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

[Engineer54]

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

Come on Eng54, you have to be taking the ---- now....
Ok, just look in the regs at correction factors for cables... they are all about heat dissipation of the cables.. Ambient temp, thermal insulation, buried in the ground.....
Look at the tables for the current capacity of cable eg. Table 4E4A the more surface area of the cable is in contact with the air the higher the current is.. Also heat dissipation...
Therefore two cables touching will not dissipate the same heat as two cables apart. Therefore you have to de-rate the cable. Again, single circuit or multi-circuits its all the same. The physics of it cant distinguish between 2 cables supplying 100A each for one circuit and 2 cables supplying 100A for 2 circuits. They are both producing the same heat....

I don't take the ****!!

Oh i forgot, none of this applies to a ring circuit does it!! It somehow doesn't need to have the same heat dissipation requirements of a similar circuit!!

Whatever you guy's want to do when running parallel circuits is up to you lot!! You'll be, all but defeating to whole point of a parallel circuit!!
I know that i'll wont be adding a grouping factor to any parallel single circuits i'll be seeing on this project!! And i'll be happily sleeping like a baby to the end of my days, in that knowledge!! lol!!!

 

[Rockingit]

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

I think what people may be overlooking here E54 (and I'm in full agreement with you), is that when you do the maths you'll find that N + N has a greater surface area than (2N) [if you see what I mean]. This is why two parallel conductors together can still dissipate more heat.

 

[Spoon]

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

Ok mate, it looks like you are not going to change your mind. I have tried to tell you the physics behind grouping factors. I just hope that the people who search this site looking for info on parallel circuits just dont look at your post count and then mine and say "Eng54 has to be right" and they look at the whole picture.
Anyhow Im going home in 5 mins, so the kids can bully me. Ill see in the morning if anyone agrees with me. Have a good one mate.

 

[Rockingit]

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

The physics of it cant distinguish between 2 cables supplying 100A each for one circuit and 2 cables supplying 100A for 2 circuits. They are both producing the same heat....

a
Sorry mate, I'm with E54 on this. They're not producing the same heat! Two parallel conductors of identical nature carrying I current at U voltage along R resistance will both be identical, whereas two different circuits will be operating at different lengths and voltage drops, hence they will never create the same amount of heat - hence one can induce heat in the other, whereas our parallel conductors are incapable of transferring heat energy one to the other as the figure is the same for both of them to start with.

If I put a jug of boiling water next to another jug of boiling water I don't create 200 degrees in between them, I've still got 100 degrees. But if I place one jug at 50 and one at 90 side by side then we get (in a perfect sealed system) 70 degs resulting between them.

 

[Darkwood]

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

I think what people may be overlooking here E54 (and I'm in full agreement with you), is that when you do the maths you'll find that N + N has a greater surface area than (2N) [if you see what I mean]. This is why two parallel conductors together can still dissipate more heat.

Im always open to debate and just as you posted this i was thinking the same ... the surface area of 2 cables is larger than that of 1 conductor of twice the size thus could omit the grouping factor this would be in agreement of E54's posts but up until this point was made it was hard to understand the reasoning but yep E54 can sit down with his cuppa and slightly smug smile il possibly re-think this area of the regs and may explain why it dosn't actually mention grouping factor directly in association of a multi cable parallel supply ..... but hey all ive wasted is a few half charlies spacing the swa's before overfill so if im gonna be wrong id rather it not effect the integrity of my previous work lol....

Browsing the tinterweb though i get conflicting results so assume some countries base ccc calculations different to ours and thus do apply this grouping factor for parallel cables of 1 circuit as their initial ccc of cables may be higher but subject to more derating.

 

[Chr!s]

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

Im always open to debate and just as you posted this i was thinking the same ... the surface area of 2 cables is larger than that of 1 conductor of twice the size thus could omit the grouping factor this would be in agreement of E54's posts but up until this point was made it was hard to understand the reasoning but yep E54 can sit down with his cuppa and slightly smug smile il possibly re-think this area of the regs and may explain why it dosn't actually mention grouping factor directly in association of a multi cable parallel supply ..... but hey all ive wasted is a few half charlies spacing the swa's before overfill so if im gonna be wrong id rather it not effect the integrity of my previous work lol....

Browsing the tinterweb though i get conflicting results so assume some countries base ccc calculations different to ours and thus do apply this grouping factor for parallel cables of 1 circuit as their initial ccc of cables may be higher but subject to more derating.

Work it out, 1 cable 300mm and 2 150mm, work out the mW/m, the larger cable will emit more heat.

When enclosed or grouped, the radiated heat loss is dramatically reduced, this is why we derate.

Practical tests were used with upto 37 grouped cables, if all were parallel, how would you expect the cables in the middle to perform?

There is also the theoretical models, all of which the table within BS 7671 is based upon.

I sure the NEC use the same method too, they just calculate In differently.

 

[Engineer54]

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

Im always open to debate and just as you posted this i was thinking the same ... the surface area of 2 cables is larger than that of 1 conductor of twice the size thus could omit the grouping factor this would be in agreement of E54's posts but up until this point was made it was hard to understand the reasoning but yep E54 can sit down with his cuppa and slightly smug smile il possibly re-think this area of the regs and may explain why it dosn't actually mention grouping factor directly in association of a multi cable parallel supply ..... but hey all ive wasted is a few half charlies spacing the swa's before overfill so if im gonna be wrong id rather it not effect the integrity of my previous work lol....

Browsing the tinterweb though i get conflicting results so assume some countries base ccc calculations different to ours and thus do apply this grouping factor for parallel cables of 1 circuit as their initial ccc of cables may be higher but subject to more derating.

I'm not being in anyway smug here, It's what and how i was taught, and what i've been applying for too many years to be told any differently now!! I've also never once seen a calculation placed in front of me, by any contractors Engineer, for approving parallel cable sizes, that have included a grouping factor. I work with quite a few different national standards, and too my knowledge, none of them apply or require a grouping factor for paralleled single circuits either!!

You haven't wasted anything, if your talking about spacing cables in a trench, I do the same. But my reasoning is for any future ground movement. I never run cables in trenches straight either, they are always laid with slight snaking along it's length for the same reason and to allow for any seasonal expansion/contraction that may occur etc....

 

[Rockingit]

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

Work it out, 1 cable 300mm and 2 150mm, work out the mW/m, the larger cable will emit more heat.

When enclosed or grouped, the radiated heat loss is dramatically reduced, this is why we derate.

Practical tests were used with upto 37 grouped cables, if all were parallel, how would you expect the cables in the middle to perform?

There is also the theoretical models, all of which the table within BS 7671 is based upon.

I sure the NEC use the same method too, they just calculate In differently.

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!

 

[Darkwood]

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

I'm not being in anyway smug here, It's what and how i was taught, and what i've been applying for too many years to be told any differently now!! I've also never once seen a calculation placed in front of me, by any contractors Engineer, for approving parallel cable sizes, that have included a grouping factor. I work with quite a few different national standards, and too my knowledge, none of them apply or require a grouping factor for paralleled single circuits either!!

You haven't wasted anything, if your talking about spacing cables in a trench, I do the same. But my reasoning is for any future ground movement. I never run cables in trenches straight either, they are always laid with slight snaking along it's length for the same reason and to allow for any seasonal expansion/contraction that may occur etc....

The smug was just humour and im not worthy ....:mickey:

 

[Knobhead]

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

Like E54 I've run countless parallel feeds, never applied a de-rating to them. As has been said it would make the whole exercise pointless.