UKH

I have a lamppost in the front garden that I'd like to reconnect and get working.

Currently it is hooked up with some ancient looking pyro cable that I would not think suitable for any modern installation.

I'm proposing to run a buried armoured cable from a fused spur (with isolation switch). I assume this is best done off an existing RCD-protected ring main circuit as opposed to a lighting circuit? If it was run off a lighting circuit then I assume this must have an RCD on it (either at the consumer unit or spur?

Is any of proposed work notifiable or should I just go at it?

It will be low voltage led anyway.  So why not run low voltage to it and you have no problems ?

I hadn't considered low voltage as an option. Any downsides - can I get 100W equivalent on 12 or 24V?

I want this system wired in properly rather than 'plugged in'. I assume I can get a 24 V transformer to run off a fused spur. Do I need RCD as the transformer effectively provides isolation does it not?

There are no theoretical downsides. The current will be low, so voltage drop not a problem.

Practically the market is catching up with led lights and there are limited products available for new uses.  Apart from strip lights, there are not many omni-directional 12V leds.  I found some :

https://www.amazon.co.uk/Medium-Voltage-Lighting-Interior-3000K-Pack/dp/B07...

I have bought 12V 5A laptop charger type power supplies for £10, or use an old one.  No RCD required. Completely isolated and safe.  One is running my pond fountain.

If your existing consumer unit is up to date with current regs, lighting should already be on an RCD. Adding a further RCD would be unnecessary and kinda goes against the principle of selectivity in that there's no certainty which device would trip if there's fault.

Adding a spur is not notifiable provided it's not in a special location (e.g. bathroom or room with a swimming pool should you be lucky enough to have one). Pre 2013, outdoor stuff was notifiable but I believe this is no longer the case as part P of building regs was simplified, so basically only stuff in a special location or involving messing with the consumer unit is notifiable.

If you're burying a cable, iirc BS7671 (wiring regs) gives some guidance about the depth but I think it's a bit vague (something about a suitable depth or something, so perhaps for a garden deeper than the blade a spade? Don't quote me on this...) It should be marked with tape buried shallower than the cable. If burying SWA (steel wire armoured) cable, it needs to be terminated properly i.e. the armour is earthed. Basically just use a proper copper gland for SWA cable rather than a plastic one and fit properly, lots of info on how to do this on the web.

Anything outside needs to be suitably IP rated and resistant to the environmental conditions (e.g. don't use twin and earth cable as the insulation will eventually break down from UV exposure).

Cable size should be appropriate for the load. There are cable size calculators on the web that will give you minimum cross section based on load, cable type, distance and installation method (i.e. direct buried in this case). For a LED light the load is pretty minimal so unlikely to be an issue.  Downstream of a fused spur you need 1.5mm cable as this is the minimum size that is rated to 13 amps i.e. the largest fuse you can use in the spur. I'm not sure you'd even be able to get SWA cable smaller than this. Upstream should be sized appropriate for the protection at the consumer unit, e.g. 2.5mm if it's a ring main on a 32amp MCB. Personally I've used a 1amp MCB fed from a fused spur with 0.75mm cable downstream for lighting circuits, as the MCB provides sufficient protection for the cable. I'm fairly sure this complies with BS7671 but I may well be corrected on this. 

Just as a disclaimer I used to work as an electrical engineer a long time ago. Wiring regs have been updated since I worked in the field (17th edition at the time, now on 18th) and I didn't work on domestic stuff so not professionally familiar with all the applicable bits of the wiring regs or building regs beyond satisfying myself I'm not breaking the law when doing DIY stuff at home, plus I was never qualified to sign anything off as being compliant anyway. Hopefully someone more current and qualified will come along. Worth keeping in mind that getting electrics wrong can be fatal, and advice from strangers on the internet might not be any good...

Thanks for the input. I'm an electronic engineer (by training) so feel able to filter good quality responses (like yours) from bad!

Quick question on that if I may? I have an old SWA feed from the consumer unit (separate MCB with RCD protection) to a smaller consumer unit in a shed, where it splits to feed lights and sockets.

It's correctly terminated in the house (SWA to gland & tag to earth bar), but whoever installed it has just chopped the SWA sheath inside the shed and just connected the inner earth cable to the smaller consumer unit.

The SWA is earthed, but I wonder if I should connect it to the consumer unit earth, just to have an additional earth route for the shed sockets and lights?

I'm afraid I don't know the correct answer to this. 

Using a proper termination would seem 'better'...

I would say make sure the SWA is anchored/clamped properly at the shed CU. Obviously an appropriate gland would achieve this.

Thanks. Think I'll properly terminate the SWA in a separate box in the shed next to the consumer unit, then 2.5 T&E plus a separate earthwire for the SWA to the shed consumer unit.

I think it's probably ok as it is, but would look neater.

I'm assuming you don't mean a council owned lamp post...

Did something similar when we put a water feature in the new garden.

Kitchen ring has a spur that goes out to provide power to the garage, with a handy junction box on the back wall. Replaced that with an IP66 fused spur to the water feature (below). Shouldn't really need the RCD, but I didn't want any failure with the water feature to kill the fridge/freezer if we were away (garage bypasses this fuse).

https://www.screwfix.com/p/british-general-ip66-13a-weatherproof-outdoor-sw...

Steel wire armoured cable then runs inside some thick plastic pipe under the new patio, so no chance of putting a spade through it - the trunking is mainly there so that the cable won't be stuck and I can pull it through if I need to replace it, without having to dig up the patio. That then connects to an IP66 switch (below) to connect to the water feature cable, which is standard non-armoured stuff.

https://www.screwfix.com/p/lap-ip66-10ax-1-gang-2-way-weatherproof-outdoor-...

This switch is a bit overkill, but as it's in the corner of the flower bed, I liked the rubber cover over it, which keeps the muck out of the actual switch.

I looked at the notification requirements, but decided they didn't apply and I'm confident enough in my wiring skills to not need a pro to check it.

Have you two RCDs on the circuit then (one on consumer unit, one on the fused spur)? As pointed out above you cannot guarantee which will trip first in this scenario.

If you're making an addition to an existing circuit it's not notifiable unless it's a special location, which the garden of your home is not.

RCD not strictly necessary as you're not supplying a socket outlet, but I treat it as good practice for anything outside. As others have said, RCD not necessary for ELV.

Jim (electrician).

PS don't knock old pyro, if it's mineral insulated and the glands are in good nick then it's got surprising longevity. Not designed for direct burial in the ground mind you.

Actually, I'm talking arse. Outdoor lighting counts as a special location and is therefore notifiable, unless it's fixed to the outside of a building and supplied directly from the internal wiring. 

Ok that's interesting . This is a free standing 'vintage looking' lamp post in the front garden.

Sounds like a low voltage approach, with transformer hard-wired in might be easiest solution all round (assuming I find find a bulb with a traditional style with high enough output).