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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...
Discuss Cable ratings/sizes for SWA in the UK Electrical Forum area at ElectriciansForums.net
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...
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).
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.
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.
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.
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.
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?
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