extending black and red circuit with blue and brown ?

[George Miles]

Is it allowed to extend an old circuit using black and red wiring with new blue and brown. (the grey cable, not the rubbery black stuff)

Isn't the CPC wire size bigger in blue and brown?

I have several circuits (sockets and lights) running from a 'submain' fusebox, I'd like to use them but powering each from a separate MCB and blue and brown cable coming from the consumer units at the other end of the house.

Thus all fuses and trips will be in one location.

But is it allowed to mix different cables in one circuit? And how would I calculate the maximum size of MCB per circuit?

And for lighting circuits can I run the main power line in 1.5mm and then spurs to individual lights in 1mm squared?

Eventually I want to do full paperwork and testing and try to get it all approved by the council under building regs.

George

[spuorgelgoog]

Yes.

No.

Yes

Maximum demand after diversity, but if you're not an electrician you probably don't need to go beyond the standard circuits:

6A lighting on 1 or 1.5mm 20A radial sockets on 2.5mm 32A ring sockets on 2.5mm 32A radial sockets on 4 mm

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.

Owain

[Andy Burns]

But you should put a warning sticker on your CU(s)

[tabbypurr]

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

[Andrew]

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.

[Robin]

With some exceptions? 2.5mm² T&E had only 1mm² CPC in the 70s

Is that a "should" as in "best practice" rather than as in "must"?

I hope so as I've rewired lighting circuits using 1.5mm (for robustness and to keep voltage drop low) but left some switch drops in 1mm where it is in good nick, has a CPC and meets the conditions for overload etc - just to avoid having to channel the walls. And of course noted it on the form.

[FMurtz]

In Australia we can go up not down, the cable size can not be smaller than the size at the fuse or breaker but can be bigger elsewhere .

[ARW]

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).

[ARW]

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.

[Brian Gaff]

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

[Dave Plowman (News)]

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.

[charles]

you should, ideally, put new colour sleeves over the old colours where they become visible.

[Robin]

D'oh!

[nothanks]

I'll be doing the same in the foreseeable future and AFAICS (and from what others have said) there is no problem. How do you plan to extend the cables? There seems to be a lack of agreement on whether insulated crimps are adequate for non-accessible areas so I was wondering about using uninsulated crimps with solder (belt'n'braces) and suitable HS sleeving ... but insulated crimps would be less hassle ...

[Tim Lamb]

In message , charles writes

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

>

[PeterC]

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!

[John Rumm]

Yes. (it would have to be, you can't buy red/black new now)

Once you have an installation with multiple colour codes installed, you should have a warning label at the CU highlighting this.

Not with any recent cable.

Very old red/black 2.5mm^2 T&E however did have 1.00mm^2 CPC rather than the 1.5mm^2 of more recent cables.

So basically remove the intermediate fusebox, and extend each of the circuits back to the main CU?

(you could probably re-use the sub main cable for one of the circuits)

Different colours? Yes.

Different cable sizes? Yes but with the proviso that the circuit protection is set to protect the smallest cable CSAs in the circuit. Would also be wise to have a label on the CU indicating circuits with smaller than apparent cable sizes.

If they are wired in typical sizes for the circuit type (say 2.5mm^2 T&E for sockets and 1 or 1.5mm^2 T&E for lighting, then normal 32 and 6A MCBs would be appropriate).

If you want. Any particular reason why?

[Huge]

W-e-e-e-e-e-e-e-e-e-e-elll.

:o)

[John Rumm]

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.

[John Rumm]

Good quality insulated crimps are ok. You will still need some form of enclose for them though - so either the joint being in a box of some sort, or the whole cable protected with heat shrink at the join.