Paralleling conductors

[overskilled]

Just bouncing something off people here..

Got a job on, customer currently has a single phase 230v 16Amp commando socket he wants changed for a 32A ( Seemingly has a few machines in mind with a larger than 16A draw but below the full 32 that he's considering buying in...hasn't decided which yet tho...)
Its currently supplied via 1.5mm2 6491X singles in plastic conduit (checked the book 4D1A, Ref method B and its good for 17.5Amps) run is sub 10 metres, but a pain in the backside to alter (heavy immovable stuff in the way for starters)
conduit is 20mm, so no way I'm going to pull 4mm2 or 6mm2 singles down there...and because of obstructions..I can't change out the conduit for bigger nor run another piece of conduit..
Socket has to go where its going.

So my only option it seems....pull in another 1.5mm2 line and neutral and parallel them (for some unknown reason the CPC is 2.5mm2 (who knows why...I didn't install it whenever it was done)...which should give me 35Amps so more than enough to cover matters...be a royal pain to pull it in more than likely...but with some extensive blue language...it should be doable. I don't especially like the idea...but I can't see any other way around the problem....

Anyone got anything to add that I've missed here?

 

[Richard Burns]

The paralleling of the cables would be acceptable and should be OK for CCC.
However if you can get two 1.5mm² cables in there why can you not pull in two 4mm² cables to replace the existing 1.5mm²?
There should be more space to do that than pulling cables against each other to pull in another two 1.5mm².

 

[overskilled]

partly - I'm concerned about 4mm "hooking up" on a couple of hidden bends (had an issue with that recently on a diff job, end kept catching on the entry or the exist of the bend, fish tape will work its way through and the 1.5s should pull through given they are thinner than 4s and a lot more flexible too..decent amount of lubricant will help matters too

Ideally I'd have the 20 out and replaced with 25 or better yet SWA...but neither is an option in this case...

 

[Lucien Nunes]

I'd also go for pulling new full size cables in rather than doubling up. If you are having to push a tape round bends past existing cables, there's a chance of damage and the new cables will end up interwoven with the old, making them harder to pull. Consider using fine stranded cable for flexibility, and ferruling at the termination. If you use the old cables as the draw wire and make a gently tapered joint to the new, smoothly taped with tape lapped the right way there's no reason for them to catch. FWIW (and I know it's not a legitimate conduit loading) I once got IIRC 4x 4mm + 6x 1.5mm through 20mm conduit with two 90's and a couple of light sets.

4D1A, Ref method B and its good for 17.5Amps.... ..pull in another 1.5mm2 line and neutral and parallel them...which should give me 35Amps

What about Cg = 0.8? Takes it the other side of 32A...

 

[overskilled]

Fair point on the cg. ...knew I was missing something (looking at calcs and knowing it didn't make sense given 4 is rated at 32 and 2 X 1.5 was coming out at 35...hence posting this...knew there was a flaw in my cunning plan.....lol)

Hmm some rethinking required here methinks. ...

 

[overskilled]

I did actually consider pulling 4 mm2 flex down it and ferruling the ends but didn't seem "right" if you know what I mean. Might end up being the easiest option tbh....

 

[Spoon]

What about Cg = 0.8? Takes it the other side of 32A...

There is not grouping factor for a parallel circuit. 0.8 is for 2 circuits, a paralleled circuit is just 1 circuit.

 

[Hellmooth]

Surely tying 4mm to existing 1.5mm cables and pulling through the conduit is going to be easier than attempting to add another set of 1.5mm through, or am I missing something?

 

[Spoon]

Just out of interest. Also please keep this clean..... What is the best lubrication for pulling cables through? I have seen washing up liquid used. What can you use and what can't you use? (e.g. products that react with the PVC)

 

[Hellmooth]

Yellow 77

Ideal Yellow 77 Wire & Cable Pulling Lubricant 950ml | Electrical Conduit | NoLinkingToThis

 

[Lucien Nunes]

There is not grouping factor for a parallel circuit. 0.8 is for 2 circuits, a paralleled circuit is just 1 circuit.

By the definition of a circuit as being fed from one OCPD, yes it's one circuit. But for grouping, ten pairs of cables each carrying 1/10 of a 100A circuit, will heat each other up as much as ten 10A circuits. The ten separate circuits would need derating to 4.8A, why would the ten paralleled pairs still be able to carry 10A each?

 

[Spoon]

By the definition of a circuit as being fed from one OCPD, yes it's one circuit. But for grouping, ten pairs of cables each carrying 1/10 of a 100A circuit, will heat each other up as much as ten 10A circuits. The ten separate circuits would need derating to 4.8A, why would the ten paralleled pairs still be able to carry 10A each?

I like you. I has the exact same discussion on this forum with Eng54 about 2 to 3 years ago. The thread went on for a bit, as some may remember.
I was sure I was right in that you had to apply grouping factors. Eng54 and Rockingit said I didn't.
I was wrong.

 

[overskilled]

Any link to that thread?
Sounds interesting

 

[Spoon]

Any link to that thread?
Sounds interesting

You did ask......
http://www.electriciansforums.co.uk...-under-buildings-backed-up-200-amp-fuses.html

 

[ImpededLoop]

You did ask......
http://www.electriciansforums.co.uk...-under-buildings-backed-up-200-amp-fuses.html

That link opens at the last post of the thread that finishes with one of Eng54's trademark "lol"s - which actually made me lol!

 

[Spoon]

It may be a good thing that it opens up at the last post. I pity the person who read through the whole thread. It nearly drove my crazy.

 

[overskilled]

I read it and I understand it....mostly....
But still losing the will to live

 

[overskilled]

On a side note saves me buying 6 mm for my own garage. ...Got lot of 1.5 6491x going surplus. ...

 

[HandySparks]

By the definition of a circuit as being fed from one OCPD, yes it's one circuit. But for grouping, ten pairs of cables each carrying 1/10 of a 100A circuit, will heat each other up as much as ten 10A circuits. The ten separate circuits would need derating to 4.8A, why would the ten paralleled pairs still be able to carry 10A each?

Quite.

I like you. I has the exact same discussion on this forum with Eng54 about 2 to 3 years ago. The thread went on for a bit, as some may remember.
I was sure I was right in that you had to apply grouping factors. Eng54 and Rockingit said I didn't.
I was wrong.

Just because one or more 'respected members' with high post counts argue against you, doesn't mean that you're wrong.

BS7671 doesn't appear to cover the subject explicitly (which is a bit of a mistake, in my opinion), but those arguing against de-rating of single-circuit parallel conductors need to read BYB Appendix 4, para 2.3.1 and 2.3.2 plus the notes below table 4C1.

"2.3.1: Methods of Installation A to D...
Current-carrying capacities given in tables 4D1A to 4J4A apply to single circuits consisting of :
(i) ... two single core cables or one two-core cable
(ii) ... three single-core cables, or one three-core cable."

Which could imply that they don't apply to single circuits consisting of other arrangements, ie parallel conductors.

"Where more non-sheathed cables ... are installed in the same group, the group rating factors specified in Tables 4C1 to 4C3 need to be applied."

I'm assuming that (i) and (ii) refer to single phase and 3 phase respectively.

2.3.2 Covers de-rating for other installation methods.

 

[Lucien Nunes]

OK I've just skimmed that thread and think I see where the all the disagreement came from, everyone is at cross purposes and all sorts of irrelevant arguments are thrown in along the way. I'm typing a reply that I think will settle it.

 

[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.