extending black and red circuit with blue and brown ?

On 29/12/2017 14:48, Robin wrote:

I would say that it is neither a "should" or "must".
eg if you have a 4mm 32A radial circuit what sized cable are you allowed to used for an unfused spur powering one single or one double socket? (app 15 of the regs is your clue).
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On 29/12/2017 15:28, ARW wrote:

D'oh!
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Robin
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Robin wrote:

There's two overlapping requirements covering this.
You need to have a fused connection where cable sizes change, and a circuit has to be fused to protect the smallest cable in the circuit. So, if your whole circuit is fused to protect 1mm cable I would accept a larger cable as a main feed to 1mm spurs. I have done this where I typically have a larger cable running from the consumer unit to a convient breakout point, where 1.5mm cable goes off to individual lighting circuits.
I've used fuse-at-cable-change typically when spuring from a 32A ring to provide a loft light where the lighting circuits are inaccessible or there's a need to have a non-lighting light so you can see when the lighting circuit is switched off. Similarly, I make sure there's an upstairs socket within extension lead reach of downstairs and vice versa.
jgh
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On 30/12/2017 08:43, snipped-for-privacy@mdfs.net wrote:

While true for things like your example of taking a feed to run a light fitting from a 32A socket circuit, its not universally true... For example, an unfused 2.5mm^2 spur from a 4mm^2 radial protected by a B32 MCB.

There is also some nuance in "protect" here. The fusing etc must provide fault protection to the whole circuit, but may not be required to provide overload protection to it all. (a spur from a ring circuit being one example - the 32A MCB will provide fault protection for the single run of 2.5mm^2 cable on the spur, but not overload protection. That job is delegated to the design limitation on the spur of only feeding one single or double socket).

With 6A lighting circuits, the cable sizes are over specified anyway with regards to maximum operating current (for reasons of mechanical durability, and limiting voltage drop).
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On 30/12/2017 14:16, John Rumm wrote:

I'm now more puzzled than before. I could look at the book but its not to hand.
Earlier it was said "No, cable should be the same size throughout the circuit. 1mm is actually adequate for most 6A lighting circuits in houses where the cable length isn't too long."
I thought this wasn't true at the time. Can we confirm that as long as the smallest conductor is still fault and overload protected by a MCB, then all is well?
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On 30/12/2017 15:16, Fredxx wrote:

As a general principle you would normally have just one size of cable in a circuit, but there are some exceptions as highlighted.

Which bit?

Yup generally. Fault protection in particular is the one critical, must be done at the origin of the circuit, requirements.
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On Friday, 29 December 2017 14:01:13 UTC, George Miles wrote:

of course

sometimes. You're not required to upgrade though

yes

lighting 6A, sockets 32A ring, 20A or 32A radial depending on cable

of course, both are fine on 6A mcb

NT
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On 29/12/2017 14:01, George Miles wrote:

It changed in about 1983-ish from 1mm to 1.5mm AFAIK, but ARW or John Rumm will no doubt confirm this. Something to do with guaranteeing that even a non-rcd protected circuit would trip or blow its MCB or fuse in the required minimum period (?40 msec).
So you'll probably be ok with an RCD/RCBO or maybe even just an MCB, but not an old-fashioned fuse arrangement.
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On 29/12/2017 14:43, Andrew wrote:

The RCD protection will be a requirement for any new sockets and for all T&E cables for any circuit that are buried in a wall behind plaster.
In 6 months time the regs will have changed and will require all new domestic lighting to be RCD protected.
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On Fri, 29 Dec 2017 15:36:40 +0000, ARW wrote:

To what value? My board has 100mA to lights and a few other things, then 30mA to sockets, cooker etc. I've always liked this because if the socket is tripped the lights will probably stay on - if I've just had a shock I'd rather not be in darkness as well!
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On 29/12/2017 22:26, PeterC wrote:

I would expect 30mA (i.e. for shock protection rather than infrastructure / fire protection).
However "all RCBO" installs are much more cost viable these days, and even if just using the normal 17th style CU you would normally ensure the power circuits in a given place are on the "other" RCD to the one doing the lights.
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On Sat, 30 Dec 2017 00:51:54 +0000, John Rumm wrote:

RCBOs are OK for a new installation but probably wouldn't fit on the Wylex board fitted c1992, judging by the space occupied by the MCBs. I'd definitely go for 100mA/30mA RCDs and then RCBOs in a new installation.
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On 29/12/2017 22:26, PeterC wrote:

If you have a TT installation that is about right (assuming that the 100mA RCD is time delayed if in series with the 30mA RCD).
However it will soon be a requirement for lighting to have 30mA RCD protection on new installations. Although this has pretty much been the case since the 17th edition came out.
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On Sat, 30 Dec 2017 21:39:10 +0000, ARW wrote:

That's a pity - I've had an experience of enough of a shock to make me jump back - in darkness (assuming that the lighting RCD tripped) I could have suffered an injury on various edges etc.
I can't recall if my 2 RCDs are in series or not; I suppose if 2x30mA are in parallel, then the lighting should stay on if there's a fault on the ring.
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On 29/12/2017 14:43, Andrew wrote:

The rule change was a bit before than I believe, but there could well have meant some installations still being later than that.

400 ms usually (TN systems)
There is a problem on the old 1mm^2 CPC cable when you have a spur on a ring protected in particular by a BS 3036 re-wireable fuse.
The single 1mm^2 CPC is undersized to ensure a fault to earth is cleared (you need about 1.25mm^2 - the ring itself with 2 x 1.00mm^2 is ok, but the spur is vulnerable)

The "instant" trip current required for a BS3036 fuse is something like 450A compared with 160A for the B32 MCB.
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On 29/12/2017 23:02, John Rumm wrote:

When I moved into my present house, the vendors had taken all the fancy brass sockets and switches (it was a while ago) and put back the original 1976 white contract stuff.
But they had managed to snap the 1mm cpc in two of the sockets, which were on a single ring (house less than 100 sq m), and badly connected the live to a third, so that it had no cpc at all and the ring was actually two spurs running from a 30 amp wylex rewireable fuse.
They also fitted a new kitchen and drilled right through the cooker live wire, and then repaired it with a bit of choc block which was hidden behind a fitted cabinet. I noticed the burn mark above the fuse wire carrier, that had actually melted part of the metalwork but it was 13 year before I removed the kitchen cabinet and found the 'repair'.
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Well if its just for practical purposes I'd say there is really no problem but if you are going to change the protection method, you might come unstuck against the twitty regulations. Brian
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Just who is going to stop you? If worried, ad a label to the CU saying both standards are in use.

No - the CPC size changed during red and black production times. But if it is bigger than your existing CPC, no harm.
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*How many roads must a man travel down before he admits he is lost? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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you should, ideally, put new colour sleeves over the old colours where they become visible.

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from KT24 in Surrey, England

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I have a fair bit of red and black on the scrap pile if you don't mind white outer insulation:-)

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Tim Lamb

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