I know we've agreed T&E should be ok but this got me looking at web sites selling cables. I notice that there is 0.75mm^2 flex rated at 6A and rated for 300V, whilst a more expensive version is still rated at
6A but rated for 500V. This surprised me because I was (naively?) expecting that if a cable with the same CSA carried more amps, it woul;d have to do so at a lower voltage. Is the maximum voltage completely unrelated to CSA? Is only the maximum current dependent on that?
What is the advantage of 500V rated flex? Is 300V used domestically and 500V used in 3-phase (450V?) supplies?
One last question, I've also found "heat resistant flex" for sale. Some of it is intended to be used around boilers, boilers are hot, so I can understand that but it also says for use on light pendants. I guess heat rises and 100W light bulb gets hot. But then I also see other flexes (particularly two core) listed as for use with light pendants and they are not specifically labeled "heat resistant". So should a pendant use heat resistant flex or can it be any old flex? I'm confused!
I had to think about that. Are you saying that if there is a hex shank on the bit, the grub screws go further in than on bits with round shanks?
I was only going from what I had read on the Screwfix review IIRC, which said the screws either stuck out too far or fell out in use!
I thought you could only group light cables through the same hole? If two ring main cables go through the same hole, don't they have to be derated? I thought the derating took them below the useful value for a ring? But I haven't read the OSG to check. I could be wrong ;)
Yup, and not only that, they don't need to be done up particularly tightly to grip the shank either.
I have no idea if the SF one is any good or not. But ISTM that if they grip the shank well well enough, then how far they stick out hardly matters (with larger spade bits)
Grouping factors usually apply to cables which run together for a distance - a couple of inches of close proximity is not a problem. Note also that you don't include grouping for cables in the same circuit. So a "to" and "from" pair forming part of a ring would be ok anyway.
Not sure your observation ties in with the example given (i.e. both cables rated at 6A).
In general the current capacity is going to be closely related CSA all other things being equal. Ultimately the current capacity is dictated by maximum conductor temperature, and so is influenced by other factors that restrict how fast a cable can shed heat.
The voltage limit will be a reflection of the insulator material, and its breakdown potential. So a posh 0.075mm^2 flex may carry the same current as the cheaper one (same CSA after all), but have a better quality insulation safe for higher voltages).
Any heat generated in the wire will be proportional to its resistance and I^2, the voltage does not really come into it.
Perhaps that you can use the higher rated one in proximity to other circuits where there is a phase to phase OD exceeding 300V. Say cables running in a duct that are connected to different phases.
Heat resistant is good for boiler and immersion heater connections, or other appliances that get hot. Using it for lamp flexes might be overkill unless you have a particularly hot running halogen or something that is prone to scorching normal flex.
T&E is ok in sunlight, theres lots of it currently in such use. It does degrade but only very slightly over decades, not enough to cause a problem. If youre feeling paranoid, a coat of household gloss can block uv, and it bonds to the cable well.
I never realised that cables in the same circuit could be grouped. I saw a bit of a recent DIY SOS episode and it looked as though they were running two cables through one hole, so that would explain how they could do that. What is the reasoning behind that? I thought it was all to do with cables getting hot, so why would it matter whether the wires were on the same circuit or not? If anything, wouldn't being on the same circuit mean they both get hot at the same time? Thanks.
Voltage rating and current rating for cables are somewhat related.
In simple terms, the current rating of a cable is based on the maximum current that the cable will pass before it the core temperature of the copper reaches a certain point. There are a number of standard "points", the two lowest being 60 deg C and 85 deg C.
Note (as an aside) that the fusing current of the same cable is very much higher than the maximum continuous current in-service.
A cable may or may not be insulated and may or may not be sheathed (and indeed armoured and/or screened and over-sheathed).
Basically, the insulation exists to stop current flowing where you don't want it to flow and the sheath has a number of functions, viz:
to bunch together a number of insulated conductors to form a multi-core cable.
to provide additional insulation (from a "belt and braces perspective").
to protect the insulation from mechanical abrasion.
to protect the insulation from external heat.
Now, the dielectric strength of PVC is about 21kV per mm, so the insulation rating of a domestic PVC cable has little to do with the thickness of the insulation per se (which is always going to be adequate), rather it is a matter of perceived risk or shock if the insulation is breached by abrasion. That is to say a cable which is officially rated at 300V is probably good for 10kV as an insulator, but not good enough for the perceived risk of a 500V shock in terms of its abrasion resistance.
In summary, the current/voltage rating of a cable is a function of heat and abrasion resistance and informs the csa of the copper and the type and thickness of the insulation and the type and thickness of the sheath.
It can be used on three-phase supplies. On single phase supplies, for the same current-rating it is physically more robust.
Is 300V used domestically
Essentially, yes, but you may still wish to use 500V rated cable on a single phase supply for extra physical robustness.
A pendant should use heat-resistant flex. Two-core non heat-resistant flex can be used (e.g.) for double-insulated table lamps, where the bulb is "cap-down" and the heat rises away from the cap.
Thinking about it, I have used an extension piece in the past and that was with a bit with flats on the shank and I didn't have any problems. Perhaps it was just one particular make that attracted bad reviews? I can see how protruding screws could get in the way with narrow spades but like you say, how often is a narrow hole useful? 16mm seems a good size for cabling.
Oh, I thought we were saying the same thing: that the maximum current is limited by the CSA and voltage is irrelevant so that's why both were rated at 6A because they both had the same CSA?
I think the two I found were:
formatting link
only difference appears to be the rated voltage.
But in a domestic situation there wouldn't be another phase, so I guess for the uses I would encounter, the 300V cable would be fine?
Do people ever change the pendant flex I wonder? I would guess it's easier to change the whole pendant. So I suppose the conclusion here is that for lighting there's probably no need to pay the premium for the heat resistant flex. Around a boiler can see that it is a very good idea. I thought there was some discussion here though that even heat resistant pvc wasn't enough for immersion cables and that they should be rubber?
Generally, on building sites Yellow = 55-0-55 (aka 110V) and blue = 240V.
Orange is sometimes, but not always "arctic grade" and is used for domestic garden machinery flex and for camping/caravanning flex.
The main problems with PVC are relatively poor abrasion resistance and relatively high stiffness at low temperatures. If you've tried to use an old PVC extension outdoors in the winter, you'll know what I mean.
Arctic grade PVC addresses the issue of low-temperature flexibility, but has no better abrasion resistance.
PVC generally has poor heat resistance, poor chemical resistance, is not considered to be fully waterproof for long-term fully-submerged use, has moderately good UV resistance (good UV resistance if black) and under good conditions, indefinite life.
Natural rubber is better than PVC in every way except rubber has poor UV resistance, similar abrasion resistance and only moderate life, (particularly) where exposed to lots of heat.
Synthetic rubber AKA PCP, polychloroprene, Neoprene or "that stuff they make wetsuits out of" is better than PVC in every way, except perhaps overall longevity (and the jury's out on that).
Note that pretty much all cables get less flexible as they age. This is due to a combination of degradation of the polymers and age-hardening of the copper.
I was struggling to remember the reasoning for my previous assertion. The best I can come up with is that books such as Whitfield draw a distinction to grouping applying per circuit, not per cable - but then fail to justify why. The wording in the current on site guide makes reference to circuits or multicore cables - i.e. no distinction.
However, some more solid information: grouping factors assume that the cables in question are carrying close to their maximum load. So two legs of a ring circuit, ought not be de-rated by the full grouping factor since even if the circuit were fully loaded, the current per leg would still be well short of the cables capacity (assuming not other factors at play here).
Also cables that carry 30% or less of the cables current carrying capacity can be ignored altogether (so in domestic situations that would include most lighting circuits).
The impact of these is that grouping rarely comes into play in domestic situations.
You mentioned moderately good UV resistance, so perhaps that explains why some people have reported success with T&E outside? I was surprised to read that rubber has poorer UV resistance, as TLC suggested I used rubber when I asked which flexes were most UV resistant. I was also surprised to learn rubber has moderate life when exposed to heat; all the immersion heater cable I have seen has been rubber, or perhaps it is a special type of rubber? All very interesting stuff, thank you very much.
A quick look up on the TLC web site says that 1.5mm^2 T&E is rated for
18A (obviously that's clipped onto the wall and it would be lower if in conduit, under insulation, etc). Even with 100W light bulbs, that's a lot of lamps before you reach 18A, so yes, I had heard before that grouping could be ignored for domestic light circuits.
I hadn't realised the same applied to ring mains because I thought it would be fairly easy to load these. For example in the winter in a house with only one ring: a kettle, tumbler drier, and fan heater could all be getting on for 3kW each. I realise there are holes in this argument: the kettle only being on temporarily, using CH instead of fan heaters, etc.
Looking again at TLC, it says 2.5mm^2 T&E is rated at 24A. Since a ring has a 32A MCB, each leg will carry 16A at full load, which is
24-16=8A below the limit of the cable. 8/24=1/3, or as you said 30% below the limit. So it would appear that you are right; they can be grouped together. I never expected that.
Perhaps it is only when the cable has to be derated because it runs in conduit or insulation that you have to be more careful in a domestic setting?
ISTR that a radial in 2.5MM^2 is protected by a 20A MCB, in which case
20/24 > 1/3, so perhaps a radial near full load needs more consideration? But OTOH a radial only has one cable so there is not another cable to have to group it with, so perhaps that solves that?
Don't worry, I'm not planning on rewiring the house or anything, just it is interesting to know these things. If I ever did do any major work I would rely on something more official than the TLC web site ;)
That's interesting, thanks. I would have thought that if a cable got abraded enough to expose the cores and risk a 300V shock, that would be just as nasty as a 500V shock! Or is it that 500V is more likely to arc than 300V?
So the difference is the 500V flex is probably a bit thicker.
That explains it; I had forgotten that not all light fittings are on the ceiling! What makes the heat-resistant flex different: do they add something extra to the PVC?
But a radial circuit can branch out into more than one leg. The situation might arise where the circuit splits at a junction box in an accessible position but the two cables run alongside each other through a few joists before diverging in a position where it would have been impracticable to locate the junction box.
There's also the situation where a single radial cable doubles back on itself before going to the next accessory.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.