Electrical question

[dmahon]

I need to get power to my observatory and sheds - I've done a lot of searching and I will be getting an electrician round to do some of the work, but is there anything I haven't considered?

I have a TN-C-S supply.

I have a high integrity consumer box with a spare non-protected circuit

I have 2 sheds plus obsy. Wood / fibreglass. I'd like a light and a double socket or two in each (more in the obsy, but maximum power draw low - I suspect a maximum of only about 10A would be used at any one time, in total across all three and usually much less (the first shed is a rabbit hutch, the third is the observatory) all won't be using power at the same time.

I perhaps need 10m of cable to get from the consumer unit to where it leaves the property

The first shed is 5m from the house

The second is another 30m

The obsy is another 5m

The plan:

Use spare non RCD protected circuit with a 32A MCB

Either 10mm T&E in metal trunking or 10mm 3 core SWA to edge of property

10mm 3 core SWA 450-600mm underground in 50mm ridgicoil trunking

Metal adaptable box in shed connecting the SWA, via T&E, into garage 30ma RCD incomer (with 16A radial circuit MCB and 6A light circuit MCB) and SWA back out to next shed. Same again for each shed.

The 3rd core of 10mm SWA used to export/bond earth (however no pipe work etc in sheds). I believe this is is the right size for TN-C-S if there is external metalwork (there isn't now, but you never know). As 10mm will be needed anyway for a 50m run with lighting based on 22A (I'll not use 22A, but just in case), I may as well run 3 core rather than have a pants TT earth.

One question: I believe I shouldn't use two of the same RCDs in series, hence my plan to use a plain MCB at the house from the unprotected (RCD) part of my consumer unit. I will have RCd protection in the garage CU, but the SWA cable will not be RCD protected. An alternative is to split with a Henley Block into a second CU in the house, with a 100ma RCD incomer which would discriminate. That could then feed the sheds/obsy as before. Is it worth protecting SWA in this way - any cut to the cable will automatically be earthed through the sheath anyway. Would that just be a waste of money (the parts are going to be expensive enough anyway)?

I know it's part P notifiable (new circuit [i believe outdoor work isn't anymore]) so I will be getting an electrician in to connect the circuit up. I'll dig the trenches, lay cable and wire up the sheds and pay the electrician to check that over too. Might have to get him to gland the SWA. I'll get one in before I start in case he/she (are there many female sparks yet) wants me to do it differently.

[Islander]

The main reason for not using 2 RCDs on the same circuit is one of discrimination - a fault on the shed circuit could trip the house RCD. It isn't necessary to protect the SWA with an RCD although one in the shed is a good idea.

Exporting a TN-C-S earth requires a 10mm2 cable so 10mm is the minimum spec for the SWA. 10mm2 is more than adequate for the current draw and length of cable. You could draw up to 45A on this.

If you're going to be doing the cable glands yourself then invest in some Pirhana nuts to replace the banjo earth connectors - they're much better. Do speak to your electrician before you do anything - they may want to see the cable in the trench before it's properly buried and they may also want to see any first fix wiring before any terminations are made.

[DrRobin]

Hi,

It all sounds very sensible to me. Without checking the 17th Edition it sounds like you are on the ball. I installed an external power supply to a distribution box on the rear of the house, to run outside lights and sockets, I fitted it with an RCD.

When I became interested in astronomy and we bought a summer house, I took a SWA cable from here out to the summer house and fitted a mini-distribution unit in the summer house with another RCD. I took the SWA from the protected side of the RCD in the distribution box on the rear of the house, so I actually have two RCDs protecting the summerhouse.

I agree that you wouldn't really want your main RCD tripping so it is probably best to feed your SWA from a spare MCB that isn't RCD protected. The reason I ended up using two is that is what I had already installed and at the end of the day there isn't any problem with having two. I also agree that the SWA doesn't need an RCD, just one on the end to protect the sockets.

The electrician who signs off the works might want to see the SWA cable for inspection, but equally he will run an insulation and earth loop impedance test so he might not really be all that bothered. Either way you are supposed to keep a drawing of the route of the cable and I photographed mine before filling in the trench.

You can do all the installation works, but if you wire the sockets, leave them open for inspection and of course don't make the final connection in to the consumer unit at the house end. The glands are straight forward enough to do, but again you can always leave the cable un-terminated for your electrician to do.

Best of luck digging your trench, when I started mine (600mm down) it took me several afternoons to go a short distance due to all the old roots and stone. I then borrowed a special trenching shovel and it was 3x easier, but there was less roots in the second half.

Robin