16mm SWA

[Barney12]

Hi All

 

So I’m busy planning the final aspects of the landscaping plan. I need to get power to the bottom of the plot for a garden room /gym. It’s not going in straight away but now is the only chance to get services down there due to other aspects of the landscaping. 

 

The run is 100m so my calcs show that I need 10mm SWA but I’d rather put 16mm in as there is a chance a hot tub might go in. 

 

Whats worrying me me is just how difficult is 16mm to handle manually? It will just get dumped on site on a pallet. Moving it on strops with the digger will be easy enough but there ain’t no way it’s going to roll off the drum  

 

Has anyone had the pleasure of manhandling 16mm SWA?

[dpmiller]

what about split concentric in a duct?

[Nickfromwales]

I’ve just had 47m of 25.0mm2 3-core dropped off on site and they put it onto a wooden drum for delivery. 

It was easily put upright and rolled downhill to the rear of the build by myself without any hint of breaking into a sweat. 

100m with a hot tub should be fine at 16.0mm2, and at most you’ll need a second pair of hands to roll it down your site. 

Not anywhere near the issue we had moving your Sunamps lol. 

You'll have no probs at all, as long as it doesn’t gather speed and Foxtrot Oscar down the valley

[Nickfromwales]

1 minute ago, dpmiller said:

what about split concentric in a duct?

Explanation please? Not heard of that phrase before tbh. 

[Nick]

Done a small run of what I recall being 16mm SWA, 3-Core. 100m should only be around 100KG and comes off the coil easy enough.

[ProDave]

I would be using 2 core SWA and putting a rod in for the earth that far from the house.

[Jeremy Harris]

I pulled 3 core 25mm² SWA through the duct from our meter box outside, under the slab and up to our first floor OK on my own, with no real problems.

 

I'd also go with @ProDave's suggestion, it's what I did with my workshop, which is only around 40m or so from the meter box - I didn't like the idea of exporting the earth that far.

[dpmiller]

30 minutes ago, Nickfromwales said:

Explanation please? Not heard of that phrase before tbh. 

Split concentric is used for submains and incomers, it carries neutral and earth *around* the live conductor

like this:http://www.doncastercables.com/cables/6/28/SWA-and-Mains-Distribution/PVC-Insulated-Split-Concentric-Cables/

 

 

[Barney12]

Thanks guys useful info. 

It will certainly pick up considerable speed rolling down my garden?

[Barney12]

1 hour ago, Nickfromwales said:

Not anywhere near the issue we had moving your Sunamps lol. 

 

Ive not been able to bloody sweep my floor since you cut my broom handles into rollers! ?

 

[ProDave]

1 hour ago, dpmiller said:

Split concentric is used for submains and incomers, it carries neutral and earth *around* the live conductor

like this:http://www.doncastercables.com/cables/6/28/SWA-and-Mains-Distribution/PVC-Insulated-Split-Concentric-Cables/

 

 

But is a grey area not formally being recognised in the wiring regs and not recommended for direct burial (though the DNO do sometimes use it as such)

[scottishjohn]

3 hours ago, Barney12 said:

Hi All

 

So I’m busy planning the final aspects of the landscaping plan. I need to get power to the bottom of the plot for a garden room /gym. It’s not going in straight away but now is the only chance to get services down there due to other aspects of the landscaping. 

 

The run is 100m so my calcs show that I need 10mm SWA but I’d rather put 16mm in as there is a chance a hot tub might go in. 

 

Whats worrying me me is just how difficult is 16mm to handle manually? It will just get dumped on site on a pallet. Moving it on strops with the digger will be easy enough but there ain’t no way it’s going to roll off the drum  

 

Has anyone had the pleasure of manhandling 16mm SWA?

you sure 10mm is big enough ?

what amperage you allowing for with hot tub 

32amp would make 16mm minimum size for voltage drop.  according to my calcs,not oversize at all

suggest you double check to be sure 

earth should not be a problem if you use correct gland packs  as you can also connect the armouring to the earth circuit at each end 

Edited April 23, 2019 by scottishjohn

[dpmiller]

8 hours ago, ProDave said:

But is a grey area not formally being recognised in the wiring regs and not recommended for direct burial (though the DNO do sometimes use it as such)

Grey indeed, especially as the linked one says

" These cables are designed to be installed in air, or for burial in free draining soil conditions "

[Onoff]

2-core 25mm is about the same weight as 3-core 16mm...put the garden room on its own TT earth as above I would. 

 

The 16mm will allow just under 40A. 

[ProDave]

2 hours ago, dpmiller said:

Grey indeed, especially as the linked one says

" These cables are designed to be installed in air, or for burial in free draining soil conditions "

The argument is the E and N "outer" are usually copper on split con, and as N is classed as a "live" conductor, there is no steel armour protecting it, so it's not considered safe in the same way SWA is.

 

Termination is always a b it messy, it does not just go into a nice gland like SWA does.

 

That doesn't seem to stop people using it, but I would not like to be the one justifying why it was used when someone has just dug through it........

[dpmiller]

Agree completely Dave. Except for the termination bit, I've just done mine and it looks pretty smart with it's snazzy heatshrink triple boot.

[Jeremy Harris]

The direct burial thing is just another difference between the regs that apply to electricians (BS7671) and those that apply to the DNOs, AFAIK.  The DNOs don't have to comply with the same physical protection requirements as anyone else, or at least their interpretation of the level of physical protection required is different.  BS7671 isn't very clear about levels of protection, either, which has led to people erring on the side of caution, which has now become custom and practice. 

 

If anyone did want to directly bury concentric, rather than run SWA, then there'd always the get out that if the cable manufacturer specifies it's suitable then that should trump whatever is in the regs.  I remember having a similar argument with an electrician years ago over the use of NYY-J, supplying power to a garden shed, that was clipped directly to a garden wall.  He argued it needed to be SWA, I argued that, as NYY-J was specified by the manufacturer as being suitable for use outdoors, or underground come to that, it was fine.   Interestingly, it seems that NYY-J is now the standard cable used for car charge point installations.

 

I was tempted to use 25mm² split concentric instead of SWA for the run from our meter box into the house, as it's smaller and more flexible.  That run is well protected, as it's inside a duct that's buried underneath a path and the house slab.  In my view the added protection of SWA wasn't really needed, but in the interest of not having something that might be considered contentious I ended up just using SWA.

 

.

[joe90]

I accidentally hit my incoming concentric cable with my JCB when digging, I didn’t “cut” it but folded it tightly, the outer plastic covering was not damaged but it still tripped the local supply. When the DNO arrived they confirmed the cable damaged and jointed a new section. They were great, very understanding and seeing me struggle to self build on my own reported the fault as somewhere else and I never got a bill ?. I cut open the “folded” section and the inner insulation had been split by the folding and shorted with the outer. I am surprised this could happen with a bona Fidi buried cable?. The SWA I installed from meter to house was far more substantial.

[Barney12]

Spoke to my tame sparky today. He’s recommending three core 16mm SWA plus the earth rod.

I think I followed his technical description in that the earth would be connected at the supply end only to provide earth protection to the cable but then the rod for earthing at the load side. 

[Jeremy Harris]

9 hours ago, Barney12 said:

Spoke to my tame sparky today. He’s recommending three core 16mm SWA plus the earth rod.

I think I followed his technical description in that the earth would be connected at the supply end only to provide earth protection to the cable but then the rod for earthing at the load side. 

 

If doing this then there's no point in running three core, as the PE conductor in the core won't be doing anything.  All that's needed is two core, with the armour connected only at the supply end.

[ProDave]

^^ I was just about to type what @JSHarris has just said. Don't waste money on 3 core.

[Ed Davies]

Would there be any actual harm in connecting the armour to the earth at the garden room end, as well? Just thinking of the practicality of the normal SWA termination connecting the armour to the box. If that's metal…

[Onoff]

7 minutes ago, Ed Davies said:

Would there be any actual harm in connecting the armour to the earth at the garden room end, as well? Just thinking of the practicality of the normal SWA termination connecting the armour to the box. If that's metal…

 

Potentially yes. A lot depends on the source earthing system too, especially PME. This imo is quite a good read:

 

https://professional-electrician.com/technical/stroma-certification-supply-chain/

 

 

[Jeremy Harris]

There's no reason why SWA cannot be terminated with a plastic cable gland when the armour should not be connected to the local earth.  Just trim the armour back to level with the outer sheath and secure that in the cable gland, with the double insulated inner core coming through to wherever the line and neutral connections need to be made off.

[Ed Davies]

Thanks both, but what would the actual harm be? For example, @Onoff's reference says:

 

Quote

If the remote building has a wooden floor and there are extraneous conductive parts inside the building, and subject to any other risks present, it may be acceptable to use the exported PME earth, providing the extraneous conductive parts are bonded to a Building Earth Marshalling Terminal (BEMT) inside the remote building. In this case, the distribution circuit CPC will also have to act as a bonding conductor and the CSA must comply with Table 54.8 of BS7671 in addition to meeting the CSA requirement for a CPC.

 

So, what's the difference between using a TT earth which just happens to also be connected to an exported PME earth and using a PME earth which is connected to an extraneous conductive part which maybe has just as low an impedance to earth as the TT's rod (assuming the CSA requirements are met)?