Limiting long cable runs for large domestic rewire

[citylife]

I am about to rewire a large, 9 bedroom domestic refurb that has gained multiple 2-floor extensions and new loft conversion.
Original footprint was 135m2 ground floor, 96m2 second floor, new footprint is 180m2 GF, 127m2 FF, 76m2 SF. Concrete ground floor

Meter cupboard and original consumer unit located ground floor rear entrance TN-CS supply

I am considering sub-mains and separate consumer units to limit long cable runs positioned ground floor meter cupboard to cover circuits left hand side of building ground & first floor. Consumer unit ground floor front entrance to cover ground and first floor circuits right-hand side of building and a consumer unit in loft for all loft circuits. Sub-mains in 16m T&E plus additional 10mm GY. No large loads required such as induction hobs, electric showers, etc. Kitchen and Utilty on board supplied by mains

Any thoughts and considerations?

 

[Octopus]

Not sure I would run sub mains in t&e as they would need RCD protection.... Would you want to go to the ground floor to reset a RCD?

 

[telectrix]

i'd think about using 16mm 3 core SWA for the feeds to the subs.

1. you have a 16mm coc/bondong conductor within the cable. no need for an additional cable.

2. The SWA would not require RCD protection.

 

[telectrix]

beaten to the draw by superfast murdoch.

 

[citylife]

Cheers for your comments.
I couldn't use a 100m/a time delay on T&E sub-mains?
Would I have to separate switches that are supplied from separate boards?

 

[telectrix]

you can't protect a T/E that's buried in a wall <50mm deep with anything other than a 30mA RCD.

 

[Wilko]

Interesting problem - I'm guessing you've already worked out voltage drops on 4mm / 1.5 won't work? For TNCS at Ze 0.35 you can have a nice long rfc before you hit max Zs and even more for a 20A radial.

 

[Ian1981]

you can't protect a T/E that's buried in a wall <50mm deep with anything other than a 30mA RCD.

You can install cable in earthed metal conduit/containment or mechanical protect the cable as an alternative to a 30mA rcd.
Not the most practical I know!

 

[citylife]

i guess that's why I posed the question - if running 4mm for sockets, 1.5mm for lights from a single do comes under the zs thresholds should I use this method or make life easier for myself with a couple of sub-mains?

 

[Ian1981]

i guess that's why I posed the question - if running 4mm for sockets, 1.5mm for lights from a single do comes under the zs thresholds should I use this method or make life easier for myself with a couple of sub-mains?

If you have access to the iet design guide amend 3 it gives great examples and explanations of maximum cable runs before zs is exceeded and the max volt drop for circuits

 

[HandySparks]

...you have a 16mm coc/bondong conductor...

Love it!

 

[Octopus]

Is surface trunking not an option ....

 

[Leesparkykent]

I don't really like multiple boards in different locations in domestic. The square meters you've stated aren't huge really. As a starting point for design I would look at splitting the gf in to 3 ring final circuits (1 being kitchen) and two ring final circuits for the 1st floor and two for the second floor.

 

[Ian1981]

You will probably struggle with effective discrimination of protective devices if you say run a distribution circuit using a 63amp mcb for example with a downstream 32amp protective device (depends on what circuits are being supplied with what mcbs)
The 32 amp if develops a fault may trip the distribution mcb thus disconnecting the whole db being supplied by the device.
Unless your going to use a bs88 fuse to supply them.

 

[SJD]

I also don't really like boards in multiple locations, and would look at either having more circuits (especially the sockets, where a lot of the cable length is in the drops from the ceiling to socket height) and using heavier duty cable.

As an example, a recent large new build, we put in four RFCs on the ground floor (Kitchen, Utility, and 2 others circuits) partly to keep cable lengths down, so we could still use 2.5mm2 T&E. For lighting, we used 1.5mm2, and used 2 circuits per each of the 3 floors. The top floor sockets, we put 2 circuits on 20A MCBs, since the usage is expected to be light (you don't have to put RFCs on a 32A MCB).

 

[Ian1981]

Can always use a 16way db for a large number of circuits if required

 

[AndyL]

If you have no large loads why the need for a 16mm sub, could that not be 10 or even 6mm. I am assuming all the main bonding comes from the met at the origin

 

[Toneyz]

A lot will depend upon the building construction we had 4 £2.5M houses in Ascot solid floors so put sub boards on each floor storage cupboard as very long runs for every circuit to go back to the garage location of the intake & main D.B. used 16mm T & E as not in contact with insulation or in wall each installation needs to be designed on its situation as well as economic pressures.

 

[LankyWill]

I wouldn't bother with multiple boards as its a bit messy and not worth the hassle in your case. Your Sqm. is not large enough to accommodate such a design anyway, i recently completed 6x 6 Bed Barn conversions and we used a Hager 36 Way Stackable fuse-board. All Lighting circuits in 1.5 and Ring Finals in 2.5, just keep your ccts small and use your best judgment. I had 6x rings in total and 5 Lighting circuits plus a load of other ccts due to heatpump and high spec kitchen.