Paralleling conductors

[overskilled]

I'm going to go interface my head with a wall...as I see how each angle is right...but they conflict...

I'm half tempted to parallel 2 x 1.5mm2 singles and bang 32A down it and see what temps it hits...just to do some practical experimentation...nah...my head is aching too much already...

 

[Simonslimline]

An interesting experiment.


https://www.youtube.com/watch?v=bBYHtBPbBJE

 

[Lucien Nunes]

Anyone still interested, see my explanation in the other thread. The gist is that if you work from the tabulated CCC of a single cable large enough for the circuit, and split it into multiple paralleled smaller cables of the same total CSA, you don't need to apply grouping factor. But if you start with the CCC of the each of the small cables and multiply by the number of cables paralleled, then you need to apply grouping factor because you will get a combined CCC higher than the single cable due to the higher permissible current density in ungrouped smaller cables.

Here, the OP has worked the second way. Instead of taking the CCC of a hypothetical 3.0mm² cable, splitting it into two 1.5's and rating each at half the CCC (which would give Iz<32A total but no grouping factor would be applicable), he has taken the CCC of ungrouped 1.5s and doubled it, giving Iz>32A to which the grouping factor must be applied - hence my comment.

 

[overskilled]

Ambients are given as 30C, with lowest correction factor as 25C (1.03 correction factor)
My garage / workshop is insulated as an example so generally holds 15 to 18C in the summer and around 12C in the winter without heat on, with heat on...17C at a max
Bs7671 seems not to have taken account of those of us living in...more frigid climes...

 

[overskilled]

To throw a spanner in the works

Regulations 433.4, 433.4.1, 433.4.2 and 523.8

No mention of derating parallel conductors, and all prior stuff in section 523 specifically mentions more than one circuit...

I think Eng54 and rockingit are right...

 

[Lucien Nunes]

Unfortunately I am not near a copy of BS7671 and won't be for some days. However I note the following from books referencing these regulations:

Bradley, J. 2009, 'A Practical Guide to the Wiring Regulations', Wiley, P131; 'When considering current-carrying capacity and the correction factors for grouping of a paralleled conductor circuit, each conductor should be regarded as a 'circuit'.

and

Locke, D. 2008, 'Guide to the wiring regulations (BS7671:2008)', Wiley, P73; 'It should be noted that grouping factors are applicable to parallel cables.'

Having read the old thread, there were a number of arguments put forward that while correct in themselves, did not prove or disprove the need for derating. For example, one contributor mentioned the lack of requirement to derate two grouped legs of an RFC. There is a perfectly good explanation for this because the RFC is a special case: The Iz of each leg is mandated by the regulations to have an Iz > In/2, due to the likelihood of unequal current sharing (otherwise, it would be acceptable to use 2x 1.5 T+E for an RFC). Since it is therefore impossible to load both legs to Iz at once, the grouping factor is automatically applied. Another argument concerned temperature rise of two adjacent hot bodies not giving rise to a higher temperature. This was irrelevant because it referenced sources of constant temperature, not constant heat input. There were other arguments and claims put forward one way or another that didn't correctly deal with the question in hand.

What I would like to see is an argument, based on sound physics, that grouped parallel cables comprising one circuit may be allowed to carry significantly more current than those of separate circuits, all other factors being identical.

 

[overskilled]

Lucien
I think the only way to solve this one definitively is to do some realworld testing...and it ain't going to be me..I don't have the kit to generate stable amperage at relevant voltages (to get things as "real" as possible) nor accurate enough temperature measuring equipment...

Regs even state that "The current-carrying capacities given in the Tables are provided for guidance. It is recognised that there will be some tolerance in the current-carrying capacities depending on environmental conditions and the precise construction of the cables" (note below 523.2) AKA we pulled these figures out of a certain orifice and we're currently hoping we didn't pick the wrong numbers...doesn't exactly inspire confidence in their accuracy / usefulness with statements like that being printed...

This could be one of those logic defying cases that contradicts "common sense" the sort where according to the hypothesis 1+2 = 3 but for some reason it comes out as 2.25 or 7

The surface area argument I can understand and see the viewpoint of - (assuming flowrates would be equal - I'm not a plumber so I have no clue if that bears out in reality or not) stick 2 x 10mm water pipes side by side and seperately a 22mm pipe - flow the same amount of water through each, the 2 x 10mm pipes would disperse more heat than the 22 due to the larger surface area, even allowing for them touching.

 

[Lucien Nunes]

Unfortunately that water pipe analogy is again totally irrelevant since the pipes themselves do not generate heat, so the heat output is a function mainly of the surface area and inflow water temp, rather than the cross section and flow rate which are the analogy of cable CSA and current.

Another reason these 'big cable, small cable' arguments are off-topic, is that the original debate is not about one big cable vs. two small cables. It's about two small cables making up one circuit vs. two small cables making up two circuits. The cables and currents are identical, only the application differs.

I have no objection to real world testing but electricity and heat obey the laws of physics and correct analysis yields correct results in a fraction of the time it takes to rig up the experiment!

 

[overskilled]

The point I was making was one of heat dispersion...crude analogy but the best I could think of, off the top of my head...

I need to do some more reading to try and confirm my thinking and be able to formulate a clearer answer...

On a side note - One point people are negating to consider...2 paralleled line conductors will be jointed mechanically or otherwise at each end, but it doesn't mean they will be laying hard against each other throughout the run and more than likely (given slack etc) they will be separate by at least 1 or 2mm probably more, which in this case is a cable diameter (1.5mm2 6491X comes in at just under 3 mm diameter)

 

[Lucien Nunes]

I'm not neglecting to consider it - the Cg I referred to is the one for cables sharing a containment, which couples them together thermally more than if they were in free air in the same relative positions. Not only must each cable transfer its heat to the air immediately surrounding it, that air must then pass the heat to the inside of the conduit, and the outside of the conduit must dissipate it again to the atmosphere. Because this heat transfer involves a temperature difference between the inside and outside air, any cable dissipating heat will heat the others, no matter how they are lying inside the conduit. That is why the Cg values for enclosed cables are more onerous than those for cables clipped direct, as they take this and various other thermal behaviours of a randomly arranged bunch of cables into account.

 

[overskilled]

One flaw in BS7671 compared with NEC is the lack of mention of fill ratio - i.e. 2 conductors in 20mm conduit has a much higher fill ratio and therefore less free air to dissapate said heat than 2 conductors in 25mm conduit etc...

Book is getting dumbed down bigstyle with worst case scenarios being treated as gospel rather than realistic scenarios...

Physics might not change but the figures given for the cables are lower than what they can carry (else half the street would going up in flames as the DNO cables ignite.....according to BS7671 those cables are all overloaded...)

Whats the actual sustained load on the circuit going to be..will that load be sustained long enough in all likelihood to cause a risk.

Anyone like to state how many circuits are running at 100% load constantly? (Without even mentioning the heavy derating that BS7671 applies to CCA as a safety margin - 30C ambient rating is a joke, never yet been in a british house that was hitting 30C 18-23C, lower still up here in Scotland or Yorkshire )

Apologies if this comes across as cranky rather than debatory lucien, my head is pouding, feeling like death warmed up and the more I read, the more it seems that Euro regs are written to the point of making a joke out of the cable sizes...the usual nanny state rather than trusting sparks to make the right call on cable sizing and giving them (and expecting those entering the industry to use) the mathematical equations to work it out, instead we get treated like idiot savants and told to slavishly follow a set of charts that often has no bearing to reality...nor to cables that are smaller CSA by a sizeable margin than the tables are requiring that have been carrying a load as big or bigger for donkeys years and yet have no sign of thermal damage, IR test out ok - yet they are condemned because someone thinks we live in the subtropics, with ambient temperatures hotter indoors than the average Spanish home, drowning in insulation and with every circuit running flat out 24/7/365 - Then happily let 2.2Kw Kettles be sold with 0.75 or thinner flex on a 13A fused plug, ditto for hair dryers, straighteners etc...all of which are far more likely fire risks...especially with the massive amount of counterfeit cheapenese flex on the market....
Oh dear lord...I'm turning into Eng54... :lol:

 

[Marvo]

One flaw in BS7671 compared with NEC is the lack of mention of fill ratio - i.e. 2 conductors in 20mm conduit has a much higher fill ratio and therefore less free air to dissapate said heat than 2 conductors in 25mm conduit etc...

I mentioned this in a thread not that long ago and I think it was Rockingit (apologies Rockingit if it wasn't you) that said there were stipulated permissible fill ratios in the BS7671 but they were basically just ignored.

 

[Marvo]

Here's the post http://www.electriciansforums.co.uk...-circuits-sharing-same-earth.html#post1124442 and it wasn't Rockingit, it was Spinlondon.