Flexible cables in stud walls

Welcome to the forum - here is one opinion, others may follow.
Well the tables in the regs are only a guide to the most common cases, as there are a more or less infinite range of things that can be done, and in any case  are subservient to any more detailed cable makers data, ERA reports etc. So if the makers publish anything more relevant, try to use that.

But the physics of electricity heating  up cores of a given cross section of copper, and then more or less obstructing the escape of the heat and estimating a temperature rise are not that variable. As a first go you could look at the de-rating of another cable of more or less the same geometry, say 2.5mm2 solid core,  in a similar environment, and say

'ah between free air and in this location the rating drops by XYZ percentage - I have assumed a similar percentage de-rating on this other cable for which I only have a free air rating.' And then if you are nervous, round to the nearest % more in the direction of safety...

Equally, 13A sockets on 2.5mm of almost any kind are unlikely to struggle any more than the same load on twin and earth, as the saving grace is actually that plug-in  loads tend to be very intermittent and most of the time a lot less than 13A anyway.

If you know that multiple 13A loads for a long time are likely - perhaps its in a burger catering van with hotplates, toasters and tea urns.. then a harder look is needed.

If the cable has been in use for it bit, look at it - does it look cooked, has the insulation blooped or gone brown and cracked in the heat ? Unless it has, whatever number you derive, it is fair to say it is not immediately dangerous for the existing load profile.  (I have found sockets on 1mm T and E, but only supplying a couple of cash registers and a light, so as the MCB was 10A, we let it go with just a recommendation that the sockets should not be used for heavier loads.)
Reverse also true- if it does look heat damaged, even if the sums say it should not be, something needs to be changed, fatter cable, splitting the loads into multiple branches, improved cooling, or if only burnt at the last inch, bigger back box and not the cheapest socket with the thinnest crummy terminals.

Mike

(In the report if worried you could put your calculated 22A, 20A or 18A or whatever and a little *, where somewhere else it says *= estimated value for non standard situation, just  to show it is not an officially endorsed figure )

Thank you for your reply. 

T

Only table 4D5 is specific to T&E cables - all the others are general and apply to circular cables in general. The only gotcha is the additional correction factor for flexible conductors (0.95 for ≤16mm²) (see section 2.4 in appendix 4 of BS 7671). So you're probably looking at method B and table 4D2A - so 23A for 2.5mm² with an additional 0.95 factor making it 21.85A.

  - Andy.

Thank you for your reply.

Yes, I'd picked up on the correction factor and RM B, but comparing ccc's of table 4F3A with RM E of 4D2A (if that's an appropriate comparison), see that apart from for 2.5mm2, the latter has more generous allowances than the former, so wasn't sure if RM B would be a suitable comparison, because if 4D2A is not as generous with flexes, that could put it below 20A.

T 

yep, the world is not that precise - the tables say nothing about vertical or horizontal routing but it makes a good few degrees difference if the cable is pre-heated by more cable below it.

Even if the figure was 15A for the headline case, the basis of the tables is  a 40degree rise, to 70C from a 30C starting temperature, after some hours at that load to reach equilibrium. In the UK unless the location is near some heat producing kit, much more than a 25C start is unlikely, and rising to 80 instead of 70 does not explode the cable, it just goes a bit floppy, and gets painful to touch. To reassure yourself drop an offcut in your tea and watch it not melt like butter - or ideally your mate's tea. My point is there is an illusion of precision with the ratings - you may calculate 18A and agonise that the load may  be 20 occasionally- but a lot of other things have to go wrong at the same time to actually cause a real problem, even if your sum ends up being 10-20% out on the current rating.

Mike.

comparing ccc's of table 4F3A with RM E of 4D2A (if that's an appropriate comparison), see that apart from for 2.5mm2, the latter has more generous allowances than the former, so wasn't sure if RM B would be a suitable comparison, because if 4D2A is not as generous with flexes, that could put it below 20A.

The two tables are based on somewhat different assumptions - 4F3A is based I think on a 60 degree conductor temperature rather than 70 (probably more suitable for not feeling too hot if it's handled in service) and differing installation methods (flexes are more likely to be lying on a carpet rather than fixed to a wall), so there will be naturally be some variation. In your case where the flex is used much more like a fixed cable (and is suitable for running at 70 degrees), then 4D2A is likely to be a better approximation.

   - Andy.

Your point is understood. However, myself being a simple electrician, if anything were ever to go wrong with that bit of the wiring (perhaps resulting in a fire), I don't have the qualifications/experience to be able to be able to argue that I evaluated the flex to be acceptable in that scenario, whereas an experienced electrical engineer can argue their case with more gravitas, and be accepted by authorities more readily than I. I recal reading a post on the forum some time ago that perhaps unfairly said that electricians seem to think that if a cable carries more than an amp above its Iz/It, it'll burst into flames, whereas engineers know better. I don't believe we do think that, it's more that we have to go by stated values because we're not qualified to make allowances in ccc that differ from those provided.

T.  

Thank you. I'm inclined to agree with you but my reservation (perhaps wrongly) is as my reply to mapj.