Schockets and power to shed using 2.5mm NYY-J

Question

I need some advice about running power to flower beds and a shed using 2.5mm NYY-J cable and whether it complies with BS. 
 The cable will feed from the consumer unit using a 16A MCB to an outside connection box to feed both sides of plants (as diagram provided). 
 The cable size and MCB were chosen based my needs and the limited power to be drawnThe cable will not be buried but will run along the perimeter of my fence on top of a 20mm pipe (not plastic conduit) secured either with plastic or metal cable ties. 
 The pipe will secured the concrete fence posts150mm above the ground heavy-duty saddles. 
 Each bed will have a double socket IP only for LED. 
 The power to the shed be fed from second connection box to double socket. 
 I have posted this to an electrical forum, but people are discussing different things and not my specific question. 
 This is why tried the IET forum, so competent people/engineers can give me advice. If the above-mentioned setup sounds good or not, please reply. 
 
 Thank you advance.

Mike Mln · Accepted Answer

Hi, 
 Lets go through my interpretation . 
 The cable is none armoured but in a low mechanical impact area, but it is low level so might have fauna interactions, so would possibly need mechanical protection for installations outside (Section 522), the datasheet I found doesnt mention any ratings, you mention it running along the top of pipe, if it was inside Galv pipe then you could deem it suitable due to that being the protection (the protection would be for mechanical impact but also if there is possibility of fauna interaction, with it being at low level, could be assessed away if deemed unlikely enough). 
 If you did run it along the top of the galv pipe, that pipework would need to make sure it is properly bonded to your installation, if it isnt already. (Edited, see replies below) 
 SWA cable would deal with both issues. 
 As the setup is using, by the sounds of it, standard plugs I would personally want to have it designed to a standard radial final circuit (assumed that was the setup you were going for?) (Appendix 15, Fig 15B), which based on your measurements you would probably need to upgrade it to the 32A radial and 4mm2 that can cover up to 75m2, as the 20A version can only cover 50m2 (although the 2.5mm2(20A) could be argued to be suitable with the reduced expected loads), the MCB would need to be RCD protected (Section 411.3.3), if it isnt already. 
 I say I would prefer a normal socket installation, as I would be thinking this installation would be staying even after you leave the house, so would people have an expectation on the installation to behave as a normal socket installtion and a reduced MCB may cause nusiance tripping in this regard, if they overload your reduced MCB, by using multiple sockets in different ways to yourself. However if it wont be staying, then I would say as long as it is appropriately labelled, you could have a reduced MCB feed. 
 From my personal point of view and my interpretation of the regs, I think the over all idea is ok, it is just the details are a little off in my opinion or the validation of some choices not communicated from your post, I provided sections of the wiring regs that can be used for further reading on the points I was using. 
 Feel free to correct/dispute any interpretations you dont agree with 
 Mike

gkenyon · Answer

Manos said: NYY-J cable it's a very tough cable and Suitable for External and Direct Burial . 
 Not suitable for direct burial in installations to BS 7671 as it does not have an earthed armour ... see Regulation 522.8.10 of BS 7671. It may be buried if installed in a suitable underground conduit to an appropriate standard (such as BS EN 61386-24) that offers 'equivalent mechanical protection' (the latest draft for public comment of BS 7671 suggests a classification of N750 to BS EN 61386-24 offers sufficient protection).

gkenyon · Answer

Mike Mln said: The cable is none armoured but in a low mechanical impact area, but it is low level so might have fauna interactions, so would possibly need mechanical protection for installations outside (Section 522), the datasheet I found doesnt mention any ratings, you mention it running along the top of pipe 
 Agreed ... SWA would often be chosen for this purpose. 
 Mike Mln said: If you did run it along the top of the galv pipe, that pipework would need to make sure it is properly bonded to your installation, if it isnt already. 
 Why ? It would not be an exposed-conductive-part, and extraneous-conductive-parts only require bonding within buildings?

gkenyon · Answer

Manos said: When buried, they should be installed in a duct or conduit to protect them from mechanical damage. 
 When buried they shall be installed in a duct or conduit that has equivalent mechanical protection [to a cable with overall earthed metallic armour or screen suitable to act as a protective conductor] ... See Regulation 522.8.10 of BS 7671.

Mike Mln · Answer

gkenyon said: Why ? It would not be an exposed-conductive-part, and extraneous-conductive-parts only require bonding within buildings? 
 If it was running along the top of the galv pipe with non armoured I would then be treating that galv pipe as support medium (like tray), and since it has the potential to be live if the cable was damaged, I would as a preference want it bonded at some point, it would be a very fringe case for it to happen. 
 That was my thinking behind it.

gkenyon · Answer

Mike Mln said: If it was running along the top of the galv pipe with non armoured I would then be treating that galv pipe as support medium (like tray), and since it has the potential to be live if the cable was damaged, I would as a preference want it bonded at some point, it would be a very fringe case for it to happen. 
 That was my thinking behind it. 
 I wouldn't disagree with the sentiment. However, if sheathed cables are installed on tray, ladder or in trunking, the containment is not an exposed-conductive-part. Within buildings, it's usually a very good practice to bond metallic containment, and it provides advantages in terms of EMC as well. It's definitely not a requirement of BS 7671 though .. and there are cases where earthing/bonding something that's neither an extraneous-conductive-part nor an exposed-conductive-part can lead to dangerous touch-voltages and touch-currents even if there were no fault. These cases occur more frequently outdoors ... in many domestic properties, you'd be very unlucky to come across those situations.,

Mike Mln · Answer

Thank you for the information, I happily stand corrected.

gkenyon · Answer

Mike Mln said: Thank you for the information, I happily stand corrected. 
 No problem, as I said, many domestic situations should not be a problem ... I've worked on many jobs in major infrastructure, though, where you need to be careful about accidentally exporting earth potentials from one place to another. Season's greetings and have a very Happy New Year.

Mike Mln · Answer

Hi Manos, 
 Having it suit your needs, vs complying with the wiring regs, as it is becoming part of your installation, that is why some outdoor installations are glorified extention leads as they are outside of the regs and can just be unplugged when it comes to testing. 
 As I have little experience on testing installations, I cant really comment on how they would judge it. 
 Look at gkenyon' s comments around the bonding (he corrected my thoughts on the matter), as he mentions you wouldnt want to introduce touch voltages and currents to something that would normally be ok. 
 A steel flex would be more than enough mechanical protection. 
 Have a nice Christmas and New Year. 
 Mike

Manos · Answer

NYY-J us suitable for External fuxed installation and Direct Burial and is compliant with British standards. 
 BSEN/IEC 60332-3-24(c): Ensures fire performance standards 
 BS EN 60502-1: A standard that some NYY-J cables are manufactured to 
 BS7671: UK wiring regulations for burying cables in the ground 
 NYY-J cables are also compliant with other standards, including: VDE0295, IEC 60228, CENELEC HD383, and VDE0276. 
 NYY-J cables are PVC insulated and sheathed power cables that can be used for fixed installations in a variety of environments. They are resistant to moisture, UV exposure, and mechanical stress. NYY-J cables are suitable for indoor, outdoor, and underground use, but they should not be buried directly. When buried, they should be installed in a duct or conduit to protect them from mechanical damage.

Schockets and power to shed using 2.5mm NYY-J

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