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I'm with e54, why would you derate a parallel cable, it seems pointless, they are both of the same origins and circuit destination so why would you not treat them as 1 cable.?? Or am I missing something?
Discuss 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses in the UK Electrical Forum area at ElectriciansForums.net
The larger won't emit MORE heat, it will emit the same amount of heat energy dissipated over a smaller surface area than the two parallel conductors which have a greater surface area - this is basic radiator principle. If N amount of electrical energy passing down R resistance of copper will produce J amount of heat, J becomes a fixed form. So all we are left with is simply J/circumference, of which 2 x ccc's are roughly 1/3 larger than a single one double the size. Do the maths if you don't believe me!
OK guys, it looks like I'm the only one here that applies a grouping factor to cables that are run in parallel (that are touching, Table ), when they feed a single source. So please can someone explain why they don't. As I posted in #40
(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.
You de-rate the cables due to heat disipation. Example 1 cables are producing the same heat as example 2 cables. In layman's terms, so a thicko like me can understand, why de-rate example 2 but not example 1? What is the logic in that... They are the same..
I have done the Maths lol, if think you need to!!
Example Table 4E1A
Ib = 700 amp, 2 Phase, Cables touching Ref Method C
2 x 150mm CSA
1 x 300 mm CSA
Your argument is missing a few key point's
2 x 150 = ccc = 952 amps
1 x 300 = ccc = 743 amps
Power dissiapted = I2R
Exactly my point about using worst case correction factors!! Your actually reducing any heat, by using two smaller CSA cables that ''CAN'' carry considerably more current, but actually isn't!! Who in their right mind would install a circuit that is going to be on it's limits?? lol!!
Do you apply a grouping correction factor to a circuit run in trefoil too?? Adds up to exactly the same thing after all!!
I have done the Maths lol, if think you need to!!
Example Table 4E1A
Ib = 700 amp, 2 Phase, Cables touching Ref Method C
2 x 150mm CSA
1 x 300 mm CSA
Your argument is missing a few key point's
2 x 150 = ccc = 952 amps
1 x 300 = ccc = 743 amps
Power dissiapted = I2R
Exactly my point about using worst case correction factors!! Your actually reducing any heat, by using two smaller CSA cables that ''CAN'' carry considerably more current, but actually isn't!! Who in their right mind would install a circuit that is going to be on it's limits?? lol!!
Do you apply a grouping correction factor to a circuit run in trefoil too?? Adds up to exactly the same thing after all!!
Re the Trefoil, single cores, Note 5 of 4C1, if a group consists of n single core cables it may be considered as n/2 circuits of 2 loaded conductors or n/3 circuits of 3 loaded conductors. ie 1 circuit.QUOTE]
This post and my question is about 4 core cable. Note 5 of Table 4C1 doesnt apply. Also note on Table 4C1 the title: Rating factors for one circuit or one multicore cable or a group of circuits, or a group of multicore cables, to be used with current-carrying capacities of Tables 4D1A to 4J4A
Great discussion!
Even if you applied worst case group factor from Table 4C1 of 80%, the 2 cables above would be 762A.
So, your effectively derating the cable yourself.
Re the Trefoil, single cores, Note 5 of 4C1, if a group consists of n single core cables it may be considered as n/2 circuits of 2 loaded conductors or n/3 circuits of 3 loaded conductors. ie 1 circuit.
I think I'm starting to get it?!?!?!?
Yes in open air the conductor operating temp will be lower in the parallel, but when we enclose and group the cables this greatly effects the parallels cable to effectively radiate that heat, thus causing the opersti.g temp to dramatically rise, so we have to derate.
Re the Trefoil, single cores, Note 5 of 4C1, if a group consists of n single core cables it may be considered as n/2 circuits of 2 loaded conductors or n/3 circuits of 3 loaded conductors. ie 1 circuit.QUOTE]
This post and my question is about 4 core cable. Note 5 of Table 4C1 doesnt apply. Also note on Table 4C1 the title: Rating factors for one circuit or one multicore cable or a group of circuits, or a group of multicore cables, to be used with current-carrying capacities of Tables 4D1A to 4J4A
I think you'll find, they are in all cases above, referring to circuits, rather than same circuit conductors!!
Or..... we go back yet again, to circuits run in Trefoil and even the humble ring circuit!!
Your quite at liberty to de-rate a parallel circuit, fortunately those that know better will not!! Your making it sound as if a parallel single circuit radiates heat like no tomorrow, when in fact sized correctly will do nothing of the kind!! The individual cables will be carrying far less current and thus less heat dissipation, than a far larger single cable, that even in itself won't be dissipating heat to anywhere near worrying levels. Your seeing/creating overblown problems, where none really exist!!
Your quite at liberty to de-rate a parallel circuit, fortunately those that know better will not!! Your making it sound as if a parallel single circuit radiates heat like no tomorrow, when in fact sized correctly will do nothing of the kind!! The individual cables will be carrying far less current and thus less heat dissipation, than a far larger single cable, that even in itself won't be dissipating heat to anywhere near worrying levels. Your seeing/creating overblown problems, where none really exist!!
We are talking about bunching cables together, 2,3,4 circuits and enclosed at that, now it's not all just Thermal, there is a thing called the proximity effect too, all this effects the conductor temp, it's well documented, these are IEC standards.
That's my point, parallel cables bunched and enclosed don't dissipate their heat well at all, add inductive issues like the proximity effect and the cable gets hotter with less current and can't dissipate that heat as quickly causing the conductor temp to increase.
Your far more likely to find a larger single cable getting a bit warm, than it's equivalent paralleled circuit, run in exactly the same conditions. Your still seeing a problem that doesn't exist!! !
Reply to 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses in the UK Electrical Forum area at ElectriciansForums.net
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