Electrics for Shed

Depends on the RCD, but if it's a standard 30mA RCD then it could be one or other or both to trip. If you went with a 10mA RCD then you *might* get that tripping first, but if the leakage is >30mA then no guarantees that one will trip faster.

I suppose the socket is to make it a 'big extension lead', but why not hardwire after a double-pole switched FCU? If you definitely want the RCD you can get FCUs with integral RCD.

Be aware that some plugin home RCDs are not rated for 24/7 use - I had one that overheated and cracked its case when left plugged in with no load.

You should assume that anything in duct is going to be sitting in water, and potentially also exposed to UV at the ends. I'd use an outdoor rated cable

- 'hi tuff' / 'tuff sheath' / NYY-J cable is outdoor rated and more flexible than SWA where you don't need the armour (NYY-J can be rigid or stranded in larger sizes, 'tuff' is stranded).

Depends on your earthing system (TT, TN-S or TN-C-S) and whether there are extraneous parts in the outbuilding (pipes, metal structure). JW has a video on the topic:

The 'proper' way to do it would be an extra circuit in the consumer unit. I understand wanting to DIY and not involve an electrician to add the circuit, and there might be complications with that (eg cable runs or lack of spare ways) but I'd think about how much extra work it would be to do it 'properly'. Or how to DIY most of it and then get an electrician in to 'upgrade' the last bit later.

Theo

Yes, but possibly sub optimal...

No - generally you get no discrimination between RCDs - both devices will see the full amount of leakage (something you can't control or limit), and so either or both could trip.

To properly cascade RCDs you would need the upstream one to be of a time delayed type.

You can use T&E if it is protected in conduit / ducting etc.

For the application you describe, and the distances involved, then you could safely export the earth unless there is some extenuating circumstance that would make that unwise. See:

If you already have an external socket fed from the kitchen ring circuit, and that circuit already has RCD protection at the CU, then you don't need another RCD.

The only real compromise is that a RCD trip will loose power and lights in the shed, but since you are not using it as a workshop, that is not really of much concern (fitting a maintained emergency light would fix that if it were).

In many cases you can use an external socket as a handy place to tap off a feed to something else (and if you were using SWA, it is also a good place to attach it and allow it to be turned 90 degrees to enter a wall)

e.g.

This SWA is a feed into an outbuilding, and the double socket is used as a way of having something solid to make the gland connection to. However it also then feeds into the building through the back of that socket.

While I have done full sub main installs many times for outbuilding feeds, for the cited application (lighting and the occasional appliance), it would be overkill IMHO).

I was thinking of 'scope creep'. Now you have power in the shed, it would be really handy if you could run a kettle and perhaps a tumble dryer in there...

ie it may help to think about how you might do an upgraded installation even if you don't actually do it, and which pieces are worth investing in to make that possible. eg if you're currently digging a trench, a few quid to upgrade the cable to something bigger could cover potential future uses even if you don't actually do the works to enable that now. Do the thinking now, even if you choose not to act upon it.

As against sizing everything to be the minimum needed for now and have to do more work to enable that later.

Really depends on whether there's any chances of needs changing over time.

Theo

You might find this of interest, particularly with regard to depth requirements and warning tape labels:

The kettle would be fine anyway (short duration load), even the TD would likely be ok depending on how heavy the load on the kitchen ring.

If you were adding a chest freezer, however, then more care making sure it can't nuisance trip would be worth the effort.

However with feature creep it is very easy to go from some T&E in an existing duct, a bit of conduit a socket light fitting and light switch to a dedicated circuit from CU, SWA sub main, satellite split load CU in the shed, all RCBO install in the shed etc. (says the man who installed a dedicated CU in the house to feed all the external circuits and downstream CUs!)

Yup certainly - if you need to dig a trench, that will represent most of the effort, so using a larger cable than required now is only a marginal extra cost now, but could be more significant later.

You could also take the view that "good enough" right now is so cheap and easy and the sunk cost is so low, that doing it "properly" later is not going to be significantly disadvantaged that you will care about doing it "twice"

Thanks a lot Theo - comments inset

Understood

Good heads up on the home RCDs - thanks

Will look into NYY-J tuff

That's a really informative video. I have the TN-C-S installation. The shed will have no other services, e. g. gas/water, and will not contain steel framing, therefore no extraneous parts according to JW.

My understanding therefore is that a 2.5mm2 8m radial spur from the kitchen ring, incorporating a single way FCU/RCD would be appropriate (routed through duct as necessary).

Unfortunately you are right. The CU is the other end of the house entirely. Would probably need 25m surface run cable round half the house perimeter.

Phil

Thanks John - comments inset

OK

Understood

I like Theo's recommendation of the tuff cable, but haven't checked prices yet!

That seemed to be well covered in the link to JW that Theo posted. No extraneous services or metal framing, so increased earth size not required I believe.

The only issue there might be that the existing external socket is fed from a 13A fused spur, so that feed wouldn't support shed and garden power simultaneously (although that scenario is unlikely).

Agreed

Hmmm - interesting

Thanks for your insight again John

Thanks Jeff - nice and clear with pictures - just my level :)