Wiring between outbuildings - SWA needed if run in ducts?

[aesmith]

Hi,

I need to run power to our outbuildings. My question is this, if I install duct for the sections between buildings, can I get away with running normal cable, or does it still need to be SWA?

Thanks, Tony S

 

[ban-all-sheds]

What sort of ducting? Would it provide equivalent mechanical protection to steel armour?

Why don't you want to use armoured cable?

Why aren't you asking your electrician?

 

[aesmith]

Thanks for the response.

What sort of ducting?

I was thinking of something like 32mm "Polyduct", which I can source in short lengths.

Would it provide equivalent mechanical protection to steel armour

Effectively, yes, because I can route the crossings where the duct would not be vulnerable to damage and if necessary I could provide additional protection to the duct itself.

Why don't you want to use armoured cable?

I would prefer to avoid the cost and hassle, as these outdoor sections will be quite short in comparison to the overall lengths needed. The buildings are close together but not quite adjoining, so most of the cable route can be internal with only short crossings between buildings. One section of maybe 5 feet between buildings, and one maybe 10 feet.

Why aren't you asking your electrician?

At this stage I'm hoping to do these works myself but I don't have the design finalised so that's not set in stone because it's possible I'm going to find something I'm not confident with. Even if I do end up getting an electrician to do more of the work, I prefer to understand what's being installed and why, to make sure it's all suitable for my requirements.

Going back to my original question it sounds from what you say that there's nothing in principle wrong with using normal twin and earth between buildings, as long as it's sufficiently protected. Would that be a fair assessment?

Tony S

 

[rjm2k]

What's the total length? 25m of 2.5mm SWA comes in at around £42 quid, does it really make sense to introduce uneccessary junctions (potential weak spots) and a dependency on outbuildings that you may one day decide you don't want, just to save a few quid?

 

[JohnW2]

What's the total length? 25m of 2.5mm SWA comes in at around £42 quid, does it really make sense to introduce uneccessary junctions (potential weak spots) and a dependency on outbuildings that you may one day decide you don't want, just to save a few quid?

I read the OP as indicating that the purpose of this cable was to supply power to each of these outbuildings - in which case the 'junctions' obviously would not be 'unnecessary'!

Kind Regards, John

 

[aesmith]

What's the total length? 25m of 2.5mm SWA comes in at around £42 quid, does it really make sense to introduce uneccessary junctions (potential weak spots) and a dependency on outbuildings that you may one day decide you don't want, just to save a few quid?

I read the OP as indicating that the purpose of this cable was to supply power to each of these outbuildings - in which case the 'junctions' obviously would not be 'unnecessary'!

Yes, and no I suppose. I would prefer not to use junctions, so if it needs to be SWA it will be end-to-end, or if I don't then it will be twin and earth end-to-end but protected by ducts for the external parts of the routes.

To explain, I have a decent supply in one building, and I'm now planning to provide power to two others. One just needs a couple of power points, and maybe a light. The other will be a workshop area and will need proper lights and a number of sockets. I was planning to run two separate feeds from the existing supply, not chain the two buildings one after the other.

So we could be talking about one run of 2.5mm from a 16A RCBO for the first named building, total distance of around 25m of which 3m is external/underground. The workshop is further, maybe as much as 50m with two external/underground sections one of 1.5m and one 3m. That will need to be 6 or 10mm cable from a 40A RCBO. Cable sizes will be determined by voltage drop requirements and can only be finalised once the routes and the lengths are known.

Tony S

 

[aesmith]

One other question, why does the voltage drop differ for 2 core or 3 core SWA?

 

[JohnW2]

One other question, why does the voltage drop differ for 2 core or 3 core SWA?

Probably because you've used a calculator which presumes that 3-core means 3-phase (with balanced 3-phase load - hence no current, and no VD, in the neutral).

Kind Regards, John

 

[aesmith]

One other question, why does the voltage drop differ for 2 core or 3 core SWA?

Probably because you've used a calculator which presumes that 3-core means 3-phase (with balanced 3-phase load - hence no current, and no VD, in the neutral).

Kind Regards, John

Of course .. I was using this ..
http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

 

[JohnW2]

One other question, why does the voltage drop differ for 2 core or 3 core SWA?

Probably because you've used a calculator which presumes that 3-core means 3-phase (with balanced 3-phase load - hence no current, and no VD, in the neutral).

Of course .. I was using this .. http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html[/QUOTE]
Indeed. As BAS is always telling people, if you have single-phase circuits, you have to tell that calculator that it's 2-core, even if you're using 3 core, otherwise it assumes balanced-load 3-phase.

Kind Regards, John

 

[aesmith]

Any more comments on the suitability of PVC twin and earth (suitably protected) vs any requirement to use SWA? I'm interpreting the first response, from BAS, as suggesting that twin and earth would be OK, but the onus would be on me to make sure its mechanical protection is at least equivalent to wire armour.

Thanks, Tony S.

 

[ban-all-sheds]

Yup, and you'd have to check that plastic ducting does.

Also you'll need to find an appropriate installation method to use in your design calculations, and ensure that the duct remains water-free, as twin & earth is not suitable for immersion.

I can't think of one good reason not to use SWA....

 

[ban-all-sheds]

if you have single-phase circuits, you have to tell that calculator that it's 2-core, even if you're using 3 core, otherwise it assumes balanced-load 3-phase.

And tell it PVC, even if you'll be using XLPE, or it will assume you can take the cable up to 90°C.

 

[aesmith]

...I can't think of one good reason not to use SWA....

Actually I'm swithering about whether SWA might be best after all for the larger and longer cable because I might be able to take a shorter overall route, at the cost of a lot of excavation though. I need to have a good walk round at the weekend and check out all possible routes, and also decide the best place to terminate in the workshop.

Good point about the installation method, I think I should take the worst case of the reference methods without insulation. I suspect (without having calculated yet) that voltage drop will be the limiting factor rather than cable rating.

Tony S

 

[aesmith]

if you have single-phase circuits, you have to tell that calculator that it's 2-core, even if you're using 3 core, otherwise it assumes balanced-load 3-phase.

And tell it PVC, even if you'll be using XLPE, or it will assume you can take the cable up to 90°C.

It also quotes different voltage drop for PVC vs XLPE.