SWA or twin and earth overhead cable?

[tonetone]

Afternoon All

I'm doing quite a lot of notifiable wiring at home, and have LABC involved already. There's something I'm unsure of tho and I wonder if anyone can help (knowledge is good before I talk to them again...).

I am going to run an overhead cable to supply power to the garage. It's 5m from the house, and the back of the house is just over 3.5m high. The garage is about 2.5m high. I will use a catenary wire. The circuit will come from a dedicated MCB in the CU (brand new) and there will be an IP rated CU in the garage. I'm aiming for a 30amp supply, so will use 6mm cable.

The TLC website data section seems to suggest that non-protected cable (eg, twin and earth) is fine as long as it's 3.5m or more above ground ( http://www.tlc-direct.co.uk/Book/7.13.1.htm table 7.8 type 1).

Am I interpretting the regs correctly? Given the height, does the cable need to be armoured, or is it enough to twin and earth (in a plastic conduit to protect it from frost/UV). At the garage, I am going to erect a post to get the 3.5m height until I install a taller garage next year. Thus the cable will have to go down the post (in the plastic conduit still) to enter the building making it less than 3.5m high. Does this part of the install necessitate SWA?

The benefits of this are obvious, I won't have to deal with a 25m run of SWA for a 5m exterior overhead line! Routing the SWA through the house to the CU doesn't look like a nice job... On the other hand, it's going to be inspected so it has to be done within the regs.

Thanks in advance

Tony

 

[Adam_151]

I don't think you can really support conduit from catenary wire!

I'd go with either SWA or split-con because as you quite rightly point out, T&E isn't UV stable, inside parts inside the house and garage can be done with T&E though (and infact it'd be a very good idea to do so unless you have a metal clad CU to terminate SWA into)

[just gland it off into an adaptable box on an outside wall with a CW gland and bring in T&E from through the wall and the back of the box]

 

[tonetone]

Thanks for the reply.

On the weight issue, I think T+E and plastic conduit will weigh less than SWA. I'm working on the principle that the span is so short the weight won't be a problem.

Neither CU will be metalclad. As to exactly how to terminate SWA, I was going to work that out/take advice at the time, but obviously the armour can't enter a plastic CU so would need to be earthed near the CU where there's more space. Maybe a short run of SWA and T+E otherwise is better, tho I don't like jointed cables where it can theoretically be done in one piece...

Thanks

Tony

 

[Steve]

don't worry about weight. 5 metres is not far. a catenary wire will support SWA no problem over that distance. Or are we penny pinching? I would terminate the SWA inside the shed, with a metal adaptable box, and run a short length of plastic conduit onto the consumer unit. Just leave the SWA tails long in the conduit, saves un-neccesary joins.

 

[Adam_151]

I know what you mean about joints, but use a quality rachet crimper, ensure you can consistantly produce good joints by playing with a bit of scrap cable and you won't have any problems, actually you can just do that at the house end, and the garage end, put a metal adaptable box right next to the CU and take the cores right through.

You can terminate SWA into a plastic enclosure, but 6mm² into a thin plastic CU... the tail is going to wag the dog and its very inadvisable

 

[tonetone]

Thanks gents

Sounds like the way to go is SWA outdoor and in the garage, and twin and earth in the house, jointed using the adaptable box. I hadn't really considered how to get the tails out of the SWA and into the consumer unit, but that will solve the problem!

Cheers

Tony