Discuss 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses in the UK Electrical Forum area at ElectriciansForums.net
Seems to me that the first set of touching cables would always be warmer (all other things being equal), so I can't understand why it shouldn't be derated slightly relative to the spaced out cables.
What's your point?? What sort of heat increases are you actually talking about here, temperature increases that will affect current ratings, that's just nonsense!!
Are we now saying that the temperature increase of a cable doesnt affect the current rating of the cable??? This is just getting a farce....
Just look at the regs EG table 4E4A. lets take 35mm 4 core cable. How this is installed affects the current rating doesnt it.. The max being 162A (in free air...) the min being 115A (direct in ground or in ducting in the ground...) This is due to the heat dissipation of the cable or temperature.
That is why in parallel cables, if touching (and a couple of other factors) you have to de-rate the cable... Single source supply or multiple supplies, they both need de-rating..
You're talking about installation method, NOT grouping!
Chr!s - wrong sort of maths! Try ignoring the tables and use some trigonometry. The tables aren't designed for this scenario, in fact for certain types of installation methods they are woefully inadequate.
This ive got too see, so we have the "Real Heat" and the "Imaginary Heat"
I'm loosing the will to live, now. FORGET THE SODDING TABLES!! (to a point).
Basic principles -
1) the ability of a cable to radiate heat is proportional to its surface area
.............
and also the temperature gradient of the cable to its surroundings.
So two touching objects, at the same temperature, won't be able to dissipate any energy from the contact area, which will lead to a temperature increase over the rest of the surface area of the objects and arguably a temperature rise in the objects themselves.
and also the temperature gradient of the cable to its surroundings.
So two touching objects, at the same temperature, won't be able to dissipate any energy from the contact area, which will lead to a temperature increase over the rest of the surface area of the objects and arguably a temperature rise in the objects themselves.
As I have been trying to explain all along, with my rubbish english, but you have hit the nail on the head
The wrong nail, though! Even if we say for arguments sake that there is (d) surface area 'not doing a lot' (back to the calculus), it doesn't really matter as we have a much larger (33% ish) surface area to start with!
Right, at this point I'm outta here, something about water and horses springs to mind.
Before you try getting any smarter, you need to read what i actually said again!! What you seem to be talking about, is the current ratings associated with installation method, which is a totally different aspect altogether!!Are we now saying that the temperature increase of a cable doesnt affect the current rating of the cable??? This is just getting a farce....
Just look at the regs EG table 4E4A. lets take 35mm 4 core cable. How this is installed affects the current rating doesnt it.. The max being 162A (in free air...) the min being 115A (direct in ground or in ducting in the ground...) This is due to the heat dissipation of the cable or temperature.
That is why in parallel cables, if touching (and a couple of other factors) you have to de-rate the cable... Single source supply or multiple supplies, they both need de-rating..[/QUOTE]
As i've said a couple of times now, if your fool enough to employ a parallel single circuit, for all the advantages it will give you, and then start applying unwarranted factors, then you get on with it, or better still use the oversize single cable, it makes far more sense to you!!
Couldn't agree more!! It's quite amazing actually!!
They just can't get it through their noggins a parallel single circuit is just that, a single circuit!! It's not the multiple circuits they are trying to make out it is, in order to apply this crazy idea of a grouping factor!! I'll not be saying another word on the subject, until they can explain why they don't apply a grouping factor to a 3 or 4 conductor trefoil run circuit, if all the phases are equally loaded!! lol!! Or if they can show me a regulation from any internationally recognised country codes/regulations that applies a grouping factor to a parallel single circuit!! I spent some time today (foolishly) looking through German, American, and French codes, and found nothing, which was exactly as i expected!! lol!!
Before you try getting any smarter, you need to read what i actually said again!! What you seem to be talking about, is the current ratings associated with installation method, which is a totally different aspect altogether!!
As i've said a couple of times now, if your fool enough to employ a parallel single circuit, for all the advantages it will give you, and then start applying unwarranted factors, then you get on with it, or better still use the oversize single cable, it makes far more sense to you!!
I really apologise if I am coming across like Im trying to be a smart arse.... I dont mean to be... its just the way I phrase things sometimes.... just as the missus!
I would love for you to really explain it to me in my examples. I can same money here, but I cant give a reason why to my bosses as they think, like me, you apply it to both.
It does state in Table 4C1: Rating factors for one circuit or one multicore cable or a group of circuits, or a group of multicore cables.....
I really apologise if I am coming across like Im trying to be a smart arse.... I dont mean to be... its just the way I phrase things sometimes.... just as the missus!
I would love for you to really explain it to me in my examples. I can same money here, but I cant give a reason why to my bosses as they think, like me, you apply it to both.
It does state in Table 4C1: Rating factors for one circuit or one multicore cable or a group of circuits, or a group of multicore cables.....
No problem Spoon.
All these terms relate to ''circuits or groups of circuits'' not to cables making up a ''single'' circuit''!! Ask your bosses why they don't apply a grouping factor to a trefoil run cable, or to the lowly ring circuit. Both are fundamentally exactly the same situation, both have either individual conductors touching or cables touching. Like the parallel single circuit, the method of installation is the initial determining CCC factor.... Other factors may be applied, depending if and when appropriate. Grouping factors however will only apply, if there are more than one parallel single circuit being run in or on the same containment etc...
And that's as far as i'm going, until my post #119 has been answered.... lol!!
Before you try getting any smarter, you need to read what i actually said again!! What you seem to be talking about, is the current ratings associated with installation method, which is a totally different aspect altogether!!
As i've said a couple of times now, if your fool enough to employ a parallel single circuit, for all the advantages it will give you, and then start applying unwarranted factors, then you get on with it, or better still use the oversize single cable, it makes far more sense to you!!
What I was trying to say is that the current rating of cable is affected by the way it dissipates its heat. That is why the way you install it depends on the max current it can take.. Inhibiting the heat dissipation lowers the cable current capacity. EG putting it through insulation inhibits the head dissipation, that is why you de-rate the cable. Putting a cable next to another cable (touching) inhibits the heat dissipation of the cables. As Archy's wonderful post 111. Therefore this de-rates the cable. You accept this. (at least I think you do... im all confused now) What I cant get my head round is why you say this applies to example 2 of my question but not example 1.
In both examples the cables are physically the same... Everything is the same.. came current, same length, same heat dissipation. Therefore physics say that the de-rating applies to both examples.
What I was trying to say is that the current rating of cable is affected by the way it dissipates its heat. That is why the way you install it depends on the max current it can take.. Inhibiting the heat dissipation lowers the cable current capacity. EG putting it through insulation inhibits the head dissipation, that is why you de-rate the cable. Putting a cable next to another cable (touching) inhibits the heat dissipation of the cables. As Archy's wonderful post 111. Therefore this de-rates the cable. You accept this. (at least I think you do... im all confused now) What I cant get my head round is why you say this applies to example 2 of my question but not example 1.
In both examples the cables are physically the same... Everything is the same.. came current, same length, same heat dissipation. Therefore physics say that the de-rating applies to both examples.
OK, i'll answer that simple one for you!! lol!!
Simply, only because that is what is called for under BS7671, for two single circuits, so i couldn't say any different could i, not unless i opened myself up to a load more stick!! ...lol!! Not that i would necessary agree with the factor applied. Personally, i take all of the de rating factors in BS7671 with a pinch of salt, and prefer to do my own assessments based on the actual circumstances of the circuit or circuits i'm assessing. Again you need to remember that these tables are all based on worst case scenarios, (they can't be anything else with a single table) which more often than not, isn't the case at all!!
I think we're all talking at cross purposes.Basic principles -
1) the ability of a cable to radiate heat is proportional to its surface area
2) i) The surface area of a round cable is given by pi D (or 2 pi R if you will).
ii) The surface area of a cable is a resultant of pi R2.
So, let's assume we have a solid conductor that is 10mm csa.
area = 10
dia = 3.56824823
circ = 11.209982432795858
therefore, 2 x 10mm csa = 2 x circ = 22.42 mm
If now do the same maths for a conductor of csa 20mm -
dia = 5.046265044040321
circ = 15.853309190424046
Now, as covered in earlier posts, J amount of energy will produce H amount of heat.
H / 22.42 for two conductors is a lower figure to
H / 15.85 for one conductor.
So, the heat generated is exactly the same (H is a constant), but by using two identical conductors you are sharing (thanks to Mssrs Ohm & Kirchoff) the load over a greater surface area of conductor which creates a fundamentally more efficient environment. Because the energy source is the same, there is no potential difference (to use a phrase in a different way) in temperature between them therefore there is no affect on one to the other, they both heat and cool at the same rate.
The radiator effect will, of course, be affected by environmental principles, thermal conductivities etc, so other de-rates may apply, but NOT those for grouping, as they are NOT grouped cables.
I'm just amazed we've managed to get 9 pages without anyone talking about resistance in parallel, either.....
:earmuffs:
I'm just amazed we've managed to get 9 pages without anyone talking about resistance in parallel, either.....
:earmuffs:
We're not that daft!
I think we're all talking at cross purposes.
You've just neatly explained why 2 cables with the same combined CSA as a single larger cable are capable of carrying greater current without overheating compared with the single larger cable, which AFAIK nobody is arguing against.
The question as I see it is whether those 2 cables run in parallel would be capable of taking greater current without overheating if they were run spaced apart or touching.
Engineer sidestepped that question before, but essentially unless you're arguing that there would be no difference, then by definition you're agreeing that a derating factor should be applied when the cables are touching vs when they aren't regardless of whether they are a parallel circuit or not.
I get that if you're not planning the cables to carry anything like their absolute maximum current ratings, as Engineer states he wouldn't, then in reality this isn't going to make a noticeable difference, but that doesn't mean that it's right to state that no derating should be applied for touching cables when you're looking at the absolute maximum loading the cables can take.
OK, i'll answer that simple one for you!! lol!!
Simply, only because that is what is called for under BS7671, for two single circuits, so i couldn't say any different could i, not unless i opened myself up to a load more stick!! ...lol!! Not that i would necessary agree with the factor applied. Personally, i take all of the de rating factors in BS7671 with a pinch of salt, and prefer to do my own assessments based on the actual circumstances of the circuit or circuits i'm assessing. Again you need to remember that these tables are all based on worst case scenarios, (they can't be anything else with a single table) which more often than not, isn't the case at all!!
No your wrong! I have 4 cable manufacturer's literature, all requrie multiple parallel cable's to be derated, the IET, ERA, NEC(American), IEC 60287.
I'm loosing the will to live, now. FORGET THE SODDING TABLES!! (to a point).
Basic principles -
1) the ability of a cable to radiate heat is proportional to its surface area
2) i) The surface area of a round cable is given by pi D (or 2 pi R if you will).
ii) The surface area of a cable is a resultant of pi R2.
So, let's assume we have a solid conductor that is 10mm csa.
area = 10
dia = 3.56824823
circ = 11.209982432795858
therefore, 2 x 10mm csa = 2 x circ = 22.42 mm
If now do the same maths for a conductor of csa 20mm -
dia = 5.046265044040321
circ = 15.853309190424046
Now, as covered in earlier posts, J amount of energy will produce H amount of heat.
H / 22.42 for two conductors is a lower figure to
H / 15.85 for one conductor.
So, the heat generated is exactly the same (H is a constant), but by using two identical conductors you are sharing (thanks to Mssrs Ohm & Kirchoff) the load over a greater surface area of conductor which creates a fundamentally more efficient environment. Because the energy source is the same, there is no potential difference (to use a phrase in a different way) in temperature between them therefore there is no affect on one to the other, they both heat and cool at the same rate.
The radiator effect will, of course, be affected by environmental principles, thermal conductivities etc, so other de-rates may apply, but NOT those for grouping, as they are NOT grouped cables.
This is like pulling teeth!! So ''I GIVE UP''!! The disbelivers had better stick to single cable supplies, because your going to end up with 2 or more parallel cables, that have conductor CSA's bigger than the original size cable you planned to replace in the first place!!
No your wrong! I have 4 cable manufacturer's literature, all requrie multiple parallel cable's to be derated, the IET, ERA, NEC(American), IEC 60287.
Really!! lol!! ....Post the pertaining sections up then, let's have a look at what your looking at or reading in to!! lol!!
Your having a Laugh, i have IEC 60287, and you have to go through 4 pages of equations to get the watts/meter.
You will realise what goes into the tables and the likes in BS 7671 so you dont have to do all those equations. If you want to side step BS 7671 for grouping etc, then you will need to use IEC 60287 for compliance, the time isnt worth it on low voltage.
I don't believe at any point we were trying to calculate watts/meter. We were taking those as a given (H).
Reply to 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses in the UK Electrical Forum area at ElectriciansForums.net
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