Assistance with electrics to garden building

[Adam1712]

Sorry if this is the wrong location, please move or direct me to correct place if so!..

In short, I'm wondering if anyone could offer comments or advice on my intended electrical run to a garden building?

From my consumer unit in the garage (main household box) there is already a 32A MCB running 4mm cable to a junction box which then runs 4mm SWA underground to shed (this was installed a while back when I built the shed by a qualified electrician).. This had its own consumer unit with a separate lighting and socket circuit. I say 'had' because I have since demolished the shed and disconnected the 4mm at the main household box, leaving the cable run to the end of the SWA (but no longer live of course)

The electrician has told me he is happy for me to wire up a new garden building while the whole thing is dead and he will carry out a thorough inspection & test before I board up the walls etc.. so it is all still exposed.

So my intention is to do the following, please advise!!..

- Not touch the main CU, the 32A MCB (currently not connected to anything) is part of a 63A RCD controlled bank
- Maintain the 4mm T+E from CU to first junction box (about 10m linear length)
- At first junction replace the existing SWA with 6mm (in-case the garden building requires something heavier duty in future, since the SWA is buried I don't want to risk needing to dig it up 10 years down the line) SWA is about a 10m run
- Install second junction box at garden building, continuing with 4mm T+E for a short length to secondary CU
- Connections at junction boxes will be proper SWA glands and WAGO connectors
- 32A RCBO into the garden building CU, splitting into 5 fused circuits

The 5 circuits being..
1 - 2000W (MAX) AC Unit (16A / 2.5mm T+E)
2 - Garden room sockets, 24m² (16A / 2.5mm T+E)
3 - Shed sockets 8m² (16A / 2.5mm T+E)
4 - Lighting circuit (6A / 1.5mm T+E)
5 - Soffit extractor fan circuit (6A / 1.5mm T+E)

All above will be radial circuits with cable direct in insulated walls

My theory being that the AC could be in use at any time, drawing a maximum of 10A.. leaving 22A for a garden room/office + shed + ancillaries

I'm sure its clear from the above that I am no electrician, to re-iterate this will all be dead during my work and will be fully inspected and tested before connecting back to power!

One thing I'm definitely not sure on is the RCBO I'm putting in the garden room CU, is this correct or can it just be a standard MCB (since the circuit is RCD protected at the main CU?)

 

[EFLImpudence]

If electrician is happy for you to do this, then you should ask him what he will be happy with.

 

[Adam1712]

If electrician is happy for you to do this, then you should ask him what he will be happy with.

I will be doing so, but I want to ensure I'm thinking on the right lines before I waste his time getting him out to run through a load of nonsense

 

[oldbutnotdead]

There's little point putting 6mm swa in and not upgrading the 4mm at the same time.
No point having an RCBO fed by an RCD protected circuit.

 

[Adam1712]

There's little point putting 6mm swa in and not upgrading the 4mm at the same time.
No point having an RCBO fed by an RCD protected circuit.

Was only thinking to do so on the basis that the cable is already there and if I do need more capacity in future the first run of 4mm is easy to access and switch to 6mm, but understood.

And thank you I thought that might be the case with the RCBO.. Would I just feed the garden room CU through a standard 32A MCB then? I want to be able to isolate the garden building from its own CU instead of going through the garage to isolate at the main CU.

 

[flameport]

32A MCB (currently not connected to anything) is part of a 63A RCD controlled bank

That's a bad start, as you don't want 2x RCDs for the same circuit. Having an RCD at the house will result in that and whatever else is connected to it being disconnected if there is a fault in the garden building.
There is also the problems of what is already connected to that 63A RCD and what type it is.

At first junction replace the existing SWA with 6mm (in-case the garden building requires something heavier duty in future,

4mm² SWA is rated more than 32A, so unless you expect to require significantly more than that, replacing it with 6mm² is pointless. Even if it was required, the rest of the cable would also need to be changed to 6mm².

Install second junction box at garden building, continuing with 4mm T+E for a short length to secondary CU

Connect the SWA into the new consumer unit. The junction box and length of T&E achieves nothing.

32A RCBO into the garden building CU, splitting into 5 fused circuits

RCBOs are generally intended for individual circuits.
If you want 5 circuits then it would be 5x RCBOs.
Or a single RCD and 5x MCBs, however that will result in everything being disconnected if any fault occurs on any of the circuits, a poor design.

 

[oldbutnotdead]

Was only thinking to do so on the basis that the cable is already there and if I do need more capacity in future the first run of 4mm is easy to access and switch to 6mm, but understood.

And thank you I thought that might be the case with the RCBO.. Would I just feed the garden room CU through a standard 32A MCB then? I want to be able to isolate the garden building from its own CU instead of going through the garage to isolate at the main CU.

Your 'best' setup will be;
Henley blocks before house cu, initial 4mm connected to them via suitable freestanding OPD. WARNING if the 4mm is at any part of its length hidden in a wall with less than 50mm cover then it must be protected by an RCD/rcbo not just an MCB or fuse
After that, 4mm to swa. Swa emerges in building 1, terminate it in an adaptable box. Swa to building 2 also terminates in that box. Connect both bits of SWA together and connect them to building 1 cu again in that adaptable box. Either rcbos or a master RCD required in that cu.
Swa goes from building 1 to building 2. Terminate direct in your (new metal) cu or in another ab if you go for a placcie cu. Again either rcbos or master RCD required
All done.
Allowing for the caveat re the 4mm, your 3 buildings can now be individually isolated, a fault in one building will usually not affect the other buildings

 

[Adam1712]

Thank you all for the responses, my mate came around this afternoon and has broken it down a bit more simply for me.. In short we are going to run 10mm SWA off its own circuit from the main CU (he is going to cover that end as its way above me) This will run direct to the garden room CU which will split off into the circuits I mentioned in original post, except we are going to merge lighting and soffit fans into a single circuit, and make the 2 socket circuits 32A rings (the AC will still be its own radial 16A)

 

[securespark]

Use 1milli for lighting. 1.5 is a waste.