How to correctly wire a spur from a spur

[neocleous]

Hi all,

From what I've read you're not supposed to do this but all the socket is going to be used for is to run an LED strip light in some eves in a loft conversion so it isn't going to be used often so I am not worried about over loading it because it will only be pulling 3 amps.

My plan was to run a cable from the back of the spur into a 13 amp fused spur then into the new socket using 2.5mm cable.

Does this sound ok?

 

[JohnW2]

From what I've read you're not supposed to do this but .... My plan was to run a cable from the back of the spur into a 13 amp fused spur then into the new socket using 2.5mm cable. ... Does this sound ok?

It sounds as if you know the answer - as far as the regulations are concerned, it is not OK (assuming that the existing spur is not fused, and that you're talking about a spur from a ring circuit).

If you could establish for certain that the existing spurred socket is not 'a spur from a spur', then you could insert a 13A FCU before that socket, and then run as many additional things from it as you wished.

Kind Regards, John

 

[neocleous]

If you could establish for certain that the existing spurred socket is not 'a spur from a spur', then you could insert a 13A FCU before that socket, and then run as many additional things from it as you wished.

How do I determine for sure which socket is a spur and which is from the ring main?

 

[JohnW2]

How do I determine for sure which socket is a spur and which is from the ring main?

Well, for a start, you were the one who described the existing arrangement as 'a spur' - what makes you think that it is?

Kind REgards, John

 

[neocleous]

There is a socket which has 3 cables going into it, two come up from the floor and one comes out of the sock and heads in the direction of the socket that I suspect is a spur.

 

[JohnW2]

There is a socket which has 3 cables going into it, two come up from the floor and one comes out of the sock and heads in the direction of the socket that I suspect is a spur.

That makes it pretty likely that you're right, but it's not certain (it could, for example, be an 'orrible spiders web of 'spurs from spurs'). Short of tracing the cables of the entire circuit, the only way to be absolutely sure what's going on would be to get an electrician to suss it out by testing. You won't get people here to condone assuming/guessing and/or doing things which are, or may be, non-compliant with regulations, but you may wish to make your own decision.

Kind Regards, John

 

[neocleous]

Thanks for that, the loft conversion is in an extension that was built in the last few years and the socket that I think is a spur was in the room when it was originally wired up so there shouldn't be any questionable wiring in there.

 

[Taylortwocities]

there shouldn't be any questionable wiring in there.

I've seen too many strange and weird versions of electrical wiring to take anything for granted.

 

[ericmark]

Ring Final is reasonable easy to test to see if it is a ring. With power off simple ohm meter across the three pairs line, neutral and earth.

It is the radial, fused spur which is really a 13A radial, and the unfused spur which causes the problem as we have different cable sizes involved and it is so easy for an error to have been made in the past.

When teaching my son many years ago I told him always measure the loop impedance although it's not an acid test when one measures on a regular basis one gets a feel for where there is something wrong.

So step one what is the size of the automatic disconnection device be it fuse or MCB or RCBO? If it's 20A or less then your only worry is volt drop if not RCD protected the also the earth loop impedance but since adding sockets some where there will need to be a RCD.

At 32A then next is to check if a ring or radial with ring no problem with radial next is cable size. A 4mm² cable and a 7/0.029 cable look very similar and even using a blue crimp connector as a go/no go gauge it is easy to make an error. The other problem is you need to remove every socket to ensure some one has not thought they were working on a ring and extended using 2.5mm² cable.

Often with a 32A radial one finds you have to down grade to 20A as some where in the past an error has been made.

There has traditionally been a lack of paperwork with domestic electrics and so each time one has to test to find out what one has. In a two up two down house there is not normally a problem but as one moves to larger houses then cable length can result in the inability to add more.

I find that people with the best intentions have extended a ring final maybe a 1/3 of the way around and although the ELI at the new sockets is within limits it has resulted in sockets at the new centre of the ring being below the limits. In general the limits for ELI as similar to limits for volt drop so even where a RCD is used keeping within the ELI limits in general means also within volt drop limits.

The amount of cable in a modern RCD protected ring is quoted as 106 meters but that gives with a incomer of 0.35 ohms a ELI of 1.5 ohms where limit is 1.44 where no RCD is used. At 100 meters it is within the 1.44 limit and most will consider one role a cable per ring but that's not going to help where some wiring already done.

All this is fine and dandy for an electrician who carries a loop impedance tester but not so easy for the DIY guy. The duel accessory box is the DIY'er friend here still have to find that ring which only really requires a multi-meter but once done swap the back box and fit a single socket and a FCU if not already RCD protected then a RCD FCU which lifts the ELI from 1.44 to 2.42 I know we should not assume but unless the ring is faulty in first place then one should be able to add many sockets on to each FCU supply without exceeding the limits.

This is not how an electrician would do the job but electricians have the test equipment which most DIY'er lack.

As I said before when one tests at every job you get a feeling for what is right and what needs further investigation. All tradesmen tend to make it look easy but really it's not as easy as it seems.

 

[ekmdgrf]

Hi all,

From what I've read you're not supposed to do this but all the socket is going to be used for is to run an LED strip light in some eves in a loft conversion so it isn't going to be used often so I am not worried about over loading it because it will only be pulling 3 amps.

3 Amps at 240V is 720W, which, as you are using LEDs, is around the equivilant illumination of 7.2KW of incandescent lamps. Are you sure you need that much light in your loft?

Oh, are these grow lights?

 

[neocleous]

They will be running off a 50W 12V transformer so very low power

 

[ekmdgrf]

They will be running off a 50W 12V transformer so very low power

Sounds like you need a bigger transformer.

 

[neocleous]

They work fine and give off a great amount of light. The LED strips are very efficient. I have used them elsewhere and are perfect for this kind of thing because they take up very little space and given even light

 

[JohnW2]

... all the socket is going to be used for is to run an LED strip light in some eves in a loft conversion so it isn't going to be used often so I am not worried about over loading it because it will only be pulling 3 amps.

3 Amps at 240V is 720W, which, as you are using LEDs, is around the equivilant illumination of 7.2KW of incandescent lamps.

As I'm sure you realise as well as I do, I assume that what was really meant was 'pulling a lot less than 3A', or something like that.

Kind Regards, John

 

[neocleous]

I followed the cable that I thought went to the socket that I want to spur off, turns out it goes to a different socket all together.

There is only one cable going into the socket I want to spur off does this mean there is no way it is on the ring main?