Multiple Consumer Units

[PerpetualLeakingRoof]

Hi,

I'm about to embark on re-wiring a fairly large house (about 300 m sq all told). It has three high floors and a basement which is where the main CU will be installed. It occured to me that getting a ring main and lighting circuit to the first and especially the second floors would be a real mission and would require a huge amount of cable.

I was wondering, therefore, if in this situation I wouldn't be better off fitting a small CU on the first and second floors and then just taking a single cable from the main CU up to each and then distributing power from there.

The plan would be to run 60A cable to each sub-CU from a 50A breaker in the main CU, the whole property would be on a single phase. I've done a very quick estimation and concluded that the ring on the top floor will be very close to the 90m cable limit after it has snaked all over the place.

Thanks.

 

[Click-Sure]

You'll be calculating voltage drop throughtout your design process.

Nothing wrong with submains and DB's serving each floor but ensure there is one single point of isolation for the whole installation.

Personally in this situation I'd have a small main DB which has a main switch and 2, maybe 3 50/56A MCB's in it, from there I'd use SWA or XLPE to the DB's on each floor, where the RCD's would be placed.

If you are hoping to be compliant with 17th Ed you'll need to think carefully about how you provide RCD protection for the circuits as you wont be able to 'mix up' the upstairs/downstairs lighting/power circuits.

Usefull searches:

Submain, Terminating SWA, Voltage Drop, Metalclad, RCD protection, 17th Edition, Safe Isolation Procedure.

 

[Lectrician]

SWA or XLPE? Contradiction in terms

56amp mcb......typo

With a fuseboard on each floor, it would be best to then have atleast two socket circuits and two lighting circuits per floor, either on their own RCBO's, or split as required over a dual split load board (left hand lights and right hand sockets on one RCD and visa versa). This is assuming 17th edition compliance.

You would also need to be wary of the submains - if you where (heaven forbid) to use PVC/PVC submains, you would more than likely have to RCD protect the submains too - a crap method of design.

Make sure the main CU does not have the 3 or 4 50/63amp breakers adjacent each other - they would need air space around them if they are going to be loaded. You would be far better with an MCCB board.

 

[GaryMo]

Make sure the main CU does not have the 3 or 4 50/63amp breakers adjacent each other - they would need air space around them if they are going to be loaded. You would be far better with an MCCB board.

Even if they were to be fully loaded (hope not as they'll probably only have a 100A supply) then it wouldn't be for prolonged periods of time.
Surely an MCCB distribution board would be overkill?

 

[PerpetualLeakingRoof]

Thanks for the replies

The circuits wouldn't be heavily loaded, the floors that they are serving are bedrooms and or home offices so pretty standard stuff.

I would like to comply with 17th edition if possible. If I went down this route of multiple consumer units then I would probably put a ring or maybe a radial into each room (4 rooms per floor) so that I could easily isolate each one. I would do a standard split loads in each sub-CU

As for the supply to the house it's three phase but the main house will be on a single phase. The cellar workshop will be on a different phase and the worlds biggest electric oven will require a three phase supply (which will be fitted by a fully qualified sparkie - I've only got Part P).

I will ensure that there is whole house isolation by fitting a switch before the main CU. I was currently thinking of going for one of these:

http://www.electrika.com/products/itemdetail.asp?MANU=0150&C=AAN&D=AAC&E=AAD

which reads like it would be up to the job but I can't help feeling it's a bit weedy. The alternative I've been toying with is something like this:

http://www.tradingdepot.co.uk/DEF/p...ch Fuse and Isolator/MEM Isolator 100 Amp TPN

which is obviously much more expensive. Silly question I know but as it's a TPN switch does it actually switch the neutral as well?

Cheers.

 

[Click-Sure]

I'd go for the MEM every time.

Yes, it is a 4 pole isolator.