Run a second consumer unit from a first consumer unit ??

[BW2]

I’m doing a loft conversion – so completely new wiring.

The new loft is a large bedsit, kitchenette, power shower, study & will be largely independent – son & family downstairs – oldies up top.

Downstairs is all modern wiring, run off a consumer unit.

I’m thinking it might be a good idea to run the loft off a separate CU placed up in the loft - it’s a separate set of fuses, elcb, etc & don’t have to run down 2 flights of stairs if the downstairs elcb goes.

If I was to do that & run the tails back down to the house supply, that’s close on 10 metres of tails !

So I’m wondering if it would be ok, or a good idea, to run the loft CU from the downstairs CU ?

Any thoughts anyone.

 

[festa]

This is some install your planning and i would say you need to start by working out the maximum demand of the new installation ADDED to the maximum demand of the original installation is going to be too much for your main service fuse and tails.
Even after diversity is applied to both your installations i would be suprised `IF` the service fuse/tails are up to the job.
Just a warning....the DNO `CAN` charge hundreds,even thousands for upgrading your supply to be able to cope with what your planning.

 

[JohnD]

You can, it's a submain. But you might do better with a separate switchfuse, so that the annexe doesn't have to lose power if the other CU is switched off for some reason.

Presume you will be submitting plans to your BCO, so they should query the design if untoward.

 

[plugwash]

the diversity guidelines tend to overestimate for domestic installations.

If you have to get the supply upgraded then you do, i'd worry about that if and when the service fuse starts blowing but tbh generally the first time you blow it they'll put in an 80A or 100A replacement which given its characteristics you are very unlikely to blow.

As for actually wiring it i'd say the way to go would be a 60A switchfuse off the tails and then a feed of 16mm T&E or split con up to the new CU. This way avoids putting extra stress on the existing CU and reduces the chance of the new loft installation inadvertantly losing power.

 

[festa]

So from the main service fuse run a set of tails(1) to a henley double pole block....run two sets of tails(2+3) from this,one set to the original C.U and the other set to a suitably rated switchfuse,then from switchfuse run 16mm T&E to the new C.U up in th`attic?.

1.These tails must be rated to cope with both existing AND new C.U demand.
2+3.Each set of tails must be suited to demand of C.U they are supplying.
Current carrying capacity....

16mm Tails (method1)=87A
25mm Tails (method1)=114A
35mm Tails (method1)=141A

 

[JohnD]

the tails have to suit the suppliers main fuse. No matter how many CUs you put on it you can't draw more(ish).

Main fuse is usually assumed to be 100A so 25mm tails are the norm.

My preference would be for a 100A DP isolatot between the meter and the Henley, so you have a single point of isolation for the entire installation.

 

[BW2]

Thanks for all your comments – I wondered if it was a no-no, but it looks encouraging.

Looking at it a bit further - the existing arrangement is …

The supply comes into the house via a 100 amp fuse, which then goes into a split-load CU. This CU is set up as …

- 100 amp Main switch disconnector
- (unprotected section)
mains-wired fire alarm 6 amp mcb
- 80 amp RCD
- (RCD protected section)
ground floor power 32 amp mcb
first floor power 32 amp mcb
ground floor lights 6 amp mcb
first floor lights 6 amp mcb


In the supply box there’s a block for the tails, and as far as I can see, spare slots that could take a second set of tails – would they allow a connection on these ? But this would involve a very fiddly bit of physical routing to get from the supply box (on the outside wall) into the house where the 1st CU is. It was a real pain last time, but maybe it has to be.

If I were to use Henley blocks & switchfuses, I presume that they are sited inside the house – not in the supply box.

I’m just wondering if it might be easier to take a connection from the unprotected section of the 1st CU to the new 2nd CU in the loft. That way I’ve got isolation of the whole house off at the 1st CU main switch.

I originally said a power shower for the loft – what I should have said was a pumped shower – they seem to be rated at 2-3 amps.

Loft CU would be

- ?? amp main switch disconnector
- ?? amp RCD
- (RCD protected section)
small hob/oven 32? amp mcb
loft power 32 amp mcb
loft lights 6 amp mcb

Comments on any of the above very welcome.

 

[Steve]

OK, lights shouldn't be on an RCD unless you rely on an earth rod for earthing. If you can, move them to the non-RCD side. Think about it, if something trips that RCD at night, you'll have no lights to find the fault or reset the RCD. It is unusual for this to have ben done like this.

Put the submain on a 50 amp MCB on the non-RCD side. Run 10mm² cable to the second CU.

 

[bernardgreen]

OK, lights shouldn't be on an RCD unless you rely on an earth rod for earthing.

Not all lights should be on a SINGLE RCD. With two RCDs put upstairs lights on one and downstairs and loft lights on the other.

With more downlighters and similar "complicated" lighting an RCD may trip out as insulation starts to char but long before the 6 amp breaker comes out. ( 1.5 kilowatts in the melting transformer is not good )