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If the separate cables do not derive from a single OCPD etc, and do not terminate at the same point of load, then it isn't a parallel supply. So what you're describing isn't a single paralleled circuit, it's 3 separate circuits and therefore those circuits will be subject to a grouping factor!! Unless of course they comply with conditions where the grouping factor can be ignored!!


In simplistic terms, if the paralleled cables start and finish from the same single points at each end of the circuit then they are classed as a single circuit and you don't apply a grouping factor to a single circuit. Other derating factors and the cables installation method do apply, and will if designed correctly, adequately look after the circuit cables....

Respect your opinion on this but I think we may have to agree to disagree - regards
 
The point is that if they are the same (which is the definition of a parallel supply) they have no affect on each other, hence why you don't need to apply derate for that reason. Any other aspect of interference from external sources is acting on them both equally, so can be counted as one. If I placed another conductor from a different circuit that was running at say 70 degC, then obviously there is going to be a transfer of heat from that new conductor to the originals as would be the case if they were a single cable.

The reason for using parallel conductors is simply that you have a greater surface area of conductor which means you have a more efficient system - 2 x 40mm is a greater area than 1 x 80mm (do the maths if you don't believe me).

No no no!!!
Not when they are touching each other, school boy maths/physics. If there's a significant gap between them, fair enough.
I'm going with my theory in post 32.
Trouble is it contradicts the old R = (rho x l)/csa
 
Sorry for not getting back sooner on this thread, it's made a real interesting read and given me lots of thoughts.
For what's its worth, it'll be an equal load, same distance and same connection points. A proper parallel circuit
. It's actually a large three phase distribution circuit, but due to the complexity of the steel work in the concrete pour, the contractors installed five small ducts instead of a decent single one.
I didn't actually put all the details as I didn't need, or expect anyone to do the cable calc for me.
Im actually only going to apply the basic correction factors. Although it's a decent sized supply, it a very short distance. About 6m. The ducts are separated, short and equal.
I'm actually working out the cable size as a single cable ducted, then splitting it by the minimal number of cables baring in mind the size of the ducting and then checking and verifying the individual cable calcs again.
If that all makes sense, I think I'm attacking it the right way.
 
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What sort of supply current are we talking about here??

If the ducts follow the same route and are essentially the same length, then it shouldn't be a problem. The main problem you are going to face, is the terminations and conductor connections at each end of the parallel supply.... You're probably going to need extended connection bars/points, ...if you have room for them that is!! lol!!
 
Respect your opinion on this but I think we may have to agree to disagree - regards


Can't in all honesty, see what you can disagree with?? The difference between what you are describing above and a parallel supply circuit, is the same as comparing chalk with cheese. There is No comparison...
 
Can't in all honesty, see what you can disagree with?? The difference between what you are describing above and a parallel supply circuit, is the same as comparing chalk with cheese. There is No comparison...

You say it is technically the 'same circuit' but this is just terminology and has, in my opinion, no bearing on whether or not grouping should be at least considered.

I understand and agree with your point (in the other thread) about the regs often leaning too much on the side of caution, but if you are installing a pair of cables as small as you can get away with then you surely must at least consider heat dissipation, even if just for peace of mind - not life or death I know but volt drop could be an issue if the cables run hottish.

My main issue (as with Spoon) is how does this setup differ from 2 cables (separate circs) with the same installation methods/loading? Presumably you would include grouping in your design calcs with those.
 
The difference being, that the 2 or more circuits that you are talking about derive from separate OCPD's and are supplying separate loads, so whatever way you want to look at it, are separate circuits. Not so with a paralleled supply circuit with let's say 2 conductors per phase (+N), it is a Single circuit, there is nothing ''technically the same as a single circuit'' and nothing to do with terminology, it IS a single circuit, pure and simple. If that paralleled circuit ran in the same containment as other separately derived circuits then that parallel circuit would be subject to grouping factors. But still only as a single circuit, not two circuits, because it isn't!!
 
No no no!!!
Not when they are touching each other, school boy maths/physics. If there's a significant gap between them, fair enough.
I'm going with my theory in post 32.
Trouble is it contradicts the old R = (rho x l)/csa

And calculus teaches us what, exactly?
 
Two cables, fully loaded and run together regardless of whether they are 1 or 2 circuits are going to be influential to each other regarding heat dissipation (that's what I meant when I said the difference is just terminology).

The fact that it is a parallel single circuit cannot, in itself, make the cables run any cooler - but that is what you seem to be saying.
 
Two cables, fully loaded and run together regardless of whether they are 1 or 2 circuits are going to be influential to each other regarding heat dissipation (that's what I meant when I said the difference is just terminology).

The fact that it is a parallel single circuit cannot, in itself, make the cables run any cooler - but that is what you seem to be saying.

Yes but 2 as I mentioned in post 28, cables running next to each other as 1 circuit is like 2 resistors in parallel. Less resistance and therefore less heat. That it my take on it anyhow.
 
Two cables, fully loaded and run together regardless of whether they are 1 or 2 circuits are going to be influential to each other regarding heat dissipation (that's what I meant when I said the difference is just terminology).

The fact that it is a parallel single circuit cannot, in itself, make the cables run any cooler - but that is what you seem to be saying.

Right - this is my last post on this before I lose the will to live and want to find some ceremonial sword to disembowel myself with:

I really don't understand what is so hard to grasp about this, it's NOT difficult! Two conductors, starting and finishing in the same place, having the same characteristics, do not heat or cool each other as the energy required to do so is shared between them, therefore if one got hotter because the energy inside it increased, the other must have decreased, therefore they balance out. Therefore, grouping factors do not apply. End of. Period. Nada. Zilch. We've called time. The bar is dry.

They will, however, interfere with ANOTHER circuit, in exactly the same way as another circuit will interfere with them so if you had three cables in a duct, two of which were a parallel supply, then you would look at derating for two circuits and treat the parallel cables as one cable (because that's what they are).

The only time that 'bunching' becomes an issue is with possible considerations for detrimental magnetic/reactive influences with multiple 3ph conductors (see App 10) making up the same circuit, however the thermal properties remain unchanged.
 
Yes but 2 as I mentioned in post 28, cables running next to each other as 1 circuit is like 2 resistors in parallel. Less resistance and therefore less heat. That it my take on it anyhow.

Yes, yes, yes!! Which is why you can use two smaller diameter cables to carry the same current as one larger one.





** OK, so I made one more post....still looking at that sword, though **
 
Yes but 2 as I mentioned in post 28, cables running next to each other as 1 circuit is like 2 resistors in parallel. Less resistance and therefore less heat. That it my take on it anyhow.


But this depends on the respective values of the resistors you are talking about - as I mentioned earlier the smaller cables (parallel) have a higher resistance than the larger one you are replacing - so there is not necessarily 'less resistance'

not that this matters anyway because the current and resistance in all examples/comparisons we are making is constant
 
In the example I was using we are just talking about grouping factors. The cables are the same, each taking 100A example 1 is in parallel (2 cables touching) and example 2 is two circuits (2 cables touching.) You apply grouping factors to example 2 as per regs. In example 1 you dont have to as the two cables are in parallel (resistors in parallel), less resistance, less heat, no need to apply grouping factors.
 
Eng54:
I may have misinterpreted your take on this -
Are you saying that as it is not required (regs) and in your opinion unnecessary (given margins etc) - so you don't bother with it?

If that's the case we have no argument. My issue is with the view (as above) that there is some kind of difference backed up by, imo, flawed science.

Rockingit (assuming you haven't yet disemboweled yourself:

The cables (in spoons 2 examples) are identical in every respect that matters - current, resistance, power dissipation - so how can your configuration of wiring (at either end) effect/change this??
 
If you have a load that you put through a resistor the resistor gets hot... or hotter. If you then descrease the resistance then the heat will decrease. Agree? (talking simple terms now)
You are correct in that in my examples the cables are identical - current, resistance.
In example 2 say each cable had a resistance of 0.5 ohms. That is two seperate circuits giving off heat. Grouping factors needed.
In example 1 there are 2 cables in parallel, each 0.5 ohms, thats 0.25 ohms. Therefore producing less heat than the 2 cables at 0.5 ohms.
 
lets assume that these cables in both your examples are 35mm

All the cables individually have the same resistance over a given length

All the cables individually are carrying the same current (100A)

So All the cables individually will give off the same heat (power) - I squared x R


How can the ones connected in parallel (same circuit) be somehow cooler??? Where is this cooling effect coming from??? Don't forget the overall current in the parallel is 200A

I can't think of another way to explain this - perhaps someone else can explain it better - that's assuming there is anyone who actually agrees with me lol :)
 
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I am most probably explaining it in a way that you cant understand. Ill try to explain it again.

"All the cables individually have the same resistance over a given length" - correct.

The resistance of the 35mm cable is about 0.5 ohms per km. (or 0.05 ohms for 100 meters)


Lets say that the parallel circuit (example 1) is 100m long. You basicly have two 0.05 ohm resistors in parallel. Overall resistance of 0.025 ohms. Less resistance less heat.

Not wanting this to sound bad, but your comment of :

Eng54:
I may have misinterpreted your take on this -
Are you saying that as it is not required (regs) and in your opinion unnecessary (given margins etc) - so you don't bother with it?

If that's the case we have no argument. My issue is with the view (as above) that there is some kind of difference backed up by, imo, flawed science.

is what doesnt get me. You are happy to accept not applying grouping factors for a parallel circuit if its in the regs but you dont want to know why.
 
I am most probably explaining it in a way that you cant understand. Ill try to explain it again.

"All the cables individually have the same resistance over a given length" - correct.

The resistance of the 35mm cable is about 0.5 ohms per km. (or 0.05 ohms for 100 meters)


Lets say that the parallel circuit (example 1) is 100m long. You basicly have two 0.05 ohm resistors in parallel. Overall resistance of 0.025 ohms. Less resistance less heat.

Not wanting this to sound bad, but your comment of :

Eng54:
I may have misinterpreted your take on this -
Are you saying that as it is not required (regs) and in your opinion unnecessary (given margins etc) - so you don't bother with it?

If that's the case we have no argument. My issue is with the view (as above) that there is some kind of difference backed up by, imo, flawed science.

is what doesnt get me. You are happy to accept not applying grouping factors for a parallel circuit if its in the regs but you dont want to know why.

No - I agree with his reasons for not being concerned about it (cables run underground in ducting/low risk/margins etc)

You do realise that (in this hypothetical that we are discussing) each cable is carrying 100A (200A split equally between the 2 parallel cables)

Please read again my last post.
 

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