Terminating SWA "early"

[Piers Finlayson]

I'm going to be installing a new sub-main using 25mm 3-core SWA. As SWA is an absolute pig to work with I'm probably going to need to terminate the SWA before it enters the final enclosure/CU.

At one end, this is no problem - I'm going to use a metal adaptable box, in an accessible location, in a safe zone (above the CU).

At the other end I have more of a challenge. I want to terminate the SWA at another metal adaptable box above the front door. I then want to run something slighty easier to deal with round a slightly tighter route. The will be visible (i.e surface mounted).

Does anyone have any useful suggestion about how I can do this? I can probably route each core within the SWA individually, but not the internal (i.e. non armoured sheath) 3 core together. Is it permitted to run the cores indivudally like this, or do I need to manually sheath each one? If so, any suggestions as to what I should use?

Thanks, Piers

[Andy Burns]

Could you switch to 25mm meter tails and a suitably sized earth cable?

[js.b1]

I presume you do need 25mm - re distance (voltage drop to lights) or current demands? You can not use the DNO main fuse to provide protection to your sub- main, which in turn means you do not have to size according to it - ie, if you want 16A at a shed 30m away then 6mm will be perfectly fine.

The cores in 25mm are shaped and the bending radius for 25mm SWA is "big" as you have probably found. There is also a min temperature for installation of SWA (and most cables for that matter).

You can not run bare cores except in say round flexible conduit (25mm or 32mm) or PVC trunking, which would need to be quite big anyway.

25mm H07RNF 3-core exists, is quite tough, waterproof, UV proof, very flexible. It is almost certainly smaller diameter than say 3 25mm cores when stuck in trunking or flexible conduit and somewhat more forgiving re route.

[Tim Watts]

For conduit - I would recommend Kopex pliable 32mm. It's all plastic (no earthing issues), bloody tough, doesn't look bad (black, smooth-ish

*and* smooth bored, could be painted) and the adaptors make off to IP65. The OP could certainly run the single insulated cores of SWA through that provided the conduit is surface or otherwise protected (usual IEE regs about buried non RCD protected cables).

I used some for my tails to protect them going through a roof void and down over the wall plate then into the meter box outside with a waterproof connection. Thoroughly met with approval from EDF when they put my cutout fuse back.

RS sell it.

You can thread a couple of 25mm2 (prob 3 at a pinch) through if it has mild bends, otherwise make it all up on the flat (straight) then install and form as a pre-assembled bundle.

[Tim Watts]

Here's some:

[js.b1]

Re photos, nicely done.

25mm is a big sub-main so I wonder if the OP is just going for too big a cable... unless it is very very long... in which case going outside the limit for lighting (say) is possible as a noted departure from BS7671.

25mm H07RNF is =A38.61/m ex VAT from one source, but I suspect a similar diameter (32mm).

[Tim Watts]

I probably wouldn't get too hung up on volt drops if the supply nominal is good to start with (like ~240V) - if the OP had (paid) the supplier to move the meter, they'd probably run a similar length of similar or smaller CSA concentric and achieve exactly the volt drop (but outside the remit of the IEE regs of course!). The point being symmetry and a degree of common sense. I'd aim to minimise the volt drops as far as practical though.

I would personally care that (if it is a long, eg >3m) run, that I had a switchfuse at the meter end, and the SWA was sufficient for carrying the earthing conductor *and* was capable of blowing the OP's fuse (switchfuse, not cutout) in the 5 seconds required for a distribution circuit - on both a L-E and L-N fault.

[Piers Finlayson]

In response to other posters responses (for which thanks):

I strictly don't need 25mm - I'm going for 3 core 25mm because - it has been suggested that the DNO might frown on 16mm should they ever see it (I am essentially extending the supply to a new location, and am aware that I need a CU within 2/3m of the meter) ? see

- I want a 16mm earth, and IIRC the amour of 25mm SWA is about 13mm copper CSA equiv ? so can?t use 2 core - it allows headroom for future expansion (i.e. moving up to a 100A supply), where 16mm is marginal.

I?ve even toyed with 4 core 25mm for possible future upgrade to a three phase. But not sure I need to plan for that much power!

My supply is actually at around 255V. Yes, I know this is over spec, and something I may get the DNO to sort in due course.

I like the meter tail idea, given I know I can work 25mm tails OK in the space I have. How would you recommend I connect the line and neutral within a small space? Is a Henley block a sensible option or are there better options using small space?

[Andy Burns]

I know this is 63A and you were planning ahead for 100A

it looks a *bit* more decor friendly than the typical red/yellow rotary isolators, don't know if rotary ones include earth block either.

[Tim Watts]

And if terminating SWA into a plastic enclosure, the OP would be advised to use these on the gland:

't trust nut tightness on a banjo tag when bolted though compressible plastic.

I'm sure the OP is also aware, but don't forget an external grade SWA gland for outside use (prob a good idea even in a meter box).

[Tim Watts]

your first fuse assuming it is fused correctly for the length/capacity etc. But they might show an interest in an undersized earth - I would personally use a 3rd core for earth here due to the onerous requirements on the main earthing conductor - parallel with the armour on both ends.

All sounds very sensible. I went through all the calculations when I was wondering whether to do something similar - in the end I compromised on the CU position and got it close enough to the meter to satisfy the 3m rule.

[Not and expert mode on...]

Big metal crimps and suitable over insulation in an enclosure?

Or

must admit - I'm not totally convince by those sleeves in the link, but Pratley are a good make which makes me think they must know what they are doing.

Another option could be to use a 100A DP main switch as a terminal mechanism - ie a tiny 4-way CU perhaps. Also, possibly some big bastard DIN terminals (try RS) in a similar enclosure for same without switch.

Not sure what I would use - those are just there for initial ideas...

[js.b1]

That referenced isolator is a european design and I have a suspicion it permits the removal of the lid in the "on" position which may contravene BS7671. Rotary isolators need to be the oversized enclosure type to handle SWA, Piranha nut a given in this situation with a plastic enclosure.

The pratley "grub-screw-in-a-hoop" work with top-hat work fine. Just tighten them with a decent allen-key, wait a while, then tighten again because stranded will loosen. Alternatively you can copper-tube crimp - but it needs a suitable crimp tool and matching crimps which for 25mm is not so cheap. A klauke k02 is about =A380 from Farnell and the matching butt-crimps are about =A312 (you have to use klauke, do not use any other because they WILL just pull out - UK uses mostly light- duty crimps compared to EU-land heavy duty). TLC will do matching crimps & tool, as well any local electrical factor (SWA is one respected brand, you do not have to go to Cembre although you may find City Electrical Factors will hire the tool out cheaply and might even sell a handful of butt-crimps).

Pratley above/under ground boxes use their own gland system, which can be expensive. I prefer a good CW gland or even E1W which guarantee water ingress into the enclosure can not go down the cable and rot the armour on what can be a very expensive cable if long (SiteBox does E1W, Bicon & CCG are two common makes). Two 20m runs of 25mm (Peak & E7) are getting on for =A3400 so the extra for better (E1W) glands is worth having if water is ever likely to penetrate an outside enclosure in a rickety garage or such like.

[js.b1]

Re sizing cable for 100A not 60A, well perhaps.

Remember a 60A BS1362 fuse will permit 1.6x its rating to be drawn continuously - that is essentially 100A and why an electric shower "and everything else" is often run on a 60A fuse without blowing it. Modern SWA uses XLPE insulation and can thus handle more current than PVC - although remember most wiring accessories are limited to 70oC which means derating to PVC. That is unless of course you use higher temperature wiring accessories re 90oC (which for a sub-main is quite possible).

[Piers Finlayson]

I have access to a heavy duty crimp tool (25mm - 200 odd mm) via work, but I prefer the idea of the pratley connectors rather than mess around with crimping. Thanks for the suggestions.

[Tim Watts]

That's useful to know. So the "top-hat" fits quite firmly and won't fall off? That was the only thing I was wondering about...