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probably an easy answer if you know it, unfortunately I don't (or just cant bloody remember)... but how do you work out cable sizes when using more than one conductor?

Basically, and I wont go into specifics, but there's been a lovely cock-up by the ground workers and instead of putting in a nice size duct for me, they've put in multiple small ducts, meaning I'm going to have to install multiple SWA's

Is it a simple case of the rating of a single cable (with normal correction factors applied) and multiple it to reach total loading requirement. Or what?

ie. if a 4mm SWA is capable of carrying 25amps over the distance, will 4x 4mms be capable of 100amps?
 
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K

Knobhead

probably an easy answer if you know it, unfortunately I don't (or just cant bloody remember)... but how do you work out cable sizes when using more than one conductor?
I can’t remember (can’t be bothered to look) how to do your work for you.

I can forward my scale of fees for consultancy work.
 
E

Engineer54

probably an easy answer if you know it, unfortunately I don't (or just cant bloody remember)... but how do you work out cable sizes when using more than one conductor?

Basically, and I wont go into specifics, but there's been a lovely cock-up by the ground workers and instead of putting in a nice size duct for me, they've put in multiple small ducts, meaning I'm going to have to install multiple SWA's

Is it a simple case of the rating of a single cable (with normal correction factors applied) and multiple it to reach total loading requirement. Or what?

ie. if a 4mm SWA is capable of carrying 25amps over the distance, will 4x 4mms be capable of 100amps?

More to the point, will the points of connection at each end of the cable be suitable for multiple conductors?? No need for correction factor relating to number of cables, as all the parallel cables of a circuit are classed as a single cable, much like a RFC.
 

spark 68

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Noooo! lol
Oh dear E54 you have started it now, I remember the long...long thread the last time this came up. lol

I am staying out of it lol
 
  • Thread Starter Thread Starter
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Ah. Can someone point me in the direction of the long thread then :)
connection points aren't an issue. I'm purely trying to confirm the cable calls and sizes. It's been a long, long time since I've had to consider it, that's all
 
E

Engineer54

Noooo! lol
Oh dear E54 you have started it now, I remember the long...long thread the last time this came up. lol

I am staying out of it lol

So let em come at me again, ....the answers they get, will still be exactly the same!! lol!!
 

spark 68

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It was said tongue firmly in cheek, lol

Because we ended up with long theories about thermal transfers and god knows what else, partly caused by the reg book tables and definitions etc..ad nauseum lol
 
  • Thread Starter Thread Starter
  • #9
Mmmmm thermal transfers, think I'll keep away from that, that really is going back to college/uni days... Lol
i was particularly keeping away from asking anyone to do the calcs for me. Just someone to point me in the right direction before I try and dig out my 15th edition and search through my notes stuck inside the back cover :)
 
  • Thread Starter Thread Starter
  • #11
Not sure if 3am is a good time to start reading :)
you sure there isn't a quick answer lol
 

spark 68

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Reading that thread you will use up time you will never get back lol

It was partly down to the regs derating factors for parallel supply cables argued in favour of by some knowledgeable chaps on here, but on the other hand dismissed by some other equally knowledgeable chaps on here.

Both sides made some good points, me ? I have never had to do the calcs for the size of cables in that thread, if it was smaller sizes I think I would probably err on the side of the regs, the larger sizes I would probably have to get help from someone who deals with that size of cable regularly.
 
E

Engineer54

It was said tongue firmly in cheek, lol

Because we ended up with long theories about thermal transfers and god knows what else, partly caused by the reg book tables and definitions etc..ad nauseum lol
I just found it quite amazing that anyone would even consider using a grouping factor on what is a single circuit... No-one ever did post a link to any of the other international standards that supposedly added a grouping factor to a parallel single circuit. I know they don't anyway, because i bloody well use many of those standards on my projects!!

All i can say is, there must be some pretty ''hot'' parallel circuits that i've left up and running around the world, one or two with in excess of 10 single conductors per phase. Now run that through a grouping factor table, and you'll see just how bloody daft things will get!! lol!!
 

Richard Burns

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probably an easy answer if you know it, unfortunately I don't (or just cant bloody remember)... but how do you work out cable sizes when using more than one conductor?

Basically, and I wont go into specifics, but there's been a lovely cock-up by the ground workers and instead of putting in a nice size duct for me, they've put in multiple small ducts, meaning I'm going to have to install multiple SWA's

Is it a simple case of the rating of a single cable (with normal correction factors applied) and multiple it to reach total loading requirement. Or what?

ie. if a 4mm SWA is capable of carrying 25amps over the distance, will 4x 4mms be capable of 100amps?

The easy answer to your question is read appendix 10 of BS7671: overcurrent protection of cables in parallel.
Spoiler: it says sum the CCCs!

and just for contention it also says apply appropriate grouping and other factors!
Though appendix 4 tangentially mentions that the number of cables equals the number of circuits!
 

Spoon

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I just found it quite amazing that anyone would even consider using a grouping factor on what is a single circuit... No-one ever did post a link to any of the other international standards that supposedly added a grouping factor to a parallel single circuit. I know they don't anyway, because i bloody well use many of those standards on my projects!!

All i can say is, there must be some pretty ''hot'' parallel circuits that i've left up and running around the world, one or two with in excess of 10 single conductors per phase. Now run that through a grouping factor table, and you'll see just how bloody daft things will get!! lol!!
Now now eng54. I know its Monday morning but there is no need to be nasty. :)
I am grateful that you and Rockingit helped me out there. Especially Rockingit for finally commenting on why grouping factors are not required for a single circuit.
 
E

Engineer54

Now now eng54. I know its Monday morning but there is no need to be nasty. :)
I am grateful that you and Rockingit helped me out there. Especially Rockingit for finally commenting on why grouping factors are not required for a single circuit.
Eh, ...Please point out to me, exactly where i am getting ''Nasty''??? :)
 
H

Hawkmoon

Now now eng54. I know its Monday morning but there is no need to be nasty. :)
I am grateful that you and Rockingit helped me out there. Especially Rockingit for finally commenting on why grouping factors are not required for a single circuit.

That's handy - there was me, all this time, fully unwinding my extension lead to feed my 3 bar fire!
 
H

Hawkmoon

The post was about parellel circuits mate... :)
I know that, I wasn't being entirely serious -

Having read the other thread (long one), I still did not see a convincing answer to your two example comparison - If you are designing to use the smallest poss parallel cables then the mutual heat dissipation (applicable to those size cables) must at some point become a valid issue, regardless of the characteristics of the original larger one.

I understand Eng54's point about unnecessary de-rating of cables but if anything goes t*ts up then at least you can say you went 'by the book'
 

Rockingit

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ROFLMAO..... It's always nice to get a mention in dispatches....

Right - herewith a very simple analogy: I have a large tank of water at a constant 60deg C. I take two pipes from it, and feed them into another tank which is empty. So I now have two pipes with water in them, each at 60C, yes? When they arrive at the other end and mix, they are still at 60C, yes? (in a perfect circuit, granted). So, if they are side by side and touching along their journey, do they interfere with each other and create 120C? No, of course not. Why? Because it is the SAME energy that is starting and finishing in the same place, using identical means of transport.....in other words, two identical conductors, in parallel.
 
S

Silly Sausage

If, and only if, these two pipes had perfect thermal insulation. (or had zero thermal conductivity)
or the external temp was also 60 degC.
 
H

Hawkmoon

All circuits in any installation are in parallel - supply live enters building, splits and feeds circs, returns to neutral bar and exits on supply neutral...

You have three single phase circuit feeding 3 equal loads, lets say 100A each in same containment - this is still 3 circuits in parallel - can you then ignore grouping?
 

Rockingit

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If, and only if, these two pipes had perfect thermal insulation. (or had zero thermal conductivity)
or the external temp was also 60 degC.
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).
 
H

Hawkmoon

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).
Yes but you are again comparing with the one cable option after altering the characteristics of the circuit...

Your two parallel cables don't 'know' that they are linked at the ends - this is no different to two cables feeding equal but separate loads.

I agree that the energy dissipation overall is the same but you have changed the means of transport
 

Spoon

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Hi Hawkmoon. This is what I concluded from the parallel post I was involved in: Anyone... If I'm wrong then correct me. This is in basic terms.....

My question was.

Example 1. We have a circuit taking 200A run in conduit. The intention is to use 2 cables, each taking 100A. Each producing heat.
Example 2: We also have 2 additional circuits each using 100A, supplied with two cables in a ducting. (different ducting to example 1.) going to different items.
Why does the de-rating factor applies to example 2 but not to example 1? How does that work?

In post 129 of the old thread, Rockingit mentions 'resistance in parallel'.


The cable gets hot due to the resistance running thought it. The higher the resistance the hotter it gets. Therefore you apply de-rating factors to allow for the extra heat of two cables together.

Example 1 is basically 2 resistors in parallel, therefore the cables will not get as hot as the cables in example 2. In example 1 the resistance of the two cables is halved (2 resistors in parallel) so therefore there is less heat. (not quite sure if you halve the resistance you halve the heat) Therefore you don't need to apply de-rating factors.

Example 2 is: 1 resistor going into to 1 supply and another resistor of the same value going to another supply. The supplies are not the same so the circuit is not parallel. The cable resistance therefore does not change so you have to apply the de-rating factors.
 
H

Hawkmoon

Hi Hawkmoon. This is what I concluded from the parallel post I was involved in: Anyone... If I'm wrong then correct me. This is in basic terms.....

My question was.

Example 1. We have a circuit taking 200A run in conduit. The intention is to use 2 cables, each taking 100A. Each producing heat.
Example 2: We also have 2 additional circuits each using 100A, supplied with two cables in a ducting. (different ducting to example 1.) going to different items.
Why does the de-rating factor applies to example 2 but not to example 1? How does that work?

In post 129 of the old thread, Rockingit mentions 'resistance in parallel'.


The cable gets hot due to the resistance running thought it. The higher the resistance the hotter it gets. Therefore you apply de-rating factors to allow for the extra heat of two cables together.

Example 1 is basically 2 resistors in parallel, therefore the cables will not get as hot as the cables in example 2. In example 1 the resistance of the two cables is halved (2 resistors in parallel) so therefore there is less heat. (not quite sure if you halve the resistance you halve the heat) Therefore you don't need to apply de-rating factors.

Example 2 is: 1 resistor going into to 1 supply and another resistor of the same value going to another supply. The supplies are not the same so the circuit is not parallel. The cable resistance therefore does not change so you have to apply the de-rating factors.
For this halving of resistance to be relevant all resistors have to be of equal value - the individual resistance of one of the smaller cables does not equal the resistance of the larger...

I can't believe they have won you over on this one Spoon, nobody has yet explained how this differs from two separate but equal loads - as per your examples.

All circuits in electrical systems are in parallel as I explained earlier
 

Spoon

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You are using the same size cable: For example table 4E4A: ref method D, 35mm cable, 115A.
Lets forget about VD and all the other factors for now, but for my examples you could use 35mm cable.
The lengths are the same, the cables are the same size so the resistance are the same.
 
H

Hawkmoon

You are using the same size cable: For example table 4E4A: ref method D, 35mm cable, 115A.
Lets forget about VD and all the other factors for now, but for my examples you could use 35mm cable.
The lengths are the same, the cables are the same size so the resistance are the same.
But higher resistance individually than the larger single cable that they are being compared to...

Lets assume large cable is 10ohms

Smaller cables are 20ohms each

The two smaller in parallel do not equal half of the larger (in this example the overall is the same 10ohms)


All the cable in the ground is interested in is how much current it is carrying and how quickly it can dissipate heat - this will obviously be hindered by an adjacent cable also giving off heat (regardless of how each cable is connected further down the line)
 
S

Silly Sausage

Something to bear in mind is that the current capacity/unit csa increases with decreasing csa.
eg, Two 25mm[SUP]2[/SUP] can carry >2x current than that of a single 50mm[SUP]2[/SUP].
So much so, that even applying a two cct de-reating factor to the two cables, the CCC is still greater than that of the single larger cable.

Try it with some figures off Table 4D1A.
 
H

Hawkmoon

Something to bear in mind is that the current capacity/unit csa increases with decreasing csa.
eg, Two 25mm[SUP]2[/SUP] can carry >2x current than that of a single 50mm[SUP]2[/SUP].
So much so, that even applying a two cct de-reating factor to the two cables, the CCC is still greater than that of the single larger cable.

Try it with some figures off Table 4D1A.
Yes but the issue here is sizing the smallest cable possible to carry 100A while at the same time ignoring grouping.
 

spark 68

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Another thing to bear in mind chaps is the OP of this thread has multiple smaller ducts (by mistake), so may not be running his cables together physically, ie. seperate cables in seperate ducts.

Intersting thread though.
 
H

Hawkmoon

Another thing to bear in mind chaps is the OP of this thread has multiple smaller ducts (by mistake), so may not be running his cables together physically, ie. seperate cables in seperate ducts.

Intersting thread though.
lol - I think it was posts 4-5 that derailed the thread train!
 

Spoon

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Looks like the OP didn't get back. Wondering if he has sorted it out?

I thought the thread shifted from the original multi conduit to 'not having to use de-rating factors for a single circuit parallel route'? Or has it shifted again...
 

spark 68

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Not sure Spoon, The OP was looking for an answer to his problem before I had inadvertently side tracked it #5. lol
 

Spoon

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Looks like he was a bit vague about what he actually needed. Its hard to help out when you dont have all the info.
Hopefully he should get his answer by reading posts 5 onwards...
 

Rockingit

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I think Eng and Tony will probably back me up on this when I say that if the OP is dealing with having to carry different cables through different routes, therefore possibly ending up with different lengths and so on, that they wouldn't really be identical conductors which is an integral condition for parallel supply; at which point I think you probably would have to take a common sense approach to how best to apply any derate factors. That being said, if your cable resistances (as installed) were within a nats todger of each other then I don't think I'd be overly concerned - again, some common sense would need to prevail.
 
E

Engineer54

All circuits in any installation are in parallel - supply live enters building, splits and feeds circs, returns to neutral bar and exits on supply neutral...

You have three single phase circuit feeding 3 equal loads, lets say 100A each in same containment - this is still 3 circuits in parallel - can you then ignore grouping?
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....
 
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H

Hawkmoon

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
 
S

Silly Sausage

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
 
  • Thread Starter Thread Starter
  • #43
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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Engineer54

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!!
 
E

Engineer54

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...
 
H

Hawkmoon

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.
 
E

Engineer54

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!!
 

Rockingit

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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?
 
H

Hawkmoon

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.
 

Spoon

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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.
 

Rockingit

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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.
 

Rockingit

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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 **
 
H

Hawkmoon

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
 

Spoon

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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.
 
H

Hawkmoon

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??
 

Spoon

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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.
 
H

Hawkmoon

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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Spoon

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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.
 
H

Hawkmoon

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.
 

Spoon

-
Mentor
Arms
Esteemed
I do realise that each cable is carrying 100A.

In example 1 they are running through a cable ( 2 cables in parallel) having a resisance of 0.025 ohms.
In example 2 they are running through a cable ( 2 cables, 2 circuits) having a resisance of 0.05 ohms.

Therefore the head created by example 1 is going to be lower that the heat created by example 2.
 
H

Hawkmoon

I get it...

You are winding me up!

This can be the only explanation lol


If not, reread post 58 - the answers are there - the penny may drop
 
E

Engineer54

What i'm saying is, that you don't apply a grouping factor to a single circuit, end of!! Can't get a much simpler explanation than that. For those that still think you DO, then explain to me why you don't apply the same requirement to RFC's run in containment, or a Trefoil run, (3 or 4 single cables grouped/bunched together) three phase supply circuit, which will often be supplying unbalanced end loads??


Jesus, this is harder than pulling teeth!!
 
E

Engineer54

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.
I'm not saying that at all!!

No ''competent'' engineer or electrician, would ever design a circuit of any description which will run at, or that close to the cable(s) current rating. Now if you're going to play around with those sort of nonsensical scenarios, we really are just wasting our time here....
 
H

Hawkmoon

What i'm saying is, that you don't apply a grouping factor to a single circuit, end of!! Can't get a much simpler explanation than that. For those that still think you DO, then explain to me why you don't apply the same requirement to RFC's run in containment, or a Trefoil run, (3 or 4 single cables grouped/bunched together) three phase supply circuit, which will often be supplying unbalanced end loads??


Jesus, this is harder than pulling teeth!!
This is allowed for in current carrying capacities in BS7671 (3 phase cables of given size always lower value than twin)

...and the RFC issue is probably a compromise because, lets face it, the cables are hugely underloaded (2 x 26A = 52A capacity on a 32A breaker?)
 
H

Hawkmoon

I'm not saying that at all!!

No ''competent'' engineer or electrician, would ever design a circuit of any description which will run at, or that close to the cable(s) current rating. Now if you're going to play around with those sort of nonsensical scenarios, we really are just wasting our time here....
If you read my post after the one you are referring to you will note I acknowledged that I may have misinterpreted you - the parallel issue was largely from spoon and rockingit
 
E

Engineer54

This is allowed for in current carrying capacities in BS7671 (3 phase cables of given size always lower value than twin)

...and the RFC issue is probably a compromise because, lets face it, the cables are hugely underloaded (2 x 26A = 52A capacity on a 32A breaker?)
Which is exactly the same as i stated earlier, CCC capacities will be determined by the installation method used along with any other applicable correction factors, and is all that is required for any Single separated circuit!! So what's the difference then??

As far as the RFC goes, Not if we apply your previous perameters, eg runing the RFC at full load!!

So in other words you don't have an answer do you??
 
H

Hawkmoon

Which is exactly the same as i stated earlier, CCC capacities will be determined by the installation method used along with any other applicable correction factors, and is all that is required for any Single separated circuit!! So what's the difference then??

As far as the RFC goes, Not if we apply your previous perameters, eg runing the RFC at full load!!

So in other words you don't have an answer do you??

You see in my opinion your modified parallel circuit is not a situation allowed for in predetermined ccc, you have 2 cables, not one - this setup is not covered (imo) by the 'grouping factor only separate circuits' point in the regs.
With regard to full load of the cables, you won't even know what that is unless you apply relevant factors - imo this modified circuit should include at least a consideration toward heat dissipation.

I have only mentioned high loading of the cables as we are trying to install as small a cable as poss to carry the 100A (half the parallel demand)

My issue mainly has been that some are believing that heat in the cables will somehow be diminished because they are wired in a parallel formation (that's what I am arguing in post 58)
 
H

Hawkmoon

Hell's Bell's, ..... I give up!! ...lol!!
I answered your points, don't see where your frustration is...

you have chosen to 'interpret' the regs to suit your modified circuit, I have explained why I don't agree...

like I said we should agree to disagree on this :)
 
E

Engineer54

I answered your points, don't see where your frustration is...

you have chosen to 'interpret' the regs to suit your modified circuit, I have explained why I don't agree...

like I said we should agree to disagree on this :)

Eh, Where are you getting this ''Modified Circuit'' from?? Parallel supply circuits are, ...always have been and will continue to be a Standard/Normal circuit wiring method!!!!... Nothing ''modified'' about them... I'm Not interpreting the reg's to suit myself either, YOU DON'T APPLY A GROUPING FACTOR TO A SINGLE CIRCUIT!! ..End OF, it's bloody nonsense!!!
 
H

Hawkmoon

Eh, Where are you getting this ''Modified Circuit'' from?? Parallel supply circuits are, ...always have been and will continue to be a Standard/Normal circuit wiring method!!!!... Nothing ''modified'' about them... I'm Not interpreting the reg's to suit myself either, YOU DON'T APPLY A GROUPING FACTOR TO A SINGLE CIRCUIT!! ..End OF, it's bloody nonsense!!!

What if you had 10 cables, parallel circ as you describe, all tightly grouped and carrying 20A each, would you consider a grouping issue then? Oh but then it's only one circuit!

I can leave my extension lead fully wound with my 3 bar fire plugged in - its only one circuit!


Seriously Eng54, we are NEVER going to agree on this because we each believe we are right...End OF, to continue is bloody nonsense!!!
 
E

Engineer54

What if you had 10 cables, parallel circ as you describe, all tightly grouped and carrying 20A each, would you consider a grouping issue then? Oh but then it's only one circuit!

I can leave my extension lead fully wound with my 3 bar fire plugged in - its only one circuit!


Seriously Eng54, we are NEVER going to agree on this because we each believe we are right...End OF, to continue is bloody nonsense!!!
That circuit was a supply off a distribution transformer to a Switchboard, they carried a damn sight more than 20A a cable too, ....and NO, a grouping factor was NOT applied!!

Dear oh dear, perhaps you had better ask yourself ''WHY'' a roll of extension flex heats up under load, certainly has nothing to do with what is being discussed here??

To be honest, you had better stick to single cable supply circuits, just god help you if that works out to be a 1000mm, cause trying to multi parallel, and adding in a grouping factor, you're going to end up with in excess of 1000mm!!

Good idea, NO, you'll never convince me that a single parallel supply circuit needs a grouping factor applied!! lol!!
 
H

Hawkmoon

That circuit was a supply off a distribution transformer to a Switchboard, they carried a damn sight more than 20A a cable too, ....and NO, a grouping factor was NOT applied!!

Dear oh dear, perhaps you had better ask yourself ''WHY'' a roll of extension flex heats up under load, certainly has nothing to do with what is being discussed here??

To be honest, you had better stick to single cable supply circuits, just god help you if that works out to be a 1000mm, cause trying to multi parallel, and adding in a grouping factor, you're going to end up with in excess of 1000mm!!

Good idea, NO, you'll never convince me that a single parallel supply circuit needs a grouping factor applied!! lol!!

Eh? - I just made that up to explain my point - not real

Ahh - I see we are now dropping to the more 'personal' level - I am not an idiot just because I disagree with you Eng54

There will be no more input from me on this.
 
E

Engineer54

Eh? - I just made that up to explain my point - not real

Ahh - I see we are now dropping to the more 'personal' level - I am not an idiot just because I disagree with you Eng54

There will be no more input from me on this.
To be honest, that's more to do with frustration, than being personal!!
 
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