CHESTNUT

[ebee]

Here`s an "old chestnut " question that some of you might like to answer.

For each scenario two questions:-

Is it compliant?

Is it safe?

1/ A ring final 2.5/1.5 T & E with six twin sockets on a 32A B MCB?

2/ As above with a spur from each point to a twin socket in 2.5/1.5 T & E

3/ As above with five of the six spurs at junction boxes on the ring.

4/ As above with 5 spurs on the ring or at a point.

5/ As in 1/ but with one spur at the fuseway

6/ A radial 2.5/1.5 to one twin socket.

7/ A number of radials (say 6 for example) 2.5/1.5 each to one twin socket providing of course that the fuseway terminal and its corresponding N terminal will accept all the conductors mechanically & electrically.

Of course all floor areas served, insulation, ambient temperature factors have already been addressed

 

[conny]

Not a spark but willing to have a go just to see how many I get right. My first answer each time will refer to being compliant and second to safety.
Would not use 1.5mm for any part of installation unless spur was to FCU to supply say a wall light or similar and fused accordingly.

1a/ Yes
1b/ Yes

2a/ Yes
2b/ Yes

3a/ Yes if spurs are not grouped between 2 adjacent socket outlets.
3b/ Yes if above applies.

4a/ No if they are all between adjacent outlets.
4b/ No

5a/ Yes
5b/ Yes

6a/ Yes. (But only in 2.5mm)
6b/ Yes (But only if above)

7a/ No
7b/ No

Go on, how did I do?
(Pretty carp probably )

 

[flameport]

They could all be compliant and safe.

I never fit ring final circuits.

 

[EFLImpudence]

Is it compliant?

Is it safe?

1/ A ring final 2.5/1.5 T & E with six twin sockets on a 32A B MCB?
Yes. Yes.

2/ As above with a spur from each point to a twin socket in 2.5/1.5 T & E
Yes. Yes.

3/ As above with five of the six spurs at junction boxes on the ring.
Yes. Yes.

4/ As above with 5 spurs on the ring or at a point.
Don't understand.

5/ As in 1/ but with one spur at the fuseway
Yes. Yes.

6/ A radial 2.5/1.5 to one twin socket.
Yes. Yes. I presume you mean with 32A MCB.

7/ A number of radials (say 6 for example) 2.5/1.5 each to one twin socket providing of course that the fuseway terminal and its corresponding N terminal will accept all the conductors mechanically & electrically.
Yes. Yes. I presume you mean with 32A MCB. But would never do it.

 

[JohnW2]

Would not use 1.5mm for any part of installation unless spur was to FCU to supply say a wall light or similar and fused accordingly.

When he writes "2.5/1.5", I presume he means 2.5mm² L&N and 1.5mm² CPC - i.e. standard 2.5mm² T&E.

Kind Regards, John

 

[JohnW2]

Another non-electrician's attempt .... If this wasn't a 'quiz', I'd probably just answer Yes/Yes for (1), (2), (3) & (5), "don't understand the question" for (4), and (assuming 32A MCB) No/Yes for (6) & (7) (but Yes/Yes if 20A MCB). However, since it is a quiz, I'll be a bit more pedantic and explicit (in all cases I'm assuming you refer to unfused spurs) ...

1/ A ring final 2.5/1.5 T & E with six twin sockets on a 32A B MCB?
2/ As above with a spur from each point to a twin socket in 2.5/1.5 T & E
3/ As above with five of the six spurs at junction boxes on the ring.
For (1), (2) & (3) ... 'Yes' & 'usually Yes*', provided that "...under the conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable" (433.1.103) - and, of course (compliance wise), that the JBs in (3) were accessible or 'maintenance-free'. Otherwise No & No (except that I personally would not say 'No' for 'safe' just becasue of concealed traditional JBs!).
* 'usually Yes' because it would theoretically not be safe if there were a fault in a spur cable or spur socket or total loading on the socket (with or without inteference with the plug fuses) such that the current in the cable was >Iz but <32A.

4/ As above with 5 spurs on the ring or at a point.
I'm with EFLI in not understanding.

5/ As in 1/ but with one spur at the fuseway
As for (1), (2) & (3)

6/ A radial 2.5/1.5 to one twin socket.
7/ A number of radials (say 6 for example) 2.5/1.5 each to one twin socket providing of course that the fuseway terminal and its corresponding N terminal will accept all the conductors mechanically & electrically.
For (6) & (7) ... Yes & Yes if the MCB was 20A (unless installation method such that Iz<20A - in which case No/No). If 32A MCB, then technically probably non-complaint, unless cable was totally 'clipped direct' (Iz=27A) and one attempted to invoke 433.2.2(i)+434 to argue that the cables were adequately protected by 2x13A fuses in plugs at the far end of circuit (very iffy argument I would say). However, this being non-compliant is a bit daft, since none of these radials are any different from a (compliant) unfused spur from the origin of a 32A ring final circuit. With 32A fuse, the answer for safe is 'usually Yes' (*see above)

Kind Regards, John

 

[ebee]

"Another non-electrician's attempt" Ohh Thanks for that.

To clarify.

4/ would be a mixture of 2/ and 3/ that`s all it means.

In all cases 32A MCB not 20A or 16A.

_____________________________________________

Reasons for post.

A/ Some seem to think that spur at fuseway is a No No.
B/ Comparing 5/ and 6/ some seem to think that ring with spur at fuseway is OK but removal of the ring leaving a radial on a 32A MCB suddenly makes it unsafe.
C/ Although the rule is only one spur per point on ring or JB on ring for reasons of possible overheating at the connection the same rule would be applied at the fuseway. However this is not necessary at this place on the circuit because the total load whether ring/radial/spurs of the whole circuit always travel thru the fuseway no matter what the configuration of the wiring.

Just for the record I would say all are as safe as each other but can understand the feeling that the last two (6/ & 7/) would not be compliant.

 

[ban-all-sheds]

 

[ban-all-sheds]

1/ Not necessarily & not necessarily.
2/ Are any of the spur cables >3m in length?
3/ Ditto.
4/ Ditto.
5/ Ditto.
6/ Is the cable >3m in length?
7/ Are any of the cables >3m in length?

 

[JohnW2]

B/ Comparing 5/ and 6/ some seem to think that ring with spur at fuseway is OK but removal of the ring leaving a radial on a 32A MCB suddenly makes it unsafe.

Obvioulsy no different in terms of safety but, as I said, unless one invokes a pretty iffy argument (which BAS now seems to be trying), removal of the ring would probably make it non-complaint - which, as I said, is daft.

C/ Although the rule is only one spur per point on ring or JB on ring for reasons of possible overheating at the connection .....

Often asserted, and not only for the reason you state - possibly concentrating load at one point on a ring (particularly if close to one end of the ring) is not good design practice, since it theoretically could overload the cable in one leg of the ring. However, although most of us would probably usually regard it as 'not good design practice, where does this alleged specific 'rule' come from?

Kind Regards, John.

 

[JohnW2]

1/ Not necessarily & not necessarily.
2/ Are any of the spur cables >3m in length?
3/ Ditto.
4/ Ditto.
5/ Ditto.
6/ Is the cable >3m in length?
7/ Are any of the cables >3m in length?

BAS, you appear to be trying to invoke 433.2.2 in the manner I mentioned last night. However, how would that work if you potentailly had 2x13A fused plugs at the socket unless the Iz of the cable was >26A (which would only be the case if clipped direct) - 433.2.2 is taking about downstream overcurrent protection, and would probably not be interested in a (controversial!) argument that the double socket was only rated to take 12A or 20A total.

Kind Regards, John.

 

[holmslaw]

..

 

[OwainDIYer]

7/ A number of radials (say 6 for example) 2.5/1.5 each to one twin socket providing of course that the fuseway terminal and its corresponding N terminal will accept all the conductors mechanically & electrically.

Not on a 30A fuse or 32A MCB. While the cables might be satisfactorily rated and protected by the assumed 20A max load at each twin socket, this would not be the case if the radial was extended from the socket.

 

[JohnW2]

Not on a 30A fuse or 32A MCB. While the cables might be satisfactorily rated and protected by the assumed 20A max load at each twin socket, this would not be the case if the radial was extended from the socket.

Is this really something one has to consider? It's always possible for someone to subsequently modify or add to any final circuit in a non-compliant fashion, but I don't think that affects compliance of the circuit as originally installed, does it? Let's face it, future idiots could do anything to any circuit!

Kind Regards, John.

 

[OwainDIYer]

Not on a 30A fuse or 32A MCB. While the cables might be satisfactorily rated and protected by the assumed 20A max load at each twin socket, this would not be the case if the radial was extended from the socket.

Is this really something one has to consider? It's always possible for someone to subsequently modify or add to any final circuit in a non-compliant fashion, but I don't think that affects compliance of the circuit as originally installed, does it? Let's face it, future idiots could do anything to any circuit!

AIUI for a general purpose circuit with BS1363 socket outlets the design current of the circuit = the rating of the MCB, an dthe cables must be able to carry the full design current at any point in the circuit.

so 20A cables on a 32A MCB = non compliant.

Also part of the design of a BS1363 socket outlet radial is that it can be extended at any point.

There must be a reason why such a non-typical circuit layout would be used in the first place, so there must be a reason why it can't be (a) on a 20A MCB (b) wired in 4mm (c) wired in MICC (d) split into 2 x 20A circuits if the load requires it