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

I really apologise if I am coming across like Im trying to be a smart arse.... I dont mean to be... its just the way I phrase things sometimes.... just as the missus!
I would love for you to really explain it to me in my examples. I can same money here, but I cant give a reason why to my bosses as they think, like me, you apply it to both.

It does state in Table 4C1: Rating factors for one circuit or one multicore cable or a group of circuits, or a group of multicore cables.....

No problem Spoon.

All these terms relate to ''circuits or groups of circuits'' not to cables making up a ''single'' circuit''!! Ask your bosses why they don't apply a grouping factor to a trefoil run cable, or to the lowly ring circuit. Both are fundamentally exactly the same situation, both have either individual conductors touching or cables touching. Like the parallel single circuit, the method of installation is the initial determining CCC factor.... Other factors may be applied, depending if and when appropriate. Grouping factors however will only apply, if there are more than one parallel single circuit being run in or on the same containment etc...

And that's as far as i'm going, until my post #119 has been answered.... lol!!

 

[Spoon]

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

Before you try getting any smarter, you need to read what i actually said again!! What you seem to be talking about, is the current ratings associated with installation method, which is a totally different aspect altogether!!

As i've said a couple of times now, if your fool enough to employ a parallel single circuit, for all the advantages it will give you, and then start applying unwarranted factors, then you get on with it, or better still use the oversize single cable, it makes far more sense to you!!

What I was trying to say is that the current rating of cable is affected by the way it dissipates its heat. That is why the way you install it depends on the max current it can take.. Inhibiting the heat dissipation lowers the cable current capacity. EG putting it through insulation inhibits the head dissipation, that is why you de-rate the cable. Putting a cable next to another cable (touching) inhibits the heat dissipation of the cables. As Archy's wonderful post 111. Therefore this de-rates the cable. You accept this. (at least I think you do... im all confused now) What I cant get my head round is why you say this applies to example 2 of my question but not example 1.
In both examples the cables are physically the same... Everything is the same.. came current, same length, same heat dissipation. Therefore physics say that the de-rating applies to both examples.

 

[Rockingit]

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

:hanged:

 

[Engineer54]

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

What I was trying to say is that the current rating of cable is affected by the way it dissipates its heat. That is why the way you install it depends on the max current it can take.. Inhibiting the heat dissipation lowers the cable current capacity. EG putting it through insulation inhibits the head dissipation, that is why you de-rate the cable. Putting a cable next to another cable (touching) inhibits the heat dissipation of the cables. As Archy's wonderful post 111. Therefore this de-rates the cable. You accept this. (at least I think you do... im all confused now) What I cant get my head round is why you say this applies to example 2 of my question but not example 1.
In both examples the cables are physically the same... Everything is the same.. came current, same length, same heat dissipation. Therefore physics say that the de-rating applies to both examples.

OK, i'll answer that simple one for you!! lol!!

Simply, only because that is what is called for under BS7671, for two single circuits, so i couldn't say any different could i, not unless i opened myself up to a load more stick!! ...lol!! Not that i would necessary agree with the factor applied. Personally, i take all of the de rating factors in BS7671 with a pinch of salt, and prefer to do my own assessments based on the actual circumstances of the circuit or circuits i'm assessing. Again you need to remember that these tables are all based on worst case scenarios, (they can't be anything else with a single table) which more often than not, isn't the case at all!!

 

[Knobhead]

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

This is getting boring now!

 

[BrianDB9]

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

Get max demand worked out and apply what ever diversity you as suitably qualified electrical engineer decide - Voila job done !

 

[Gavin A]

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

Basic principles -
1) the ability of a cable to radiate heat is proportional to its surface area
2) i) The surface area of a round cable is given by pi D (or 2 pi R if you will).
ii) The surface area of a cable is a resultant of pi R2.

So, let's assume we have a solid conductor that is 10mm csa.
area = 10
dia = 3.56824823
circ = 11.209982432795858

therefore, 2 x 10mm csa = 2 x circ = 22.42 mm

If now do the same maths for a conductor of csa 20mm -
dia = 5.046265044040321
circ = 15.853309190424046

Now, as covered in earlier posts, J amount of energy will produce H amount of heat.

H / 22.42 for two conductors is a lower figure to
H / 15.85 for one conductor.

So, the heat generated is exactly the same (H is a constant), but by using two identical conductors you are sharing (thanks to Mssrs Ohm & Kirchoff) the load over a greater surface area of conductor which creates a fundamentally more efficient environment. Because the energy source is the same, there is no potential difference (to use a phrase in a different way) in temperature between them therefore there is no affect on one to the other, they both heat and cool at the same rate.

The radiator effect will, of course, be affected by environmental principles, thermal conductivities etc, so other de-rates may apply, but NOT those for grouping, as they are NOT grouped cables.

I think we're all talking at cross purposes.

You've just neatly explained why 2 cables with the same combined CSA as a single larger cable are capable of carrying greater current without overheating compared with the single larger cable, which AFAIK nobody is arguing against.

The question as I see it is whether those 2 cables run in parallel would be capable of taking greater current without overheating if they were run spaced apart or touching.

Engineer sidestepped that question before, but essentially unless you're arguing that there would be no difference, then by definition you're agreeing that a derating factor should be applied when the cables are touching vs when they aren't regardless of whether they are a parallel circuit or not.

I get that if you're not planning the cables to carry anything like their absolute maximum current ratings, as Engineer states he wouldn't, then in reality this isn't going to make a noticeable difference, but that doesn't mean that it's right to state that no derating should be applied for touching cables when you're looking at the absolute maximum loading the cables can take.

 

[Rockingit]

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

I'm just amazed we've managed to get 9 pages without anyone talking about resistance in parallel, either.....

:earmuffs:

 

[Silly Sausage]

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

I'm just amazed we've managed to get 9 pages without anyone talking about resistance in parallel, either.....

:earmuffs:

We're not that daft!

 

[Toonlad]

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

I'm just amazed we've managed to get 9 pages without anyone talking about resistance in parallel, either.....

:earmuffs:

We're not that daft!

That would be Epic