95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fuses

[High Tower]

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?

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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..

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

I think Rockingit has given you a more than good enough explanation!!

Again i stress, that were talking here about a single parallel circuit, not a situation where there are two or several parallel single circuits grouped/run in a containment. In this case you ''Would'' apply a grouping factor for the multiple ''circuits'', as you would normally, for running multiple ''circuits'' in a given containment.... But notice that i stated circuits and not conductors/cables!! lol!!

On another note, ''All'' these de-rating tables (correction factors) are based on the worst case scenario, which can be quite misleading to say the least, for those that lack experience. Blindly using these tables verbatim, can and will lead to grossly over sized cables if they aren't anywhere near worst case in the first place... The times i've seen a cable on paper, that has gone through a number of correction factors, without any thought being applied, ending up too large to be physically possible to connect!! lol!!

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!

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

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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..

No your not on your own, i derate to!!

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Eng54. I understand that we are talking about a single parallel circuit. If you are also refering to Rockingit post #75 then he deviates from my examples of the cables being of the same length and carrying the same current. It does not answer my question in post 83.

Chris. Regarding your post #85: My argument is about applying groupling factors to parallel cables that are touching. Not what your maths are about.. Thanks for your post #86

 

[Engineer54]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

 

[Spoon]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

Deviating from me learning here
In my examples (post 83) all the cables are carrying the same current, are of the same length, same type of armoured cable.... therefore producing the same heat.
Please explain why you only apply de-rating factors for example 2.

If you use the argument ' ''All'' these de-rating tables (correction factors) are based on the worst case scenario' and 'Who in their right mind would install a circuit that is going to be on it's limits' then you wouldn't apply the correction factors for example 1 and 2. (This is not meant to come across insulting in any way or form)

 

[Chr!s]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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!!

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.

 

[Silly Sausage]

Re: 95 mm 4 core cable, 30 m run, in ducting under buildings backed up by 200 amp fus

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

Great discussion!

Even if you applied worst case group factor from Table 4C1 of 80%, the 2 cables above would be 762A.

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!!

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?!?!?!?