Outbuildings - multiple consumer units

Henley block - fused isolator - no twin and earth - straight to SWA sized for max demand for outbuildings.

Negates the need for an initial twin and earth run which would have to be protected by RCBO. No good the RCBO in the house board knocking off before the outbuilding protective devices for the sub circuits do.

You give no indication of max load in amps expected at the destination end so difficult to go any further with this.

I'd go 10mm x 3 core SWA protected by a 40A fused switch at the house end, the terminate into a sub dist board in 1st outbuilding, then run suitably sized SWA cables to other outbuildings - no twin and earth outside please - go SWA all round for distribution and leave the twin and earth for sub circuits within each outbuilding only. Add one sub board/consumer unit per outbuilding and that's my take on it.

There are pros and cons to both the henley block, and the breaker(s) in the CU approach.

The obvious one is ease of isolation and if you want to turn the house of and leave the outbuilding on or vice versa.

You must have some sort of fuse or breaker between the meter and the long run of cable to the outbuilding, but it does not have to be along side all the others. But note you should have a clear place for isolating the whole installation, ideally a single point.

Total load also comes into it as well, if the outbuilding loads are more than a few tens of amps then you do not want them  eating up the capacity of the main CU. Depending on the layout you could feed one outbuilding from another, but this may not make sense if they are not in the same direction.

More details may clarify some of this.

Then there is earthing - do any of the outbuildings have water or gas supplies that would need earthing ? If so then you may need a 10mm2 earth bond.

Or if they have earth floors or will supply things outside like a caravan or a car charger, you may prefer to make them TT islands.



Mike.

Each distribution circuit can be taken off the primary distribution board. 'Tis as simple as that!

Chris,

If you are defining the house CU as 'the primary distribution board, how can you provide adequate discrimination via mcbs only?

Rather, I suspect you are saying exactly what I said!

Note that if you have two RCDs in series (one in the house CU and one in the outhouse CU) then there's no guarantee that the outhouse one will trip first.

Sounds sensible, perhaps I worded my original post wrongly, I've got no intention of putting any twin and earth anywhere outdoors. 


My intention in the largest outbuilding was to have a 32A ring, internal lighting and garden lighting fed from the consumer unit, then for the other two smaller outbuildings I'll be wanting a 16A radial and lighting in each. I don't anticipate ever using anything like 32a or 16a in the respective buildings but if feeding 10mm SWA to the first and 6mm to the second cables are rated more than adequately


With this and the other replies (thanks all) maybe I'll do Henley block - Fused isolator (40a or 60a) - 10mm SWA - RCD consumer unit (MCBs feeding all 1st outbuilding circuits & 2nd outbuilding Garage consumer unit) - 6mm SWA - 2nd outbuilding garage consumer unit (MCBs feeding 2nd outbuilding circuits & third outbuilding Garage consumer unit) - 6mm SWA - 3rd outbuilding garage consumer unit. The outbuildings are in a layout which makes it relatively sensible to have one feeding another to another, will end up being an L shape cable run. Doing the above should in my mind give me RCD protection through all the outbuildings -however if the 3rd has a fault all three will go out (maybe a split board with the feed to the next building not RCD protected but all the circuits in each outbuilding covered under the RCD section would resolve this?) 


Anyone have any critique on the above? 

Thanks again

Thanks for your reply Mike, there will be isolation from the meter able to isolate everything then secondary isolation for the long SWA run to outbuildings (in the form of a fused isolator) and possibly secondary isolation to the house CU. 


Don't think its likely to be more than a few 10s of amps but would like to future proof as much as possible so seems sensible to segregate as the option is there. The main outbuilding has water but it comes from the house so will be earthed there, the first may well require a car charger at some point (maybe wise for me to consider putting a larger than 10mm SWA in to accommodate for the possibility of this future load?). Depending on the earthing arrangement in the house (either TN-S or TN-C-S) I will probably earth the main outbuilding from the house via third core of the SWA and then earth the following two with one new electrode TT system


Sound sensible?

If the water pipe to the main outbuilding is plastic, it won't (can't!!) need bonding. Definitely take the possibility of a car charger into account when rating your SWA for the distribution circuit. It might give you more options with the car charger later on, if you make that outbuilding TT. I'd be inclined to make all the outbuildings TT. Use the house earthing arrangement to protect the SWA only. That depends on how easy it is to get rods in. You know the site best.


Dave

The main outbuilding has water but it comes from the house so will be earthed there

I think Mike meant bonding rather than Earthing. If the water pipes are metallic then they'll need bonding in each building they vist - you can't rely on it remaining continuous metal underground these days what with any repairs or alterations likely to be done in plastic. If newer and the pipes are MDPE with no metal into the ground then no need to bond.


   - Andy.

the first may well require a car charger at some point (maybe wise for me to consider putting a larger than 10mm SWA in to accommodate for the possibility of this future load?)

Certainly good to consider future loads. With long runs the limiting factor tends to be voltage drop rather than the current carrying capacity of the cable though - so a bit of calculation might be in order.

   - Andy.