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 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.

I've side stepped nothing!! You have still not grasped the fact were talking about a single circuit, no grouping factors apply whether touching or not touching, ...end of!! Any initial derating as such, would have already been applied by the installation method, not by any grouping factor!! Now is that clear enough for you??

Only a bloody fool would design and run any circuit, parallel or otherwise at it's absolute maximum, your now making up scenarios to suite a non-existent argument!!


Tony's right, this is getting really boring now!!

 

[Gavin A]

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

of course I've grasped that, though I only have your say so that no grouping factors apply, and you've not backed that up by any logical reasoning so far in this thread.

So to me, heat is heat is heat, and a warm cable directly next to another warm cable will reduce the rate that each cable can dissipate that heat to air regardless of whether both cables are running as parallel cable on the same circuit or not, and 2 such cables in the same duct underground will result in double the total heat needing to be dissipated via the duct into the surrounding ground, so the internal air temperature in the duct will be higher and so will the temperature of the surrounding ground.

and yes you side stepped that point earlier, as you're doing again now.

Do you agree that 2 cables of the same CSA and running close to the maximum stated loading capacity for each cable will run hotter when touching, and/or in the same duct than when spaced apart or in separate ducts even when they're parallel circuits? If not, please explain why.

If you agree with this, then please explain why you state that no derating factor should be applied, as there's a logical disconnection there.


OK, so you say you'd not design a circuit to run at it's maximum rated output, which is fair enough, but the ratings you were giving in your post that started this discussion were maximum ratings, which you stated didn't need derating for grouping because they were in parallel.

 

[Gavin A]

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

You're right though, this is getting a bit silly.

Just to take the original question back to first principles...

95mm2 copper 4 core cable x 200Amps = around 7.8W per m per phase heat = around 31.2W / m of heat being generated.

Just based on my knowledge of ground source heat pumps, I can see that that shouldn't be a problem as heatpumps are generally rated at a maximum of 50W heat extraction from the ground per m of pipe, so the ground should have the ability to wick away 31.2W per m continuous heat output without the cable getting too warm, as long as the conduit isn't too insulating, which it shouldn't be, and it's not going to be continuous output anyway.


Not sure I'd want to be pushing much more than that though continuously, as for example 250Amps per phase would give around 48.8W / m heat output, which is getting towards the level at which the surrounding ground temperature would start rising significantly if that were continuous output.


figures based on Elland Cable resistance figures.

ps I've calculated that as if the neutral has the same losses / heat output as each phase. TBH I can't remember now if that's right or not.

 

[Gavin A]

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

Is the reason for this misunderstanding that there are 2 reasons for derating grouped cables

1 - Temperature
2 - Induction / eddy currents

I can see that there shouldn't be a need to derate due to the risk of eddy currents if it's the same circuit single phase, and if it's 3 phase then this shouldn't be much of an issue anyway (as I understand it) as long as it's relatively balanced between the phases, which I assume is what Engineer is referring to.

It won't have any impact on the temperature side of things though that I can see, where 2 cables next to each other will always have less ability to dissipate heat than a single cable would, especially in an underground duct.

 

[Chr!s]

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

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!!

No your wrong! I have 4 cable manufacturer's literature, all requrie multiple parallel cable's to be derated, the IET, ERA, NEC(American), IEC 60287.

 

[Engineer54]

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

This is like pulling teeth!! So ''I GIVE UP''!! The disbelivers had better stick to single cable supplies, because your going to end up with 2 or more parallel cables, that have conductor CSA's bigger than the original size cable you planned to replace in the first place!!

 

[Chr!s]

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

I'm loosing the will to live, now. FORGET THE SODDING TABLES!! (to a point).

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.

Your having a Laugh, i have IEC 60287, and you have to go through 4 pages of equations to get the watts/meter.

You will realise what goes into the tables and the likes in BS 7671 so you dont have to do all those equations. If you want to side step BS 7671 for grouping etc, then you will need to use IEC 60287 for compliance, the time isnt worth it on low voltage.

 

[Chr!s]

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

This is like pulling teeth!! So ''I GIVE UP''!! The disbelivers had better stick to single cable supplies, because your going to end up with 2 or more parallel cables, that have conductor CSA's bigger than the original size cable you planned to replace in the first place!!

There is a lot of rounding down by BS 7671 and up, but its not a bad thing.

Iet table 4C1 , 2 cables touching, factor 0.8, the ERA who the cable Manufacturers use 0.85.

Pulling teeth, you go through IEC 60287 to get your own factor's lol.

 

[Engineer54]

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

No your wrong! I have 4 cable manufacturer's literature, all requrie multiple parallel cable's to be derated, the IET, ERA, NEC(American), IEC 60287.

Really!! lol!! ....Post the pertaining sections up then, let's have a look at what your looking at or reading in to!! lol!!

 

[Rockingit]

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

Your having a Laugh, i have IEC 60287, and you have to go through 4 pages of equations to get the watts/meter.

You will realise what goes into the tables and the likes in BS 7671 so you dont have to do all those equations. If you want to side step BS 7671 for grouping etc, then you will need to use IEC 60287 for compliance, the time isnt worth it on low voltage.

I don't believe at any point we were trying to calculate watts/meter. We were taking those as a given (H).