Twin Core and Earth....

[Derek Doormer]

Hi, Just about to re-wire the lighting circuit, with the Loop-in system, the cable from switch to light has the live switch (which I know is the black which usually has red tape attached) but ive also read of people using another colour (Yellow and Blue I think) and Ive also noticed Twin Red core. So whats the best combination to use?? I've also read that 1mm or 1.5mm cable is used, is it common practice to use 1.5mm for the circuit cable and

1mm for the switching cable? Thanks DD

[Andy Wade]

Most people use ordinary red & black for the switch 'drops', with, as you say, red marking. Sleeving is much better than tape by the way: easier to apply and doesn't fall off. T&E cable with two red cores is available but never seems to have gained widespread acceptance - probably because it sometimes helps to know which wire is the live feed and which is the switched live. (If one of the red cores had a thin white stripe or similar it might have caught on more.) There's also the argument about only needing one type of cable of course.

The other colours (yellow & blue) only come into play with 2-way or

2-way and intermediate switching, where you need 3-core & earth cable between the two-way ends (via any intermediate switches on the way). So far as BS 7671 is concerned all cores are red - i.e. should be marked with red sleeving. There's no fixed 'correct' use of the colours in the 3-core cable: I use red for the permanently live side, yellow between the commons of the two-way switches and blue for the switched live side, but many others seem to do it with the red & yellow the other way round.

You are now at liberty to use the new colours. The translation is as follows:

Red -----> Brown Yellow --> Black Blue ----> Grey Black ---> Blue

No, best to stick to one size throughout. 1mm^2 is almost always OK for

6A circuits. For 10A or 16A circuits you may need 1.5mm^2. Check your cable installation conditions and grouping factors against the tables in the OSG.

[Stefek Zaba]

Whatever you've got a 100m reel of. Most frequent is using the very same cable throughout the lighting circuit: so, red-n-black in the "old" style, brown-n-blue in the Modern colours. Best thing to identify the switched-live with is a bit of red or brown (old/new) sleeving, rather than tape - tape falls off, while sleeving stays on being trapped between the sheath and the connection. Twin red's OK (some pedants would argue it's "best"), but has the practical issue of needing testing to identify which core's which (or swapping the two wires round which you find your switch is "down" for "off"). The yellow-blue thing you're thinking of is 3core+E - yellow, blue, red in the Olde colours, brown, black, and grey in the New one - most often used domestically for 2- or

3-way switching, or for running both a permanent and a switched live to e.g. a bathroom fan: not the best of things to wire simple 1-way switch drops with.

No, again it's most simple to use the same cabling throughout. While

1mmsq is "nearly always" OK, 1.5mmsq has better "headroom" when it runs buried in or under insulation - as it often will in th'loft. The difference in cost is usually two thirds of sod all, so if you're buying fresh I'd go with the 1.5.

Stefek

[Andrew Gabriel]

Yellow and Blue are part of triple and earth, normally used for 2-way switching, but not necessarily exclusively so. The twin red is intended for the switch wire, but is rarely used in my experience. The red sleeving is supposed to be put over the black to indicate it isn't a neutral wire. Professional electricians seem to rarely bother with it, and I've seen it put on the wrong wires too often to take any notice of it myself.

I would suggest red and black T+E throughout with red sleeving (rather than tape) used on the black switched live in switch drops.

No. Normally you use the same for the whole circuit. Unless the circuit is abnormally long, 1mm is fine.

Of course, you can rewire in new colours now instead, but you should stick to one or the other.

[Dave Plowman (News)]

And that it's simply an extra reel of cable to lug around. Most domestic re-wires wouldn't use a full reel either which would make it out of favour for DIY.

I've got some, though. ;-)

[Derek Doormer]

Cheers look forward to attempting it, 1st inspection indicates it should be ok, as most cable are run through pretty big conduit and cables seem to move freely, what I cant understand tho is the bloke before rewired most the house (Even the lighting circuit) but the *ickhead rewired the lighting circuit in 2 core with no earth!!! Presumably because he had some lying around! The switching cable for two way switches is annoying, looks like I only need a few metres and buying that from one of the sheds will prolly work out more than getting a whole reel! Thanks for all the help, looks like new colours it is then, btw is all the new harmonised colours in grey sheathing? cheers. DD

[Lurch]

On Wed, 8 Dec 2004 19:48:36 -0000, "Derek Doormer" strung together this:

Or more likely because it was rewired befor lighting circuits actually had an earth.

You can use two runs of T&E and use 3 of the cores for the two ways instead of 3 core..

Yes, as is the old coloured cable! Only way to tell is if the cores are brown and blue as opposed to red and black. Some wholesalers book out the new stuff with a H suffix but that isn't always the case.

[Dave Plowman (News)]

If he did have some 'lying around' it still makes it so old it should be replaced. I've never seen PVC twin cable with no earth.

Are you *sure* it's not just had the earth cut off at termination points? You'd need to strip back to be certain...

[Stefek Zaba]

Or did it long enough ago that relying on the metallic conduit for earth continuity was still an Acceptable Method (or was harking back to the days when it was ;-)

[Ian Edwards]

Last available in the '60's? - I'm old enough to remember it. :-(

[Andy Wade]

It still is an Acceptable Method, provided that all joints are screwed (no slip couplers allowed) [543-03-06] and that the earth terminal of each wiring accessory is connected to an earth terminal in the associated mounting box via a separate wire [543-02-07].

[Dave Plowman (News)]

To reiterate. I've seem 3/0.29 PVC with no earth, but never 1 or 1.5mm.

[Dave Plowman (News)]

Yup. Should have said 1 or 1.5mm PVC twin cable with no earth. IMHO, TW&E

3/0.29 in PVC had become the norm long before metrication came along. Although many simply cut off the ECC.

[Lurch]

On Wed, 08 Dec 2004 23:24:05 +0000 (GMT), "Dave Plowman (News)" strung together this:

When you say reiterate do you mean it's already been 'iterated' somewhere, or have I missed something?

[Dave Plowman (News)]

Yup.

[Andrew Gabriel]

My parents house, which is somewhere in later half of 1950's, is all PVC T&E. The earths in the lighting circuits were not joined in the junction boxes though, they were all neatly brought around the outside of the boxes and very tightly twisted together in a 6" length. There are marks on the twisted bundles which look like it was done with some tool which did it tightly and crimped them, but without actually leaving any crimp on the bundle.

[Derek Doormer]

[Derek Doormer]

[Andy Wade]

Where the manufacturer provides an earth terminal to allow you to earth accessible metal parts then, yes, it's essential good practice to use it. This applies whether or not the incoming earth is provided by conduit.

[crb]

Dave Plowman (News) wrote: !

But see Andy Wade's response to another thread about the life of PVC T&E as follows:

"How about a technical justification for not replacing it? There's an interesting article on cable life on the IEE web site - here:

the life is highly temperature dependent (Arrhenius equation,

etc.) If you take the equation in the IEE paper, with the constants given for PVC, and massage it slightly to give the life directly in years you get:

life in years = exp(15028/T - 40.678)

where T is the absolute (kelvin) conductor temperature of the cable.

Substituting some temperatures:

- at 70 deg. C (343 K) the life is 22.6 years (as per the paper);

- at 60 deg. C the life is 84 years;

- at 40 deg. C the life is 1,498 years!

70 deg. C corresponds, in principle, to operation at the full rated current. Except for cables feeding space heating installations, domestic wiring will spend the vast majority of its life at little more than ambient temperature."

CRB