Extending a ring mains

Horlock

OK

Next question for today. This one I don't think is quite so silly. It relates to the thickness of mains cable.

(A very brief account) Recently had a house rewired, had an rcd unit thingy mabobby instead of my very old wire fuse box, and all the upstairs wiring replaced. The electrician use 1.5mm for the mains and 1mm for the lights.

Today I went to buy some extra wire/cable to add an extra socket, however, the wire in the shop was rated:

1mm 6.5amp
1.5mm 16.5amp
2.5mm 23 amp

This all sounded reasonable so I was going to buy the 1.5mm to extend the ring. After all maximum draw before circuit breaker cuts in is 32amp, can go either way back so wire can never have more 16amps flowing down it - not that it would ever have anything like this in bedroom anyway.

Then I read the small print on the 1.5mm wire and it "states suitable for lighting circuits only"

So that threw me.

Options.

1. Suitable for lighting circuits only was a misprint in the DIY store.
2. My electrician has ripped me off fitting unsuitable wire.
3. Not all 1.5mm wire has the same amp rating (seems likely)

Please do comment. Please don't just say - leave this to the professionals they know what they are doing - so far I'm far from convinced that this is always true. In the past I had an electrician connect live to neutral in a switch - interesting yes, loud yes, bright yes but probably not a sign of knowing what they were doing.

The question is: which wire/cable should I buy to extend the circuit? Does it actually matter? (Just to qualify not considering buying the 1mm wire before anyone thinks I'm about to put 6.5amp wire in a ring mains).

Just to clarify this all referring to 2core+1earth.

rich13348

Using 1.5mm in a ringmain is OK as the purpose is to reduce the overall thickness of the cable needed to provide the same voltage. As you are putting a spur it 1.5mm is fine as long as you don't use more than 16 amps.

angel_of_dartford

STOP!

DO NOT USE 1.5mm BACKED UP BY A 32A BREAKER

you have misunderstood the rating system. If the label says 16A, that is the continuous rating of the cable.Any higher, and it will overheat and possibly ignite. The breaker rating must be lower than the cable rating, so the breaker trips BEFORE the cable overheats.

I won't go into cable calcs, installation methods etc, beyond the forums fields

brightontraveller

Why you adding to a house that’s been recently rewired, If new installations wired as you say very unlikely done by an electrician ?????? 2.5 mm for ring main 1.5 mm for lighting for a verity of reasons (there are exceptions but above is the norm),

Horlock

rich13348 wrote: »

Using 1.5mm in a ringmain is OK as the purpose is to reduce the overall thickness of the cable needed to provide the same voltage. As you are putting a spur it 1.5mm is fine as long as you don't use more than 16 amps.

Thanks for the info, there seems very little difference in price, might it be worth using the thicker wire? Is there any disadvantage to using thicker wire?

Horlock

angel_of_dartford wrote: »

STOP!

DO NOT USE 1.5mm BACKED UP BY A 32A BREAKER

you have misunderstood the rating system. If the label says 16A, that is the continuous rating of the cable.Any higher, and it will overheat and possibly ignite. The breaker rating must be lower than the cable rating, so the breaker trips BEFORE the cable overheats.

I won't go into cable calcs, installation methods etc, beyond the forums fields

After your post, I was a bit scared so I've gone and had a better look. That'll teach me to try and read a wire in the attic in the dark. The 1.5 was the earth. Other two are indeed 2.5. So that sorts my problem. I do want 2.5.

That said I think that 16a cable with a 32a breaker still sounds fairly safe providing it is a ring circuit - obviously 16a in a line would be very bad.

Feeling a bit stupid that I misread/misremembered what I had seen. But at least I can buy 2.5mm cable confidently tomorrow.

As this is a moneysaving forum I suppose I should comment on money saving. Currenly pleased to be getting 50m of 2.5mm for under £30. If you know I'm being done do let me know please I wont be buying until tomorrow. But this seems a lot cheaper than CPC, wickes, B&Q, screwfix from what I can tell.

Horlock

brightontraveller wrote: »

Why you adding to a house that’s been recently rewired, If new installations wired as you say very unlikely done by an electrician ?????? 2.5 mm for ring main 1.5 mm for lighting for a verity of reasons (there are exceptions but above is the norm),

It is an extension, we were pulled up on the electrics as soon as we started the extension, and we couldn't wait until completion (which is still a long way off) to get some electrics, so this will just be adding one additional room to the ring.

27col

In my humble opinion. Anyone who refers to any part of an electrical installation as a thingy mabobby is not qualified to tamper with it. I would be interested to see a drawing of exactly how you propose to extend the ring. Using the correct 2.5 mm cable of course.

rich13348

Well in line with part P it would all have to be signed off by a qualified electrician which I'm sure the OP was going to do anyway

Horlock

rich13348 wrote: »

Well in line with part P it would all have to be signed off by a qualified electrician which I'm sure the OP was going to do anyway

Yep I'll get the electrician back at the end of the job, he hasn't tested his own work yet (by that I mean the official tests - obviously he tested that it worked, tripped etc)

Horlock

27col wrote: »

In my humble opinion. Anyone who refers to any part of an electrical installation as a thingy mabobby is not qualified to tamper with it. I would be interested to see a drawing of exactly how you propose to extend the ring. Using the correct 2.5 mm cable of course.

Making a ring bigger doesn't sound like rocket science, and I can't see why anyone would need a diagram, you simply break the original ring (in my case at the socket nearest the extension which is already pre wired - not connected yet for obvious reasons but the wires are already running internal to the walls), form an incomplete circuit around the extension, then connect the two ends of the incomplete circuit to the wires in the wall at the break point. Now have bigger circuit.