2nd CU fed from existing CU

[Christhejp]

Hi All,

In the process of planning a large wrap around extension. A ways off (builders penciled in for next year). I'm a bit of a prepper and like to understand exactly what is required, so I know what to ask for and understand what I'm being quoted for etc. Also means I can manage the astronomical costs .

I've done a bit of research and I currently have a TNS system with a plastic RCD (63A) split board CU 100A main switch fed with distributor 100A fuse.

Existing CU has on one RCD protected side - 40,32,6,6,16 MCBs B type and on the other RCD protected side - 32,32,16,6,6 MCBs B type. Would it be possible to feed the 2nd CU (metal clad with RCBOs) from the existing board, where an existing 40A MCB type B would be replaced with a 50A MCB type B (25mm2 cable approx 1M to 2nd CU)?

FYI the new extension circuits required on the second board would be; Kitchen Ring Final 32A RCBO, Kitchen Cooker 40A RCBO, Kitchen/Dining Lighting 6A RCBO, Upstairs Ring Final 32A RCBO, Upstairs Lighting 6A RCBO, Electric Shower 45A RCBO (10mm2 cable).

Does the above sound ok as a general concept or does anything I have said look completely wrong. Any advice would be much appreciated.

Thanks in advance

 

[DefyG]

The total maximum demand is questionable even before adding the extension.
25sq is OTT for a 50A breaker, 10sq would suffice at 1M.
With a 45A (10kW) shower circuit (to which no diversity can be applied) you are using 90% of the available 50A capacity so it wont take much to overload the OCD for the sub main. (potentially as the shower may not be used a great deal in the grand scheme of things??)

 

[westward10]

1M away, replace the existing CU and take the new circuits back to it.

 

[Christhejp]

The total maximum demand is questionable even before adding the extension.
25sq is OTT for a 50A breaker, 10sq would suffice at 1M.
With a 45A (10kW) shower circuit (to which no diversity can be applied) you are using 90% of the available 50A capacity so it wont take much to overload the OCD for the sub main. (potentially as the shower may not be used a great deal in the grand scheme of things??)

Thanks for the reply. I think I understand. I had assumed because the total loading of the current CU (all the current MCBs added together) is already above the 100A main switch that this must be acceptable and it must be to do with not every load being on simultaneously (but you are saying this is questionably high as is?). In my naivety, I had then taken this assumption and applied it to the second CU thinking that - as you pointed out - the shower will only be on for short periods and thought this would be ok for the same to be true In the second CU.

In that vein could a 63A be installed for the second CU instead, as the 63A is protecting/limiting the current in the cable to the second unit. The main CU is protected from over current by its existing 100A main switch.

In regards just doing a new CU, not against the idea at all but wanted to keep older/new elecs separate, electricity flowing for the misses and not look down the barrel of a whole house re-wire....

I guess an alternative would be to get the spark to install a 2nd consumer unit via a Henley block, and maybe make it large enough to initially take the extension loads, with space scope to eventually re-wire the rest of the house into it?

Thanks again for any comments

 

[DefyG]

as the 63A is protecting/limiting the current in the cable to the second unit. The main CU is protected from over current by its existing 100A main switch.

The 63A RCD and 100A Main Switch are NOT protecting devices as in overcurrent protection. They are merely rated for a safe maximum operating current as stated.

I guess an alternative would be to get the spark to install a 2nd consumer unit via a Henley block, and maybe make it large enough to initially take the extension loads, with space scope to eventually re-wire the rest of the house into it?

This would really be your best way forward but you'd best employ a sparks to assess maximum demand and an EICR on the existing as some circuits may be able to put straight onto the new CU.

 

[Christhejp]

The 63A RCD and 100A Main Switch are NOT protecting devices as in overcurrent protection. They are merely rated for a safe maximum operating current as stated.

This would really be your best way forward but you'd best employ a sparks to assess maximum demand and an EICR on the existing as some circuits may be able to put straight onto the new CU.

Excellent, il go down the second consumer unit route as a starter. At least I have an idea of what I'm asking for now when I get to it. Thanks for all the replies.

 

[littlespark]

I’m with Westy, re #3.

Replace your existing plastic, split rcd board with a new one. Metal, All RCBO, with SPD. Big enough for the new circuits.

How old is the existing wiring? It’s possibly good for another 30 years give or take.


You may also want to think about the possibility of an EV charge point, PV and battery storage… things like that, at this point. Running large enough cables for the future while the building work is being done.

Extending your smoke detector circuit

 

[Christhejp]

I’m with Westy, re #3.

Replace your existing plastic, split rcd board with a new one. Metal, All RCBO, with SPD. Big enough for the new circuits.

How old is the existing wiring? It’s possibly good for another 30 years give or take.


You may also want to think about the possibility of an EV charge point, PV and battery storage… things like that, at this point. Running large enough cables for the future while the building work is being done.

Extending your smoke detector circuit

That's a great shout regarding the PV and EV point. It's not something I need now so hadn't considered but worth at least getting the cables in whilst the place is ripped apart for the future.

Thanks again for all the comments.

 

[Brother Blue]

A second CU should not be fed by an MCB as you will not ensure any segregation (discrimination or coordination for us older/real electricians)

A replacement CU does not require an EICR, despite the modern trend for scamming customers out of money for this.

When you're planing a job such as this get quotes from a few electricians , take their advice and choose the best if them. We all have different approaches to jobs and some will suit you better than others, plus domestic work is the bottom of thr foodchain so you won't get the best of electricians whatever you do.