Cable that looks like SWA but maybe isn't

[Chris Green]

I am trying to identify the cable that feeds our house consumer unit. It was buried many years ago, before 1998 at least and quite likely some years before that.

It's not a 'company' cable, the company fuse is outside, it used to feed from an outdoor meter/company fuse, it now feeds (after a re-route under the drive) from meter and company fuse in the garage.

It might be pre-metric SWA, the outside diameter is just over 23mm and looks somewhat like black SWA. It's a three core cable with Red, Blue and Yellow insulated cores.

I'd basically like to know what size it is if possible.

I'm not *absolutely* sure that it's armoured cable, was 'non armoured' cable ever used?

[newshound]

A magnet will tell you if it is armoured.

[Harry Bloomfield, Esq.]

Chris Green formulated the question :

It could be lead/paper, which should be plumbed at the end and sealed in a compound box. It could be a tough PVC armoured cable without the steel armouring.

Why not post a photo somewhere, to help identify it?

[harry]

At one time "concentric" stranded cable was popular. Centre core is live. Outer ring is half earth and half neutral strands Neutral strands individually insulated.

Looks just like SWA externally.

[Chris Green]

It's not lead/paper, the individual leads that enter the CU are fairly normal looking PVC (I assume) insulated, Red, Blue, Yellow.

It certainly *could* be PVC only, that's sort of what I'd like to find out. I'll try the magnet idea that someone else suggested.

I'm not sure that that will help much, all you can see of the bit I have exposed (in a dry lined wall which is about to be removed) is a black plastic tube. I had originally thought it was a water pipe because, wierdly, it has plastic foam insulation wrapped around it! However all the plumbing is copper so I've decided the black plastic thing must be the incomer.

[Chris Green]

Absolutely right! Yes, it's SWA (well, a magnet sticks to it).

So, is it pre-metric then as the diameter doesn't seem to correspond with the sizes given for metric SWA. It measures, as I said, about

23mm. 16sq mm would be about 21.5mm and 25sq mm about 25.5mm.

[Chris Green]

By the way, I need to know the size of the cable because I'm moving the CU and will need to extend the incomer. I'll be crimping the add-on so need to get the correct sized tube crimps (I have a hydraulic crimp tool).

[Andy Burns]

Sounds like the sort of job where for a few extra pence, you'd do well to have the next sizes up and down on hand, just in case ...

[Harry Bloomfield, Esq.]

Chris Green submitted this idea :

What, you are adding an extra piece onto the existing bit?

Why not just get one piece long enough end to end, to avoid joints?

[John Rumm]

More commonly called "split concentric"

[John Rumm]

Depending on how much "just over" is, I would guess you either have:

From BS 6346:1997 "PVC Insulated armoured cables...", Table 9 Three core 600/1000V cables with stranded conductors:

Nominal cross section 16mm^2, Approx overall diameter 23.1mm

Of from BS5467 1997 "XLPE thermosetting armoured cables", Table 8 Three core stranded:

25mm^2, Approx overall diameter 23.6mm with an extruded bedding, or 23.2mm with a taped bedding.

[Chris Green]

:-), yes, that's probably the right approach. I guess it has to be close enough to either 16sqmm or 25sqmm to make a satisfactory crimp. The only trouble is that I'll need both 16sqmm and 25sqmm for the extension cable.

[Chris Green]

Because it's about 30 metres long and buried. These aren't meter tails. The meter is in the garage, with the company fuse and then there's a time delayed RCD (we're TT) feeding a henley block which, in turn feeds a 'garage' consumer unit and a switch fuse which goes into the underground cable to the house.

[Chris Green]

Brilliant, thanks John, I think it's nearer the second of those two so I think I have 25sqmm. I'd better get some 16sqmm tube crimps as well though, just in case, not much money. I need to check what cable I have, if any, though.

[Roger Hayter]

Personally, if that involved digging up concrete I might compromise and join it. Were actual copper sizes ever Imperial, or have they been metric ever since SWA was invented? If the wire is not metric then it would be difficult to crimp it to a metric one.

[Chris Green]

It's not under concrete but it is pretty deep under a large gravel drive and I'm not even sure of the exact route it takes. I know it's quite deep because 15 years or so ago when the supply company put the incoming supply underground they left a hole for me to join the feed to the house (which used to go to the 'meter on a pole' we had before) to a new cable run from the new meter and company fuse in the garage. If my memory serves me right it was a good meter underground. I did that join with a proper underground splicing kit but my memory doesn't extend to what size the cable was! :-)

I *might* use a henley block to do the join initially and then, having got full access to the cable I will know the size and can get the right size crimp tube and extra cable.

[ARW]

Where are you based Chris? I could call in and offer site advice depending where you are. I do travel the country quite a bit for work.