empty before the main switch for possible future inclusion. Re the MCB, I'm inclined to agree with you that it's after the main switch, thereby ensuring all modules are isolated by the switch. There is a big isolator in the meter cabinet as well, but not all houses have this so it seems to make sense your way.

@Ragnar_AT - yep you're right about Ireland. The UK seems to favour ring mains but in Ireland the electricians tend to install radials. Seems to be pros and cons either way. You've caught my attention about the MCB acting as a main fuse. When the distributor added it to the quote I did ask him about it. He said they now spec them when replacing old units, but at the same time did hint that he prefers the 'old' way. If it's prone to tripping that could indeed be an issue, hmmm? I'll see what our electrician thinks when he's here.

Still pondering on how the two busbars are 'linked' - any hints welcome :)
Last edited:
I can see why some one who's first language is English asking for help when living an a non English speaking country. But they do speak English where you live, so ask on local forum as I think Irish rules are strict as to what can be done DIY and we may give wrong advice.
Your Registered Electrical Contractor (REC) will sort this for you. The OP cannot design this.
Sponsored Links