Fire Hazard? 2.5mm 20A Cable Changing To 4mm 25A Cable

[Tozzy]

what rating of circuit breaker would you use in both those situations? that's the key really.....

Just off the top of my head, I think I've got diagram 2 wrong. I don't know what i was thinking. Surely it doesn't make it any better to have 2.5mm after 4mm because the current will still run from the 4mm through to 2.5mm, therefore overloading it.

Very true, the key is which circuit breaker to use. I'll try and ammend if I have time. But i think at the end of the day, you have to use MCB on the weakest cable rating, resulting in mixing cable sizes not very useful? Because the weakest will keep tripping before you've reached anywhere near the peak load of the 4mm cable.

Misleading?

Indeed so - I presume that 'we' have no control over those pages - so to whom do we complain?

Kind Regards

You can probably see now why I was confused because I'm a newbie when it comes to electrics, learning a bit more each day though and that's where I looked for cable amp ratings. I would say it needs revising or at least some elaboration.

 

[JohnW2]

Just off the top of my head, I think I've got diagram 2 wrong. I don't know what i was thinking. Surely it doesn't make it any better to have 2.5mm after 4mm because the current will still run from the 4mm through to 2.5mm, therefore overloading it.

No, if everything else was OK, then that final length of 2.5mm² cable would also be OK, provided that (as indicated in your diagram) it was supplying just one socket - it then counts as an 'unfused spur', which is allowed to be in 2.5mm² cable, even if the main part of the circuit is wired in 4mm².

Very true, the key is which circuit breaker to use. I'll try and ammend if I have time. But i think at the end of the day, you have to use MCB on the weakest cable rating, resulting in mixing cable sizes not very useful? Because the weakest will keep tripping before you've reached anywhere near the peak load of the 4mm cable.

There are two consdierations, even if one does not consider the specific circuit arrangements mentioned in (but not mandated by) the regulations. Firstly, the MCB must have a rating no greater than that of the cable used for 'main part'of the circuit (i.e. excludfing any 'spurs'), taking into account the installation method of that cable. In practice, that is likely to mean a 20A MCB if the initial part of the cable run is in 2.5mm², or 32A if it is 4mm². However, again even forgetting regulations, the MCB obvioulsy has to be rated at least high enough for the load - about 23A with what you describe, although theoretically someone might come along and plug something requiring much more current in place of the TV.

The bottom line, therefore, is that with a 32A MCB, your diagram 2 would be OK, but that your diagram 1 would not be suitable.

Misleading?

Indeed so - I presume that 'we' have no control over those pages - so to whom do we complain?

You can probably see now why I was confused because I'm a newbie when it comes to electrics, learning a bit more each day though and that's where I looked for cable amp ratings. I would say it needs revising or at least some elaboration.

I agree. That page is probably well-intentioned, but it is an over-simplification which can,a s you have discovered, mislead. It needs revising.

Kind Regards, John.

 

[EFLImpudence]

I think I've got diagram 2 wrong. I don't know what i was thinking. Surely it doesn't make it any better to have 2.5mm after 4mm because the current will still run from the 4mm through to 2.5mm, therefore overloading it.

No it's fine because, in effect, the 2.5mm² is a spur (although the term doesn't apply to radial circuits) and cannot be overloaded because it only has to carry the current drawn by the tv. (Though I don't think it wise to advise such on a DIY forum.)

Also current is drawn by the loads - not pushed through regardless.

The cable must, however, be capable of handling any fault current.

the key is which circuit breaker to use. I'll try and ammend if I have time. But i think at the end of the day, you have to use MCB on the weakest cable rating, resulting in mixing cable sizes not very useful? Because the weakest will keep tripping before you've reached anywhere near the peak load of the 4mm cable.

Sort of. That's why circuits have to be 'designed' properly.


I think the problem is you are doing it the wrong way round.

Assess the demand (load current), say 30A
Use a Circuit Protective Device which can handle this current (obviously a bit higher), say 32A.
THEN choose a cable which can carry 32A or a bit more allowing for any derating, say 4mm² (CCC 37A if clipped) or 6mm² (CCC 38A if in conduit).
(If running through thermal insulation (with a derating factor of 50%) 10mm² would only just be sufficient.)

Ring final circuits allow 2 x 2.5mm² (5mm²) but there are restrictions.

 

[JohnW2]

No it's fine because, in effect, the 2.5mm² is a spur (although the term doesn't apply to radial circuits) ...

As I recently wrote, I agree - but, as a point of information, the term 'spur' actually is used, in relation to radial circuits, in Appendix 15 of the regs.

Kind Regards, John.

 

[EFLImpudence]

No it's fine because, in effect, the 2.5mm² is a spur (although the term doesn't apply to radial circuits) ...

As I recently wrote, I agree - but, as a point of information, the term 'spur' actually is used, in relation to radial circuits, in Appendix 15 of the regs.

True, I can't, therefore, disagree but I don't think it is really correct.

The term (spur) in appendix 15 would appear to apply only to the reduced csa connections but the definition in part 2 does not distinguish between varying csa.
So, all branches are spurs.

So, do I need to fuse a spur with more than one socket? Not necessarily ???

 

[ban-all-sheds]

Thanks ban-all-summerhouses, *cough cough*, I mean ban-all-sheds

Ah - no, I mean this:

not this:

One of them is £70! That's crazy

That'll be the wiring regulations.

If you want to do design and installation properly you have to have a copy.

I'd get this one instead:

Instead of the Wiring Regulations?

It's not a suitable "instead of".

 

[JohnW2]

No it's fine because, in effect, the 2.5mm² is a spur (although the term doesn't apply to radial circuits) ...

As I recently wrote, I agree - but, as a point of information, the term 'spur' actually is used, in relation to radial circuits, in Appendix 15 of the regs.

True, I can't, therefore, disagree but I don't think it is really correct. The term (spur) in appendix 15 would appear to apply only to the reduced csa connections but the definition in part 2 does not distinguish between varying csa. So, all branches are spurs.

I'm still not sure I understand your original comment, or your statement (above) about 'not really correct'. As you say, Part 2 defines any branch of ring (or radial) socket as 'a spur' - so I'm not sure in what sense you felt that the word 'spur' did not apply to radial circuits.

I certainly agree that I would personally always describe a 'same csa' branch of a radial circuit simply as 'a branch' (even though Part II calls it a spur). As you say, despite Part 2 definition, Appendix 15 only talks of spurs in terms of 'reduced CSA' branches - which I personally find logical and helpful (unlike the Part 2 definition), don't you?

So, do I need to fuse a spur with more than one socket? Not necessarily ???

Not necessarily with a radial circuit if you go with the Part 2 definition, since that 'spur' might simply be a ('same csa') branch.

The corresponding situation with ring finals is one we've debated before. I'm personally inclined to the view that, provided one can get all the conductors into an accessory, a 4mm² unfused spur off a 32A RFC which supplied multiple sockets probably ought to be OK in engineering terms (particularly if connected near the middle of the ring), the only real engineering issue being that of 'point loading of the ring' - which is not much of an issue near its mid-point.

Kind Regards, John

 

[EFLImpudence]

I may not have worded it as well as I could.

In general I think the word 'spur' brings to mind, albeit contrary to the definitions, a 2.5mm² cable connected to a ring where...
...whilst the cable is the same csa as the ring, its ccc is less (than the ring).

This does not apply to a radial.

I think I'm on a loser here but I knew what I meant.


A branch on a radial would only seem to be termed so by virtue of being shorter than another... 'branch'.

 

[JohnW2]

I may not have worded it as well as I could. In general I think the word 'spur' brings to mind, albeit contrary to the definitions, a 2.5mm² cable connected to a ring where...
...whilst the cable is the same csa as the ring, its ccc is less (than the ring).

No argument with that - I think that what you describe is precisely what comes to all our minds although, as you say, contrary to definitions.

This does not apply to a radial. I think I'm on a loser here but I knew what I meant.

You might be on a slight loser, in as much as a corresponding thing 'comes to my mind' in relation to a radial circuit - i.e. a (fused or unfused) 'branch' of lower csa than the 'main' radial circuit.

A branch on a radial would only seem to be termed so by virtue of being shorter than another... 'branch'.

I would go further than that, since I don't really think that 'branch' (singular) can really be defined for a radial circuit. If the radial is anything but 'linear', it's fine to say that it has 'branches' (plural) - but I would then call each of them (beyond the first point of branching) 'a branch', be that a 'shorter(er) branch' or a long(er) one - just as each branch which comes off the trunk of a tree is 'a branch', regardless of length or position.

Kind Regards, John

 

[EFLImpudence]

This does not apply to a radial. I think I'm on a loser here but I knew what I meant.

You might be on a slight loser, in as much as a corresponding thing 'comes to my mind' in relation to a radial circuit - i.e. a (fused or unfused) 'branch' of lower csa than the 'main' radial circuit.

Yes, but a spur, of reduced ccc, on a ring is the same csa.

I'll give up now. All branches are spurs; all spurs are branches. No trunks.

 

[JohnW2]

This does not apply to a radial. I think I'm on a loser here but I knew what I meant.

You might be on a slight loser, in as much as a corresponding thing 'comes to my mind' in relation to a radial circuit - i.e. a (fused or unfused) 'branch' of lower csa than the 'main' radial circuit.

Yes, but a spur, of reduced ccc, on a ring is the same csa.

Not really. Even if you are thinking just of 2.5mm² spurs off a ring, they represent an effective reduction in csa (from 2x2.5mm² to 1x2.5mm²). Furthermore, I imagine that you would regard a 1.5mm² fused branch off a ring (per Appendix 15) as being a 'fused spur', wouldn't you, despite the reduced csa?

Kind Regards, John

 

[EFLImpudence]

Yes. I give up.

 

[ban-all-sheds]

So which are branches/spurs? (All cables the same size)

 

[JohnW2]

So which are branches/spurs? (All cables the same size)

As I said, as far as I am concerned, all connections other than the one between CU and 'A' are branches (which includes 'branches of branches').

As for 'what is a spur', I'm afraid you just have to pay your money and take your choice. Part 2 of BS 7671 effectively says that the two words are synonymous. However, as I've said, I would only personally think of a branch (fused or unfused) of a radial as being a 'spur' if it was of lower CSA than the 'main circuit'. What about you?

Kind Regards, John.

 

[Tozzy]

Wow, I've missed all the excitement , I've got some reading up to do.

Well, this is what I think but please correct me if I'm wrong (I'm sure I probably am anyway):

CU > A = Circuitry
A > B = Spur
B > C = Spur
A > D = Branch
D > E = Spur
B > F = Branch

I was thinking that a socket consisting of 2 cables is a spur and a socket consisting of 3 is a branch. What do you think? And what does CSA mean?