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

The post was about parellel circuits mate... :)
 
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'
 
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.
 
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, 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).
 
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
 
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.
 
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
 
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.
 
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)
 
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.
 
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.
 
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.
 
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!
 
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...
 
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...
 
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.
 
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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