Garden room electrics - protect armoured supply cable with RCD/RCBO?

[Nick Thomas]

So I've got an electrician coming out Monday to wire up the garden room and give it a certificate. We talked through a couple of different ways of wiring it up to the consumer unit in the house, and I was hoping to double-check what we arrived at.

 

The CU:

 

 

So just the one slot left, protected by the "downstairs" 80A / 30mA RCD.

 

The electrician has proposed moving that RCD and all its fuses left one, so the free slot is next to the 100A main switch. It would get a 16A fuse, then 4mm² SWA out from there, along the side of the house and into the attached garage at about shoulder height. From there, it'd join to another cable I've already laid - also 4mm² SWA, and 25M long - which goes out of the back of the garage, along the line of the fence in the back garden (in plastic conduit, for all that's worth), then under the decking - again in plastic conduit - then under the garden room and into it through some 68mm drainpipe I put through the wall. There'd be a tiny consumer unit on this side, with a 16A fuse and RCD protecting 2x double sockets. No lighting circuit. Total SWA run probably ~30M.

 

The reasoning was that if there's a fault along that cable, or with an applicance in the garden room, it won't affect anything inside the house, which seemed compelling at the time. The more I think about though, the less I like the idea of that 30M cable run just having the 16A fuse on it.

 

What do the good people of BuildHub think? Is this a sensible thing to do, or would RCD or RCBO protection for this cable in the house CU be worth it? If so, would you tag it onto an existing RCD or find a way to free up more slots, to make room for it to have its own?

 

The leftmost slot 4 is the heat pump,and its cable goes the same way as the new cable will go, so if I did need to free up space, I was thinking I could have a CU in the garage (40A?), and have the HP go from that. I assume if I did that, I'd still want the CU in the garden room as well?

Edited October 2 by Nick Thomas

[ProDave]

He is making it into a "high integrity" CU with as you describe one way not protected by either RCD which is okay for the SWA.

 

He needs to have the appropriate extra cables inside the CU.  Some will argue if it is not supplied like that by the manufacturer then it is no good.

[goodbyegti]

How are you earthing it? I did something similar and I had to install an earth rod for the new CU in the shed. I hit some rocks but it wasn't that bad. Took a few goes.

 

I think in many cases you can export the earth from the house, but you need a large section cable, 10mm2 minimum I think. 

 

I have the shed running off a 16A RCD in the house (legacy thing), with a 10A RCD protecting the sockets in the shed. It always trips first in the shed, which is good, but I'll be upgrading the SWA at some stage so I can get some more amperage in there.

Edited October 2 by goodbyegti

[Nick Thomas]

I didn't think to ask about earthing. I'll mention it when he shows up on Monday; maybe he's planning to pop a separate earth in, I don't know.

[Temp]

I found this guide some time ago. Haven't checked if it's still current though...

 

https://electrical.theiet.org/media/1695/electrical-installations-outdoors-a-supply-to-a-detached-outbuilding.pdf

 

I think mine was done as per figure 3 eg..

 

MCB in the house CU.

SWA cable to a "Garage CU" in the outbuilding.

Garage CU contains an RCD and two MCB (lighting and power)

Earth provided at the outbuilding via a rod.

[Nick Thomas]

Thanks. It's done and I've got my certificate; no earth rod, and it seems he's just used the house earth through the 4mm² SWA.

 

I questioned him lightly about it and he said that the separate earth might have been needed if the garden room had any metal in the construction ("external-conductive-parts" in Temp's PDF, I guess?) but since it didn't, he thinks it's fine as is.

 

In the end it's gone: house CU -> 20A MCB -> 4mm² SWA -> garden room CU -> RCD -> 16A MCB.