Thanks all for the thoughts overall I agree that SWA is probably the better solution. I was looking at other options as it's a TT earth and from my quick look at a very messy incoming supply area I think there is a reasonable chance that non of the submains had RCD protection. There were 10 to 15 enclosures, most with metal lid's that I couldn't access easily on the initial visit. I didn't see anything that was obviously an RCD.

Owner agrees it all needs rework, but wanted that as a future project, hence considering a sub main cable that I could use without RCD protection, but maybe the steel catenary means I need RCD protection anyway. Life would have been much easier if the customer hadn't cut the existing twin + earth, I would have been comfortable continuing to use it with a  comment on the certificate. Now whatever happens we are looking at having to get something like a HIAB in to enable replacement of the cable.

Clearly need to go back to site to look a bit more closely at the incoming area and many other things.

if you have to hire a cherry picker to replace it you might be better abandoning the catenary and using split concentric which can self support over reasonable spans. There are a few threads on here that discuss the limitations of this cable but its lightweight and relatively inexpensive.  If you do go this way get a couple of preform wire 'dead ends' to support it.

Thanks for the idea. The span is in the region of 20 meters, don't see avoiding the catenary as an option.

20m is quite a span unless the cable is quite chunky- what copper cross-section would you be looking at ?

Mike

20m is quite a span unless the cable is quite chunky- what copper cross-section would you be looking at ?

Mike

Not quite sure at the moment, trying to pin customer down on max demand. If the total cable run is 25m with 60 amps, looks like I can get away with a 10mm cable, which gives a 3.07% voltage drop. Clearly need to do some actual measurements on length of run. Given the number of other buildings running on a 100 amp single phase supply even this is pushing our luck with the main supply fuse.

Sadly most of the manufacturers of split con dont list span data anymore but this WPD document OH4K_1-2.pdf says single phase services up to 30m are OK for concentric.  They are probably talking Al central core.  I think (and it was some time ago ) that 16mm2 was OK up to 23m with a suitable sag.  If you are stuck you could ask the manufacturers they will still have the data somewhere.

The maximum unsupported span of a cable depends to a degree on its cross-sectional area, although annex D of the OSG gives the single figure of 3m,
This is very conservative and can safely be exceeded by a significant margin for higher cross-section cables up to about 20m for 6mm2. However, it is critical that the cable is not over-tensioned if the copper cores are the only strength. The tension rises to infinity if one attempts to pull the cable straight, and the exact tension can be solved by assuming it forms a coshine (hyperbolic cosine) curve, or to with about 20% by pretending it is a simple  triangle of forces with all the weight of the cable in the centre. (https://www.youtube.com/watch?app=desktop&v=OBoPpTExyBI )

In practice to be safely self supporting over more than about 10m often requires levels of central droop of about 75cm to 1m and that can give problems with clearance for vehicles beneath etc, and then a steel tension wire is preferred anyway to allow a more level run.

Even so be careful what you attach the end anchors on to -  it can be  embarrassing if it pulls out part of the top course of bricks, or removes a soffit board from the gable ends or something ;-)

Mike.

(the breaking strain of soft copper wire is about 200N/mm2 (20kg force per square mm )but outdoors stay well below  20% of that after allowing for a 2mm layer of ice all-round and a 100mph wind....)