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.....