I'm confused about junction box rules

[securespark]

This is what the 13th Edition from 1st September 1955 has to say about "spurs".

Regulation 114 B (ii)

When spurs supplying outlying socket-outlets are connected to a ring circuit, not more than two socket-outlets or one fixed appliance shall be fed from each, and not more than half the number of points installed may be served by spurs.
Spurs shall be connected to a ring circuit in socket outlets or in suitable joint- or junction-boxes.

The conductors forming the ring circuit shall either be unbroken where they pass through socket-outlets or joint- or junction-boxes, or, alternatively, if the conductors are cut, the electrical continuity of the ring circuit shall be ensured by joints complying with the requirements of Regulation 208.


Regulation 208 (A).

Where joints in cables are required, they shall be mechanically and electrically sound and, except in buried cables, they shall be accessible for inspection. They shall be made either by soldering or by means of mechanical clamps which shall securely retain all the wires of the conductors.
NOTE.-- The resistance of a jointed conductor should not exceed that of a similar unjointed conductor.



Notice that for Reg 114, there are no fused spurs, no allowance for connecting unfused spurs at the origin of the circuit and only half the number of points may be spurs.

 

[EFLImpudence]

How long was it before they realised all that was unnecessary?

 

[securespark]

All what?

 

[EFLImpudence]

Everything in red.

 

[JohnW2]

The single point I was talking about here was literally that: multiple spurs being taken from the same single point on a ring final. Your single point scenario is not quite that.

It could be. As I said, the theoretical (albeit hypothetical) very best situation (for the cable) is to have all loads connected at the single 'point' which is the mid point of the ring - and that is just as true if the loads are all connected through multiple spurs originating from that one point as if there were (hypothetically) multiple sockets on the ring at that point.

Further, the very best situation of which you speak often does not happen.

Of course it doesn't - but it is the extreme case and I thought it would illustrate the fact that it is nonsense to suggest that having multiple spurs originating form the same single point is necessarily 'bad' - it is only 'bad' if that single point is fairly close to one end of the ring.

 

[ebee]

Very interesting Securespark and pretty much as what I thought.
What was the idea about half the number of points or even no more spurs than on the ring itself?
I`ve never been totally convinced about any reasoning other than encouraging neatness thereby robustness or to avoid encouraging spurs at the outset perhaps.
Indeed when I plan a ring my intention is all sockets on the ring and only later additions, if any, to be spurs and then I would limit to one unfused spur max per point on ring plus one at fuseway (but more at the fuseway would not matter) and perhaps one JB spur to each length of ring cable.
If someone wanted to run a ring purely of junction boxes and then spur from a JB to a socket then they only objection I can see is intrucing more points of possible failure in a circuit but that should not happen anyway.

 

[EFLImpudence]

Regulation 114 B (ii)

When spurs supplying outlying socket-outlets are connected to a ring circuit, not more than two socket-outlets or one fixed appliance shall be fed from each, unless the conductor is large enough to carry the current and not more than half the number of points installed may be served by spurs. No electrical reason for that - Bungalows?
Spurs shall be connected to a ring circuit in socket outlets or in suitable joint- or junction-boxes like any other conductor.

The conductors forming the ring circuit shall either be unbroken where they pass through socket-outlets or joint- or junction-boxes or not , or, alternatively, if the conductors are cut, the electrical continuity of the ring circuit shall be ensured by joints complying with the requirements of Regulation 208.


Regulation 208 (A).

Where joints in cables are required, they shall be mechanically and electrically sound like any other joint in any other circuit and, except in buried cables, they shall be accessible for inspection. Get away! They shall be made either by soldering or by means of mechanical clamps which shall securely retain all the wires of the conductors like any other conductor in any other circuit.
NOTE.-- The resistance of a jointed conductor should not exceed that of a similar unjointed conductor.

 

[JohnW2]

A good approximation to start with is: ... A/ loads divided equally fairly equally around to ring and avoiding unequal loading of the two legs.

I can't disagree with that

or ... B/ Most loading centred around the midpoint - the middle one thiird of the ring length would usually be considerd acceptable

I agree with that qualitatively, but your 'rule of thumb' is rather conservatives/pessimistic in the most common situation of a 32A ring final with Method C 2.5mm² cable (CCC=27A). In that situation even a full 32A load would only overload the short leg of the ring if it were within about 18.5% (of total ring length) from one end of the ring - which means that "the middle two-thirds" (actually about 'the middle 63%') would be OK. ... and, of course, as I've been saying, the theoretically 'ultimate' case of that (the best possible situation) would be to have all the loads connected AT the midpoint of the ring (which, of course, would then effectively become a 5mm² radial!)

or ... C/ Combining A/ and B/ above.

That's really a repeat of 'B', which already says '... centred around the midpoint.

Kind Regards, John

 

[JohnW2]

How long was it before they realised all that was unnecessary?

Quite. Interestingly, of course, some of that persists in today's OSG, even though it disappeared from BS 7671 decades ago.

 

[JohnW2]

What was the idea about half the number of points or even no more spurs than on the ring itself?

Very good question but, as I've just observed, that persists in today's OSG ... and, even worse, it still has ...

As a rule of thumb for rings, unfused spur lengths should not exceed 1/ 8 the cable length from the spur to the furthest point of the ring.

What on earth is that all about?
@securespark - was that ever in the regs?

 

[SUNRAY]

Very good question but, as I've just observed, that persists in today's OSG ... and, even worse, it still has ...

What on earth is that all about?
@securespark - was that ever in the regs?

I have it firmly in my mind there was a limit on spur length, something like 10ft. However that was 'hearsay' from schooldays, before I had anything to do with regs books.

Oh and the bit about uncut conductors, I had a bit of a discussion about that at work - would have been early to mid 80's with our works electrician who tried to tell me I was wrong - I showed him the superseded book a few days later and again he said something about what a load of rubbish, but bearing in mind he was somewhat older than me it is something he should have been aware of.

 

[JohnW2]

I've been thinking more about the OSG's ...

As a rule of thumb for rings, unfused spur lengths should not exceed 1/ 8 the cable length from the spur to the furthest point of the ring.

When it says "the furthest point of the ring", do you think it means "furthest from the origin of the circuit" (i.e. the mid-point of the ring) or "furthest from the spur"?

I personally took it to mean the former (i.e. the mid-point of ring) but, if that is the intention, then the length of a spur originating AT the mid-point should "not exceed zero" and any spur originating anywhere near the mid-point of the ring would only be allowed a 'negligible' spur length

If, on the other hand, it meant the furthest point from the spur on the ring, that would always be equal to half the total ring length - which again would seem pretty odd (and, in any event, could have been expressed in such terms, had that been their intended meaning)!

 

[securespark]

I have to point out that Reg 208 (as you may be able to tell by its number) is not purely in reference to Reg 114, but relates to all electrical circuits.

 

[SUNRAY]

I've been thinking more about the OSG's ...

When it says "the furthest point of the ring", do you think it means "furthest from the origin of the circuit" (i.e. the mid-point of the ring) or "furthest from the spur"?

I personally took it to mean the former (i.e. the mid-point of ring) but, if that is the intention, then the length of a spur originating AT the mid-point should "not exceed zero" and any spur originating anywhere near the mid-point of the ring would only be allowed a 'negligible' spur length

If, on the other hand, it meant the furthest point from the spur on the ring, that would always be equal to half the total ring length - which again would seem pretty odd (and, in any event, could have been expressed in such terms, had that been their intended meaning)!

My immediate thought was 1/8 of the halfway point, ie if the ring has 60m then 1/8 of 30m or 3.75m

EDIT: But thinking about it further that would mean for a ring of 100m the restriction would be 6.33m but a really tiny ring of 8m (yes they can just about exist) only 0.5m. Which would be somewhat counter intuitive to me. it kinda feels like it should be the inverse.

 

[JohnW2]

My immediate thought was 1/8 of the halfway point, ie if the ring has 60m then 1/8 of 30m or 3.75m

Exactly - as I said, that's what I've always assumed - but, as I pointed out, it leads to ridiculous situation

EDIT: But thinking about it further that would mean for a ring of 100m the restriction would be 6.33m but a really tiny ring of 8m (yes they can just about exist) only 0.5m. Which would be somewhat counter intuitive to me. it kinda feels like it should be the inverse.

Yes - but, as I wrote, even if it is a very long ring, spurs originating anywhere near the midpoint of the ring would be allowed to have little (or, ultimately, NO) length