Judging imperial submain T&E cable

[Dartlec]

Doing an EICR on a top floor flat recently and run into an issue I often do - which is judging imperial sizes when they are larger than the standard 1/1.5/2.5mm equivalents.

In this case, the flat is fed by a Twin and Earth cable that is somewhere around the 10-16mm range equivalent, with an earth that appears to be around the 6mm equivalent, but I'm struggling to tie down exactly what it is to confirm suitability.

Pic attached is of the consumer unit - Circuit 1 is a 10mm t&e shower, The incoming earth is in the far right earth bar connection, next to 2 x 10mm bonding cables.

To my eye, I would put the earth at larger than 4mm, but smaller than 10mm (so 6mm approx). The places I've found that list what sizes were available in Twin and Earth don't quite seem to match up.

The two options for the Twin and Earth I've found in an internet search could be 7/.052 (9.35mm2) for the Live conductors, and 7/0.44 (6.45mm2) for the earth.

However, the Live conductors look a little larger than that, in which case they could be 7/.064 (14.52mm2) and the earth 7/0.52 (9.35mm)

Any experienced people out there who could judge which it is from the picture?

Either way, my calculations suggest the earth would still comply under the adiabatic equation even at 6mm2, though it obviously does have an impact on cable rating for the main incomers. (Currently from a BS88 Fuse isolator that says 60A, but actually has an 80A fuse in it)

Other than having a micrometer on hand to measure the diameter of an individual strand, is there a rule of thumb that works for old cables like this? Based on my experience there are still a lot of them out there in older houses or converted flats, and most of them still seem to have life left in them, unlike the nasty rubber stuff shudder

Any thoughts welcomed from the more experienced among you.

(For clarity, I've noted lots of other things on the EICR that are clear in this picture - the Europa MCB, the possible overload of RCD 2, etc - this post is really about the incomer, but feel free to comment on any other codes you see for interest)

 

[buzzlightyear]

1.0 = 44 thou = 1.12mm
1.5 = 54 thou = 1.37mm
2.5 = 70 thou = 1.78mm
4.0 = 34 thou = 0.86 mm (7 Strands)
6.0 = 41 thou = 1.04mm (7)
10.0 = 53 thou = 1.35mm (7)
16.0 = 67 thou = 1.7mm (7)
25.0 = 84 thou = 2.13mm (7)

looks to me a sub main is 16mm looking at the picture the only way is guessing .
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but feel free to comment on any other codes you see for interest)

yeah, tin of spaghetti comes to mind .

 

[7029 dave]

Hopefully it goes back to a 60 amp switchfuse.

 

[Julie.]

Why C type - what's it feeding?

 

[Dartlec]

Hopefully it goes back to a 60 amp switchfuse.

It goes back to an elderly switch fuse marked 60A, which helpfully contains a BS88 80A fuse. Just goes to show that believing what's marked on the outside is sometimes misleading....

I can change it to a 60A of course, but wanted to be sure the cable was adequate size for that before I went that route...
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1.0 = 44 thou = 1.12mm
1.5 = 54 thou = 1.37mm
2.5 = 70 thou = 1.78mm
4.0 = 34 thou = 0.86 mm (7 Strands)
6.0 = 41 thou = 1.04mm (7)
10.0 = 53 thou = 1.35mm (7)
16.0 = 67 thou = 1.7mm (7)
25.0 = 84 thou = 2.13mm (7)

looks to me a sub main is 16mm looking at the picture the only way is guessing .

Thanks - My view was that it would make sense that it was equivalent 16mm when it was put in, but it's difficult to judge when there is little to compare it to.

From what I've seen elsewhere, the twin and earth equivalent 7/0.64 would actually be 14.5mm, so should 15mm go in the box or the nearest equivalent modern option at 16?

Agreed with the spaghetti, but I've seen far worse
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Why C type - what's it feeding?

It's an electric shower, recently installed by another electrician (EIC supplied). No idea if he did calculations to consider C type, but more likely it was the one that was available off the shelf. It does comply with tripping requirements, so I didn't raise it as an issue on the EICR.

 

[Lucien Nunes]

My impression from the picture is that it is 7/.064, and I would say that this was a much more commonly used cable than 7/.052. One reason for that would be the rarity of final circuits over 30A in domestic work before electric showers became popular, making 7/.044 (34A) the largest T+E cable used in quantity. Another would be a historical preference for even-numbered SWG sizes (0.064" = 16SWG, 0.052" = 17SWG). 7/.064 a.k.a. 0.0225 sq. ins. was a popular incoming service cable size too.

 

[Mark Wright]

sorry am I being totally stupid again but I can only see one Red and Black cable and it doesn't look T&E ?

 

[telectrix]

i'd also go for 7/.064, andi hope that CU is isoated at source with the cover and busbar shroud off. ????

 

[Lucien Nunes]

it doesn't look T&E

Presumably the OP saw more of it than we can, in order to know for certain (and we know he's looked in the switchfuse at the other end.)

 

[Dartlec]

My impression from the picture is that it is 7/.064, and I would say that this was a much more commonly used cable than 7/.052. One reason for that would be the rarity of final circuits over 30A in domestic work before electric showers became popular, making 7/.044 (34A) the largest T+E cable used in quantity. Another would be a historical preference for even-numbered SWG sizes (0.064" = 16SWG, 0.052" = 17SWG). 7/.064 a.k.a. 0.0225 sq. ins. was a popular incoming service cable size too.

Thanks, that makes sense - and is exactly the sort of personal knowledge I was hoping for. For whatever reason, there seems to be very little info available on this sort of thing, when there must be a huge number of houses still wired (and working acceptably under testing) on such wiring, at least with sub mains etc.

It means the cable will likely not need to be replaced at least, though I'll need to change the switch fuse 80A HRC fuse for a 60A...
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i'd also go for 7/.064, andi hope that CU is isoated at source with the cover and busbar shroud off. ????

What busbar shroud? Not sure if these boards are supposed to have one, but it's often a luxury that isn't supplied in my experience, particularly in flat CUs. In this situation the flat was empty and the CU about 6 inches off the floor in a cupboard, so I have to admit that I didn't put the board cover back on every time while live testing.....

 

[telectrix]

What busbar shroud? Not sure if these boards are supposed to have one, but it's often a luxury that isn't supplied in my experience, particularly in flat CUs. In this situation the flat was empty and the CU about 6 inches off the floor in a cupboard, so I have to admit that I didn't put the board cover back on every time while live testing.....

let you off then . .

 

[Dartlec]

sorry am I being totally stupid again but I can only see one Red and Black cable and it doesn't look T&E ?

Sorry, the pic isn't that clear - the incoming Red and Black cables are indeed part of a supplying T&E - the earth can be seen going into the right hand earth bar terminal. If indeed the cable is 7/0.64 then that suggests the earth is 7/0.52, so equivalent about 9.4mm.

Just out of interest, this is the switch fuse on the other end. It's still not entirely clear, but exiting it is the other end of the twin and earth that ends up at the consumer unit. (And yes that adds a whole new can of worms, but not all relevant to the flat in question....)

 

[Lucien Nunes]

The line conductor insulation looks a bit melty, quite apart from the lack of anything around the outside of it.

 

[R-fur]

Don't know if this helps, it shows the relative relationship between metric and imperial cables. Sorry about the quality of the picture.

 

[Dartlec]

Don't know if this helps, it shows the relative relationship between metric and imperial cables. Sorry about the quality of the picture.

View attachment 59280

Thanks, that's actually a really useful way of showing it - will be saving that to my folder of useful info!

Seems like info that should be much more widely available, given how mujch imperial stuff is out there. I suspect a lot of EICRs are going to be incorrectly filled in over the next few years with the new letting requirements and people who don't bother to follow up when they run into something that is outside their experience.
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The line conductor insulation looks a bit melty, quite apart from the lack of anything around the outside of it.

Agreed - I don't think it's recent to be honest, and seems very stable now. Fortunately the landlords are in the process of obtaining the freehold for all 3 flats, and I'll be redoing the whole lot I imagine - the feed from the meter is the lovely braided stuff you can see in the foreground and the DNO's tails look undersized to me (though who knows if they will change it when asked...)

 

[ChipFanDave]

Whats powering the LHS RCD?

 

[Mark Wright]

when was imperial cable last in use?

 

[R-fur]

I think imperial cables were last made about 1970. When I started in 76 it was all metric and had been for a while.

 

[Mark Wright]

I think imperial cables were last made about 1970. When I started in 76 it was all metric and had been for a while.

Ok Thanks, When was the rubber insulated stuff last used?

 

[R-fur]

Rubber disappeared a bit earlier, you can get imperial PVC cables but no metric rubber as far as I know.
The give away is the conductors, tinned and stranded = imperial, solid with no tin = metric.
You also got Imperial PVC twin with no earth.

 

[7029 dave]

I think imperial cables were last made about 1970. When I started in 76 it was all metric and had been for a while.

It was about the same time as decimalisation.1970

 

[Mark Wright]

Thank you sirs.

 

[pc1966]

Whats powering the LHS RCD?

Was wondering the same - no sign of a cable into the RCD (or it has a painful bend radius we can't see).