spur to spur lighting?

Beefpotnoodle · 28 May 2015

Hi, just wanted to check some advice I've had. I'm not a qual electrician, but will be having all my worked checked and cert'd. I have an old house with an internal entry between the house and integral garage. There is no light in the entry and the light in the garage is a spur.
I font like spurring, but I've been told a spur is fine. I'd prefer a loop.
I've been told I can spur off a spur, in lighting, is this correct?
Or, what Id prefer to do, at a bit more expense and fair bit more work...spur from the house and make it a loop with the garage spur.
Does that make sense?
Thanks, Beefy

Risteard · 28 May 2015

Your lighting circuit won't have spurs.

JohnW2 · 28 May 2015

Beefpotnoodle said:
There is no light in the entry and the light in the garage is a spur. ... I font like spurring, but I've been told a spur is fine. I'd prefer a loop. ... I've been told I can spur off a spur, in lighting, is this correct? ... Or, what Id prefer to do, at a bit more expense and fair bit more work...spur from the house and make it a loop with the garage spur.
Does that make sense?

Lighting circuits do not use rings/loops - so all domestic light circuits are radial, often with branches ('spurs'). It sounds as if your understanding of these matters is such that it might be best to get an electrician to do the work.

Kind Regards, John

ban-all-sheds · 28 May 2015

Beefpotnoodle said:
will be having all my worked .. cert'd.

What do you mean by that, i.e. what do you think you'll be getting?

PrenticeBoyofDerry · 28 May 2015

Beefpotnoodle said:
Does that make sense?

It would make more sense, if you spoke with the person who is going to certificate this work for you!

ericmark · 28 May 2015

The word spur and radial are hard to define the regs say " Spur. A branch from a ring or radial final circuit." but a branch off a 2.5mm² radial with 2.5mm² cable I would call a radial. I would only call it a spur where the current carrying capacity of the cable is reduced to below the capacity of the parent supply. However we also refer to a fused spur which personally I would have called it a radial so to the layman I can understand the problem working out what it's called.

What is important is that the current carrying capacity of the cable is not exceeded and the volt drop is not exceeded and where no RCD then the earth loop impedance is not exceeded. With a RCD it is unlikely the 200Ω Note 2 Table 41.5 is exceeded so with RCD it's volt drop and cable current carrying capacity. With lights ceiling roses which also act as junction boxes are normally rated at 5 or 6 amp and there is no cable permitted which could not handle 5 or 6 amp so the only limitation is volt drop.

So we are looking at over 29 meters before we have to worry about volt drop. The exact distance is hard to work out but if we assumed the load is even spread then we are talking about around 60 meters before the 6.9 volt (3%) is reached. That's 196 feet in old money. It's unlikely you will in a domestic ever exceed those limits.

With 1.5mm² we are looking at 91 meters assuming even spread of load i.e. calculated at 3 amp rather than 6 amp.

JohnW2 · 28 May 2015

ericmark said:
The word spur and radial are hard to define the regs say " Spur. A branch from a ring or radial final circuit." but a branch off a 2.5mm² radial with 2.5mm² cable I would call a radial. I would only call it a spur where the current carrying capacity of the cable is reduced to below the capacity of the parent supply.

As you say, the definition in the regs of a 'spur' is just plain ridiculous in relation to a radial with the same cable CSA throughout - if two cables of the same CSA leave the same point on a radial circuit, which is a continuation of the radial and which is the 'spur'?!!

For that reason, I always talk about 'branches' in relation to radials, certainly when all cables concerned are the same CSA. I suppose that a reduced-CSA branch could be called a 'spur', but I usually just call it a "reduced-CSA branch"!

With ring circuits, the terminology is obviously straightforward. Any single cable leaving the ring to feed something is a 'spur', no matter what its CSA and regardless of whether it is 'fused' or not.

Kind Regards, John

ban-all-sheds · 28 May 2015

ericmark said:
The word spur and radial are hard to define the regs say " Spur. A branch from a ring or radial final circuit."

I wonder if the people who wrote that could tell me where (if any) there are spurs in this circuit:

JohnW2 · 28 May 2015

ban-all-sheds said:
ericmark said:

The word spur and radial are hard to define the regs say " Spur. A branch from a ring or radial final circuit."

Click to expand...

I wonder if the people who wrote that could tell me where (if any) there are spurs in this circuit:....

Indeed - as I wrote:

JohnW2 said:
As you say, the definition in the regs of a 'spur' is just plain ridiculous in relation to a radial with the same cable CSA throughout - if two cables of the same CSA leave the same point on a radial circuit, which is a continuation of the radial and which is the 'spur'?!!

Kind Regards, John

Beefpotnoodle · 29 May 2015

Thanks for all the replies.
I have a mate is a sparkie but says he cant fit me in (in time limits I have), I suspect its more because he's a lazy git or doesn't like taking money from a friend. Anyway he tried to explain this over the phone, I've only read up on wiring sockets before so this method of wiring lighting sounds wrong in the context.
He was just going to check my work once it was done, for a few beers at mine. When he was taking it sounded like spurring to me, which I find a lazy opinion. I wanted a second opinion to make sure it was correct.
Its wired up no, not live and will be checked over the weekend.
Thanks again

PrenticeBoyofDerry · 29 May 2015

Not much of a mate if he is unconcerned about your safety!

ericmark · 29 May 2015

In the main lighting is done as radials the limiting factor is how many wires will fit into a ceiling rose. The rose is designed for power in, power out, and switch wires any more and it means two wires in one hole. The holes sizes vary make to make but taking two power outs from one ceiling rose is more dependent on being able to physically do it than any rules or standard practice.

As to a fused spur I know that is the term used in the regs but personally I would call it a radial as from the fuse it is a new circuit. The FCU may be on a spur but the cable from the FCU must be a radial it's a circuit in it's own right so there is nothing for it to spur from.

Taylortwocities · 29 May 2015

If it helps your understanding, BAS's very useful diagram above shows a very typical layout of a lighting circuit.

ban-all-sheds · 29 May 2015

Beefpotnoodle said:
I've only read up on wiring sockets before so this method of wiring lighting sounds wrong in the context.

You really, really, really ought to do some more reading before getting stuck in to actually doing any more electrical installation work.

Edited following John's post.

JohnW2 · 29 May 2015

ban-all-sheds said:
You really, really, really ought to do some more reading before getting stuck in to actually doing electrical installation work....

The horse appears to have bolted:

Beefpotnoodle said:
Its wired up now, not live and will be checked over the weekend.

Kind Regards, John

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spur to spur lighting?

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