1mm Twin earth cable query

I'm currently redoing my kitchen where i'm at the stage of putting a new ceiling using timber and plasterboard.
I'm not a qualified electrician but have done some wiring in the past, Before I put up the plasterboard i'm feeding a 1mm twin earth cable to the otherside of the kitchen to be used for the kitchen wall unit under lighting. The red 1mm cable has been fed through to the switch (via flush conduit) that will be used for the wall unit lights and i'm going to connect a live cable from the main ceiling light switch to the wall unit light switch.
My question is, if the current wiring is 1.5mm can I use the 1mm wire to connect to the wall unit switch and a 1mm live wire to feed off the current 1.5mm cable?
Am also looking at wiring the ceiling downlights with 1mm cable to be fed off the current 1.5mm cabling.
Can this be done or will there be an overload?
TIA
Pedge
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Assuming you have the standard 6 amp MCB on the lighting circuit, 1mm will be fine.
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*I believe five out of four people have trouble with fractions. *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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this:

TBH, most people that say this shouldn't be doing any electrical work at all.

Obviously the basic understanding of electrical theory has escaped you. Please let someone else do it.
--

SJW
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strung together

Whilst I understand your sentiments which are often well applied, I think you are bring a bit over-zealous here. The OP asked a reasonable question which Dave confirmed was okay.
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together this:

Probably.
The 'single red to the switch' bit got the alarm bells ringing, (coupled with the statement I quoted in my first post).
I could have probably worded it better, but I'm a little dubious about the OP's competence. Sorry if not everyone agrees with me.

Well, Dave answered the question, but did he really........
(BTW, I'm trying to word this post without sounding too agressive, but it's not working so I'd like to point out I'm not having a go at you, just explaining my reasoning).
--

SJW
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Then there's no point in this DIY group attempting to answer wiring questions?
--
*Never test the depth of the water with both feet.*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Fri, 21 Jan 2005 22:57:03 +0000 (GMT), "Dave Plowman (News)"

Well, I may have been a bit hasty with the shortness of my reply, it's just that phrase scares me. If the OP hadn't put that statement in I probably would have answered in a more useful way. The whole post seemed a bit non-sensical to me so decided I'd rather not have to let another electrician find the (what sounded like) bodge that was being installed by the OP.
I may be wrong, but I'm edging on the side of caution today!
--

SJW
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strung together this:

I understand what has been discussed since my original posting and the work being done electrical, it is a dangerous area to touch if not competant enough.
I've done wiring before in other rooms of the house which involved wiring downlights + transformers. This obviously needed extra wiring from the main supply into the room. Pretty simple if you know what you're doing.
My only concern here is that if the live supply into the room is a 1.5mm cable can it be extended to the kitchen wall unit lighting using a connection block and 1mm cable.
Dave has answered yes as long as there is a 6amp MCB. The fusebox only has a 5amp fuse for the lighting circuit and not 6amp. On the calculation of the lighting wattage throughout the circuit in the house dividing it by the voltage it should be safe. These guidelines were checked on web resources.
The 1.5mm cable can take upto 18amps and the 1mm 14amps.
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Pedge wrote:

Yes, that's the starting point.
Cable capacity is reduced by various factors, though. Since the first consideration is 'will this cable get too hot carrying its normal load', the first set of derating factors is to do with heat under normal load. So, if a cable runs somewhere it's harder to get rid of the (natural, inevitable, nothing wrong with it) warming, we allow less current through it: examples of such spaces are running through thermal insulation - the Regs have detailed tables and call out lots of different ways a cable might run. Related to this is ambient temperature (so derate the cable if it runs through an airing cupboard, or round the back of a built-in oven, as it starts off warmer with no load), and grouping with other cables (if it's close to other warm cables, they'll warm each other up and can't be expected to lose as much heat as if they were well apart.)
The second set of limitations isn't to do with the normal (design) load, but rather performance of the cable under fault conditions. Here, you're looking at what happens when there's a short-circuit across live-to-earth and live-to-neutral. Although when thinking about the 'normal' case you treat cable conductors as having negligible, indeed zero, resistance, when thinking about performance under these conditions we examine the small but definitely non-zero cable resistances closely, to work out (a) how much current will flow, (b) therefore how long it'll take the fuse or MCB to react to that fault current, (c) how hot the cable will get during that fault-clearing time. This is where not just the thickness of the cable conductors but their total *length* comes into play: the longer the run, the higher the resistance, the lower the current flowing, BUT therefore the longer the fault takes to clear so the longer the cable has to heat up. For these (rare, short-lived) faults we allow the temperature to go quite a lot higher than for normal loading (e.g. for PVC cables, the limit temp is 160 degrees C for the fault condition, but 70 degrees for normal loading).
Finally, there's one more limit on cable length, which is working out whether too much of the supply voltage will be dropped in the cable and not make it to the load - the conventional limit is to lose no more than 4% of the supply voltage along the way.
All these factors need, in principle, to be taken into account each time a 'final circuit' - a run from your consumer unit - is designed. This is why it's often not possible to answer a simple-sounding query - 'will it be OK to wire my <lights/heater/shower/garage> in <1/1.5/2.5/4/6/10> mmsq' - with a simple yes/no answer.
In practice, though, there are 'conventional circuits' where the calculations are done in advance, and these are tabulated in the IEE On-Site Guide, along with their maximum lengths. And in further practice, lighting circuits are usually well over-specified - as you've found yourself, the nominal capacity of 1 and 1.5 mmsq cable is up around 14 and 18A respectively, given ungrouped, normal-ambient, not-surrounded-by-insulation, yada yada yada. So in your case, it's massively unlikely that 1mmsq would be at all wrong for your short side-runs, even if the main part's done in 1.5mmsq; and the main part being done in 1.5mmsq is itself unusual enough that it might be just what whoever was wiring it had to hand, or becuase for a part of its run it's surrounded by thermal insulation which won't be the case on the bits you're adding in...
HTH - Stefek
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<snip>

And possibly lethal if it turns out that the person isn't as competent as they thought...

<snip>
Then you are not competent, think about it, can you vouch for all your other work if you have failed to understand the basic theory ?
I think you have encapsulated the thinking behind 'part P' !
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It's possible to simply copy exactly other similar wiring without really understanding all the theory. I've been doing this for years. ;-)
The far more dangerous approach is the IMM technique of truly believing he knows what he's talking about, but plainly doesn't.
--
*Never underestimate the power of very stupid people in large groups *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

I could just use 1.5mm like I did last time but as somebody gave me a left over reel of 1mm cable i'm going to use this instead.
This 1mm cable will be used to power 3x 25watt kitchen wall unit underlighting and a seperate 1mm cable will power 3x 50watt halogen downlights. Both 1mm cables will be joined to the 1.5mm live current into the room.
Kitchen unit lighting= 3 x 25w= 75w= 75w / 240v= 0.3125A Halogen downlights= 3 x 50w= 150w / 240v= 0.625A
Total ampage = 0.9375 Amps
Cable load = 14 Amps
So in the above calculation it's ok.
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As was said by Stefek, in general you don't have to worry about cable sizes on a domestic 5 amp lighting circuit as either 1.5 or 1mm will be fine. However, it's worth totalling up the entire load to avoid having the fuse or breaker go - it's quite easy these days to exceed 5 amps for one floor in a house.
But do be aware the 14 amp rating for 1mm is under ideal circumstances where the cable is kept cool by adequate ventilation - it's not an absolute rating for all circumstances. However, for your application it will be just fine.
--
*Warning: Dates in Calendar are closer than they appear.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Pedge wrote:

In addition to _some_ of the points above, please do use a proper junction box, not terminal strip, to make any joints in your system.
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Thanks to all for the advice. Will use proper 4 inch junction boxes for this.
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I didn't see anywhere in this thread whether the bulbs were 240 volt or low voltage (12V ?) and the thread replies seemed to assume 240V
In the 12 volt case, the current will be a bit higher (in the low voltage part of the circuit...... ;-)
but I haven't got the earlier replies or the original posting available....
Nick
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nick smith wrote:

In this context 240V *is* low voltage (LV), 12V would be classed as extra low voltage (ELV)
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Hmm. Regardless of power transmission jargon or whatever, LV means to most its safe to a human. And 230v most certainly isn't. 50 volts used to be the accepted maximum - although I notice my Fluke meter puts up a warning flag at 30.
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On Sun, 23 Jan 2005 09:28:13 UTC, "Dave Plowman (News)"

It's the 'jargon' used by 16th Edition...!
--
Bob Eager
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wrote:

But how many on this group are going to work with 25Kv distribution lines...
I can see the point Dave is making, to describe 240v as 'low voltage' in a DIY group without explanation is daft at best and lethal at worst.
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