1.5mm2 cable for single socket?

[plugwash]

(and, indeed, even if there is no upstream OPD at all!).

Well an overcurrent protection device normally provides both "overload protection" and "fault protection", even if you don't need the former you usually do need the latter, unless it's a short run *and* you can argue that damage is unlikely.

 

[SUNRAY]

Many of our guts probably feeling the same - but that merely illustrates how our gut-feelings can be irrational.

1.5mm² cable with downstream 13A protection is, rationally/electrically (and per regs) OK no matter how high the rating of any upstream OPD (and, indeed, even if there is no upstream OPD at all!).

Kind Regards, John

Agreed and yes I've fitted a temporary 13A socket on a 100A service, also on a 300A circuit during major control panel changes BUT it was under MY personal and full control ( I locked the enclosure with my padlock) But I wouldn't dream of doing anything like that elsewhere. HOWEVER circuits in distribution are usually smaller cables connected to a busbar and 2.5mm² as the link between the a several 100'sA busbar and 16A fuse is very standard and theoretically fully controlled unlike a 13A socket which can easily be abused.

 

[SUNRAY]

That downstream overload protection is not needed on a 20A circuit for the 1.5mm².

It could easily be, dependant on installation.

As John says a single socket on a 1.5mm² T&E on a 20A circuit is fine as we are permitted to use downstream overload protection, ie the single 13A fuse in the single 13A socket will limit the current to the level that a following 1.25mm² flex is correctly protected.

That's not an "i.e." nor downstream protection, it applies to (just about) everything with a UK plug.

Reading it back now it didn't read very well. My intended point is to state 13A is generally correct to protect a 1.25mm² flex and by default will generally be correct as downstream protection of 1.5mm²

I'd be hesitant to support the use of 1.5mm² on a 32A breaker/fuse. Only because it doesn't feel right to me, not because there is anything contrary to regs.

Mmmm. I think I can see why.

I've seen several examples where an undersized circuit gets extended with a larger cable...

 

[JohnW2]

Well an overcurrent protection device normally provides both "overload protection" and "fault protection", even if you don't need the former you usually do need the latter, unless it's a short run *and* you can argue that damage is unlikely.

I wasn't suggesting otherwise, and certainly was not suggesting that any circuit within an installation would ever not have any OPD - I was merely pointing out that if it can be argued that downstream overload protection is necessary, that does not rely on there being any upstream overload protection.

Whilst it is true that fault protection is still required when overload protection is 'downstream', it is only in TN installations that it is "normally provided by" and OPD. In most TT installations, fault protection relies upon RCDs.

Kind Regards, John

 

[JohnW2]

That downstream overload protection is not needed on a 20A circuit for the 1.5mm².

It's not (provided that it is Method C 1.5mm²). However, I mentioned the single downstream 13A (max) fuse (in the message to which SUNRAY responded) because that was the situation described in the OP.

Kind Regards, John

 

[JohnW2]

I've seen several examples where an undersized circuit gets extended with a larger cable...

That's the sort of "possible future idiocy" which, if we felt we had to take steps to 'prevent' it, would probably result in our concluding that no electrically installation could be safely installed

Kind Regards, John

 

[plugwash]

So 1.5mm2 is ok?
I ask for future reference

If the installation counts as "method C" and there are no derating factors then 1.5mm² has a current rating of 20A so would be ok on a 20A circuit.

If there are any derating factors or the cable comes under another installation method then the cable rating would be less than 20A

I'm not 100% sure what rating method is appropriate for cables loose inside a hollow wall without thermal insulation, the descriptions I can find online of the installation methods don't seem to cover that case.

If the cable is rated less than 20A, and it's a single socket, then you could try and justify it by saying that the 13A fuse in the plug provides overload protection. Personally i'm of the opinion that while this may be strictly speaking compliant it's not a good idea.

That's the sort of "possible future idiocy" which, if we felt we had to take steps to 'prevent' it, would probably result in our concluding that no electrically installation could be safely installed

You can't totally protect idiots from their own stupidity.

But equally i'm not convinced it is reasonable to expect people to notice that a particular cable is one size down from the rest of the circuit. One size step is only about a 1.2x change in core diameter! and even if they do notice it is one size down how will they know what installation method applies to it.

In an ideal world our installations would carry diagrams of all the wiring installed along with comprehensive documentation of the cable sizes used and the calculations/justifications that lead to those cable sizes being chosen, so the person making an extension to the system can make an informed choice, but we all know that doesn't happen for normal installations. At best we get a certificate with a vague description of what was installed and a results table that has no places to document varying conditions around that circuit, at worst we get nothing at all either because the documentation was never produced or because a previous occupant of the property lost it.

Thus IMO unless one is prepared to create and maintain proper comprehensive documentation and somehow ensure that said documentation is maintained long into the future one should try and perform their installation work in a way that is as conventional and error-resistant as possible.

 

[JohnW2]

I'm not 100% sure what rating method is appropriate for cables loose inside a hollow wall without thermal insulation, the descriptions I can find online of the installation methods don't seem to cover that case. ...

As you say, and seemingly rather strangely, this (which must be a very common method of cable installation) does not appear to be covered by any of the 'installation Methods' described in BS7671. However, although I may be wrong, I get the impression that most people consider this to be Method C. In fact, it would not surprise me if some people considered it to be 'in free air' (particularly if the 'hollow wall' were quite 'wide'), in which case the CCC of 1.5mm² cable would be 22A.

You can't totally protect idiots from their own stupidity. .... But equally i'm not convinced it is reasonable to expect people to notice that a particular cable is one size down from the rest of the circuit. One size step is only about a 1.2x change in core diameter! ...

I wouldn't argue with that, but, in this case, I'm not sure what sort of future idiocy/stupidity (or even just 'errors') we are contemplating ...

• All we know is that the OP was talking about ~15cm of 1.5mm² cable installed "behind a plasterboard wall" and protected by a 20A breaker. We don't know what sort of circuit is is, or what cable the rest of the circuit uses.

• If the cable in question were an unfused spur from a ring final circuit (seemingly very unlikley, given the 20A OPD), then any 'non-idiot' working on the installation in the future should understand they they could not 'extend' the circuit further from that spur, regardless of what size cable it uses.

• If it is a 20A (1.5mm² or 2.5mm²) radial circuit then, then there would be no problem with extending the circuit with (Method C) 1.5mm² cable, regardless of what size cable it is being 'extended from'.

What other scenarios are we considering in which people could be misled into doing something wrong in the future if (about six inches of) 1.5mm² cable were used for this socket?

... and even if they do notice it is one size down how will they know what installation method applies to it.

Given that the cable is apparently only about six inches long, I would imagine that the 'installation method' would be pretty obvious. However, even if it were much longer, I don't think that any future worker would have any (non-destructive) option but to 'assume' (hope!) that the installation method was such that it was adequately protected by the 20A OPD. Let's face it, even if it were 2.5mm² cable (which 'disappeared into a wall'), one could not be certain that it was not, say, Method 103 along at least some of its length and hence inadequately protected by the 20A OPD - in practice (unless there were very detailed documentation, which would be very rare), one could presumably do nothing but 'assume' that it (and/or the circuit's OPD) had been installed 'correctly'.

In any event, the above uncertainties would relate to the then-already-existing wiring. As above, if (as I assume) it is a radial circuit, then there would be no problem (aas far as the new wiring were concerned) in extending the circuit with (Method C) 1.5mm² cable, would there?

Thus IMO unless one is prepared to create and maintain proper comprehensive documentation and somehow ensure that said documentation is maintained long into the future one should try and perform their installation work in a way that is as conventional and error-resistant as possible.

I totally agree but, as above, unless I'm missing something (which is far from impossible), I'm not sure that the actual situation we are talking about (per OP) is appreciably "non-error-resistant". Am I missing something?

Kind Regards, John

 

[333rocky333]

Do not remember where i heard it, but was not there something about power circuits had to be a minimum of 2.5mm

 

[EFLImpudence]

No. 1.5mm² for T&E according to Table 52.3.

However the table is ridiculous and makes no sense.


What is a power circuit?

 

[JohnW2]

No. 1.5mm² for T&E according to Table 52.3. However the table is ridiculous and makes no sense.

Quite so. Apart from the almost arbitrary (and largely undefined - see **) distinction between 'lighting' and 'power' circuits, it would seem that, per that Table, if one uses flexible cable then one could use cable of ≥0.75mm² for anything ("any other application").

[ ** there is now a Note to Table 52.3 which says "NOTE 4: For lighting circuits and associated small items of current-using equipment, such as a bathroom extractor fan.", but that doesn't really help very much ]

Kind Regards, John