95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses

[Rockingit]

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.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

See my jugs of water analogy.

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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..

 

[i=p/u]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

If equally loaded is key word here.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

You're talking about installation method, NOT grouping!

Please note the "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.."

This relates to installation methods and grouping

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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" to find the total Heat dissipated we now need trigonometry lol?

I think you digginga hole here, you see in the tables we have "r", "x" and "z", what do you think z is?

The tables correlate to the BS for that cable type, what is it you think you know thay dont?

I tend not to use the tables in BS7671, i use the Manufacturers, who in turn use ERA guidance, theuy have differing ccc, but this is inly down to factors applied, if i apply the same factors to BS7671 then i get he same values.

Please explain your Trigonometry though thanks.

 

[Silly Sausage]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

This ive got too see, so we have the "Real Heat" and the "Imaginary Heat"

Haha....j heat, you think you're warm, but in reality you're not!

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

 

[Silly Sausage]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

hence why other de-rate factors may apply, just not for grouping as according to the marvels of calculus those two round objects never actually touch each other, do they? If you divide H / 2 by splitting it across two cables, you can't then magic some of that back over to the other one in a disproportionate way, or create H+h, the maths just doesn't allow it.

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

OP - did you ever think that your sounds-so-simple-at-the-beginning thread would end up back in the maths class at college?! Welcome to the world of the Forum. :crowngrin:

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

 

[44ALLAN]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Fascinating discussion guys.................I thought that all along ............ta :shocked:

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I'm still waiting for the guys to explain to me why they de-rate the parallel cables in example 2 but not on example 1 in my post 40 or my improved post 83 (see below). Note: The cables are all the same size, have the same characteristics and are 4 core SWA.

(example 1:same source supply) We have 2 cables (touching, in ducting) each drawing 100A (the source requires 200A) The cables are of the same size and are both 30 meters long. You are saying you would not de-rate these cables as they are supplying a single sourse

(example 2: 2 separate sources) We have 2 cables (touching, in ducting) each drawing 100A (each source requires 100A) The cables are of the same size and are both 30 meters long. You are saying you would de-rate these cables.

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

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!!

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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]

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!!

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

I will try and explain it from my end of things.
First of all I do not use trefoil runs. (if anyone is bored, try typing "what is trefoil" into Google.. Second result down is "Welcome to the Trefoil Guild homepage. ... Trefoil Guild is guiding for ADULTS – fun" No I didnt go into it...
I use multicore SWA cables mainly. This is also what was suggested the OP use isnt it? It was suggested he could use a parallel run for his supply cable if he was to renew it. I stated (post 27) that the cable would have to be re-rated, as they would be touching in the ducting. Ok I did state the wrong factor of .75 and I will hold my had up to that, as it is incorrect.

In post 28 you stated that "They are the same supply cable, be they X 2, X 4 or whatever, you don't de-rate a parallel circuit, de-rating only applies if the cables being de-rated are two different supplies!! "

So I came up with a question, in that, in example 1 you would not de-rate the cables, but you would in example 2. The cables are exactly the same, drawing the same current, having the same characteristics, but you only apply the grouping factor to example 2, where I apply it to both examples.

I was waiting for an answer.

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.....

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

:hanged:

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

 

[Knobhead]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

This is getting boring now!

 

[BrianDB9]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Get max demand worked out and apply what ever diversity you as suitably qualified electrical engineer decide - Voila job done !

 

[Gavin A]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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 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.

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I'm just amazed we've managed to get 9 pages without anyone talking about resistance in parallel, either.....

:earmuffs:

 

[Silly Sausage]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!

 

[Toonlad]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!

That would be Epic

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

I've side stepped nothing!! You have still not grasped the fact were talking about a single circuit, no grouping factors apply whether touching or not touching, ...end of!! Any initial derating as such, would have already been applied by the installation method, not by any grouping factor!! Now is that clear enough for you??

Only a bloody fool would design and run any circuit, parallel or otherwise at it's absolute maximum, your now making up scenarios to suite a non-existent argument!!


Tony's right, this is getting really boring now!!

 

[Gavin A]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

of course I've grasped that, though I only have your say so that no grouping factors apply, and you've not backed that up by any logical reasoning so far in this thread.

So to me, heat is heat is heat, and a warm cable directly next to another warm cable will reduce the rate that each cable can dissipate that heat to air regardless of whether both cables are running as parallel cable on the same circuit or not, and 2 such cables in the same duct underground will result in double the total heat needing to be dissipated via the duct into the surrounding ground, so the internal air temperature in the duct will be higher and so will the temperature of the surrounding ground.

and yes you side stepped that point earlier, as you're doing again now.

Do you agree that 2 cables of the same CSA and running close to the maximum stated loading capacity for each cable will run hotter when touching, and/or in the same duct than when spaced apart or in separate ducts even when they're parallel circuits? If not, please explain why.

If you agree with this, then please explain why you state that no derating factor should be applied, as there's a logical disconnection there.


OK, so you say you'd not design a circuit to run at it's maximum rated output, which is fair enough, but the ratings you were giving in your post that started this discussion were maximum ratings, which you stated didn't need derating for grouping because they were in parallel.

 

[Gavin A]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

You're right though, this is getting a bit silly.

Just to take the original question back to first principles...

95mm2 copper 4 core cable x 200Amps = around 7.8W per m per phase heat = around 31.2W / m of heat being generated.

Just based on my knowledge of ground source heat pumps, I can see that that shouldn't be a problem as heatpumps are generally rated at a maximum of 50W heat extraction from the ground per m of pipe, so the ground should have the ability to wick away 31.2W per m continuous heat output without the cable getting too warm, as long as the conduit isn't too insulating, which it shouldn't be, and it's not going to be continuous output anyway.


Not sure I'd want to be pushing much more than that though continuously, as for example 250Amps per phase would give around 48.8W / m heat output, which is getting towards the level at which the surrounding ground temperature would start rising significantly if that were continuous output.


figures based on Elland Cable resistance figures.

ps I've calculated that as if the neutral has the same losses / heat output as each phase. TBH I can't remember now if that's right or not.

 

[Gavin A]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Is the reason for this misunderstanding that there are 2 reasons for derating grouped cables

1 - Temperature
2 - Induction / eddy currents

I can see that there shouldn't be a need to derate due to the risk of eddy currents if it's the same circuit single phase, and if it's 3 phase then this shouldn't be much of an issue anyway (as I understand it) as long as it's relatively balanced between the phases, which I assume is what Engineer is referring to.

It won't have any impact on the temperature side of things though that I can see, where 2 cables next to each other will always have less ability to dissipate heat than a single cable would, especially in an underground duct.

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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.

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.

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

There is a lot of rounding down by BS 7671 and up, but its not a bad thing.

Iet table 4C1 , 2 cables touching, factor 0.8, the ERA who the cable Manufacturers use 0.85.

Pulling teeth, you go through IEC 60287 to get your own factor's lol.

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

 

[Rockingit]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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).

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I don't believe at any point we were trying to calculate watts/meter. We were taking those as a given (H).

We are trying to calculate the heat disipated via the cable under varying conditions, so wee need to us Watts/m as a measure, and if you want to "FORGETN THE SOODING TABLES" you need IEC 60287.

You need to consider many factor's, from the construction of the cable, to the A/C resistance taking into account the Skin effect and the Proximety effect, then you need to consider the external influences, then as i have said after 4 pages of equations you can calculate the Heat disipation of the cable, the tables etc in BS 7671 are not far off the calculted.

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Yep, good idea, forget the soddin table altogether!!! lol!!

 

[Lucien Nunes]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I think there is confusion about the need to apply grouping factors to paralleled cables because different contributors have assumed the CSAs have been arrived at in different ways, and started with different tabulated CCCs...

Those saying no derating is needed, have assumed that a single cable of sufficient Iz with CSA Qmm² is split into n grouped parallel cables of Q/n mm². This gives equal CSA and greater heat dissipating area compared to the single cable, so no derating is needed. In fact the current density can be increased somewhat.
Example: Take one fully loaded 6.0mm², tabulated CCC 41A, and split it into four 1.5's. Current in each 1.5mmm² cable 41/4= 10.25A This is well within tabulated rating (17.5A) and even if Cg=0.65 is applied to the cables (11.375A) they are still good for more than 10.25A. So you don't need to consider the grouping factor separately.

Those saying that derating is needed, have assumed that n separate cables of Q/n mm² rated at their individual tabulated CCC have been paralleled up and grouped to form one circuit. Since the Iz for a cable of CSA Q/n is much more than 1/n x (Iz of cable CSA=Q) this would result in too high a current density, therefore derating is needed.
Example: Take four 1.5mm² cables tabulated at 17.5A CCC and parallel them up to get a cable totalling 6.0mm². You cannot work this at 4x 17.5 = 70A if the cables are grouped together. But if you apply the Cg of 0.65, you can work it at 70 x 0.65 = 45.5A.

So the disagreement is all in the choice of which CCC to start with. Use that of the individual cables and you'll need to derate. Use that of a cable equal to the combined CSA and you don't.

 

[Dan]

Bump