lighting spur from a ring main/socket

kevsterdiy · 2 Dec 2007

Good morning.

Yesterday I was asked by a friend to help him wire a few connections in his shed. To cut a long story short he has 2 single sockets wired in a ring from an RCD in the shed. He wants to come off one of these to a FCU and then wire from this with a 3amp fuse to a light via a switch. The sockets run on 2.5T&E and the lighting 1.5T&E,

now would I be right in saying that the cable between the socket to be spurred from and the FCU should be 2.5T&E and then 1.5T&E from that point?

I my mind to be correct it should be 2.5T&E, but 1.5T&E can run 15amps, again is this right?

Thanks.

electronicsuk · 2 Dec 2007

Yes, the cable before the FCU in this case should be 2.5mmsq and anything after the fuse should be appropriate for the load and fuse rating. In this case 1.5 or 1.0mmsq will be fine for the lighting circuit.

kevsterdiy · 2 Dec 2007

electronicsuk said:
Yes, the cable before the FCU in this case should be 2.5mmsq and anything after the fuse should be appropriate for the load and fuse rating. In this case 1.5 or 1.0mmsq will be fine for the lighting circuit.

Thank you. so to confirm the 1.5 cable is rated at 15amps, but the reason it has to be 2.5 is that it's a spur from a ring (32).

ColJack · 2 Dec 2007

yup.. if a fault occured at the spur end, then 32 amps could flow before tripping the breaker..

if i have to spur off a socket ( and that's a big HAVE TO.. ) then i prefer to do it with 4mm...

of course, the best method is to put the fused spur INTO the ring rather than tap off a socket for it..

kevsterdiy · 2 Dec 2007

ColJack said:
yup.. if a fault occured at the spur end, then 32 amps could flow before tripping the breaker..

if i have to spur off a socket ( and that's a big HAVE TO.. ) then i prefer to do it with 4mm...

of course, the best method is to put the fused spur INTO the ring rather than tap off a socket for it..

Very sensible advice there! Not the best way i know, and 4mm the safest that's for sure. Aslong as 2.5 is safe.
Thanks.

kevsterdiy · 2 Dec 2007

Been in my house over a year now, Have an outside light that comes off the ring main as a spur from the hall, up the wall to a 3map S/W FCU. I would put money on taking off the covers to find 1mm cable from the ring....! Many many things even i know are wrong i have had to alter from previous owners.

1 interceptor · 2 Dec 2007

I agree with Coljack, because a 2.5mm T&E cable is not rated to take 32A, which under fault conditions, you are asking it to do.. Be safe and wire to it in 4mm..

Most house fires are caused by 2.5mm spurs from ring main circuits that are not correctly protected, do it right and sleep at night...

kevsterdiy · 2 Dec 2007

1 interceptor said:
I agree with Coljack, because a 2.5mm T&E cable is not rated to take 32A, which under fault conditions, you are asking it to do.. Be safe and wire to it in 4mm..

Most house fires are caused by 2.5mm spurs from ring main circuits that are not correctly protected, do it right and sleep at night...

Indeed, i notice my garage has a short ring with 2.5 cable direct from the CU, no problems there but it then has a 2.5 spur to a 3amp FCU for the lighting. As you say works but not ideal under safety/fault conditions (if the ring wasn't complete etc). Far better to treat it as a 4mm radial.
Cheers.

dingbat · 2 Dec 2007

1 interceptor said:
Most house fires are caused by 2.5mm spurs from ring main circuits that are not correctly protected, do it right and sleep at night...

You sure about that?

Electrical fires are almost always caused by poor connections, worn switchgear arcing, faulty connected equipment, etc.

There is absolutely nothing wrong with using 2.5 sq mm T&E to spur from an A1 circuit.

(You may also wish to check out Reg 473-01-04 para (ii) which allows for the overload to be limited by the load itself... in this case the fuse at the end of the spur.

)

1 interceptor · 2 Dec 2007

Poor connections, probably due to trying to put 3 wires into the same connection at the back of the socket when making a spur..lol..

The statement about spurs being the cause of most house fires, came from a case study we were shown at college, usually through overloading the circuit with electrical goodies. after all no one can see how the sockets are wired in the walls unless they have xray vision..

What we were told at college is this, if you want to add a 2.5mm spur from a ringmain, then derate the 32A ringmain protection to 20A because the circuit is only as good as the smallest conductor in it..

dingbat · 2 Dec 2007

1 interceptor said:
What we were told at college is this, if you want to add a 2.5mm spur from a ringmain, then derate the 32A ringmain protection to 20A because the circuit is only as good as the smallest conductor in it..

The college then, is guilty of making up its own rules.

You should seriously question the competence of the staff as far as technical knowledge is concerned.

If they are also the ones responsible for the 'outside lights regulations' you need to find another college.

Spark123 · 2 Dec 2007

You were incorrectly taught. There is nothing wrong with a 2.5mm non fused spur on a ring final as long as it only feeds one accessory. See page 153 of the on site guide.

ColJack · 2 Dec 2007

the key word there is "GUIDE"...

i could if i so choose to wire a fused spur with a 3 amp load with 0.75 cable backed with a 32A breaker.. then?

a spur should be wired to safely take the maximum rated current of the breaker..

even the "GUIDE" says that a 2.5 radial should be protected with a 20A breaker..

1 interceptor · 2 Dec 2007

hmmmmm, must admit i dont know where the college got its case studies, etc from.

I agree with you Spark123 in that "as long as it feeds only one accessory", but once you have finished the job and gone, how would you know how many accessories it was feeding,
at the college they were throwing health and safety at us all the time, after all if something does go wrong and you were the last know person working on the electrics, whos house do you think will be getting the knock knock..
and like Coljack rightly said, even the guide says a radial should be protected by a 20A breaker, and by adding one to a ring main then if you dont increase the cable size to the spur then derate the breaker..i think that makes sense from a safety point of view..

davelx · 2 Dec 2007

Take a look at 473-01-02 and 473-02-02. If you comply with those conditions (the FCU or plug fuse povides the overload protection, the existing 32A MCB on the ring will provide protection against faults so long as Zs & the adiabatic check out OK), then you can use 2.5mm, or even 1.5mm cable to the FCU.

I really believe that at college sutdents should ask their tutors to explain the regs from first principles. You need to understand and apply, not just regurgitate.

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lighting spur from a ring main/socket

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