Grouping factors on multicore cables

If you have 3 cables you need to look at the installation method. 4 circuits in each are well 4 circuits. This guy is trying to make some kind of point, but what is it? Does he not want to pay you, or doesn't like the "look" of the job? So far you have not shown any basic design problem, except probably that the circuits are not fused at 1.24A, so could possibly be "overloaded". This is a pretty false argument for this application as I expect each circuit feeds 1 LED luminaire. I need more info, if you don't want to post it try 07578 997199. Short calls are free to the emergency number!

Hi Dave - I may give you a call for a chat - thanks for the offer.


I think he dug himself a big hole going all-out on the cable type thing - the subject of my earlier post that you helpfully contributed to. The NICEIC audited that installation last week, reviewed all our documentation and passed it accordingly.  Now he has all that information, he's clealry looking for something else to prove that he is 'right' and we are not and/or to make us replace the cabling he doesn't like much.  I enitrely agree with you about the nature of his arguments but am just struggling to get him to shut up and go away (and pay us the money they are witholding....)!


Slight correction to my OP - each core of each multicore cable is used and wired from an individiual socket outlet on each of 4 lighting bars.  The sockets are paired up at the switch pack so it is a 12 way pack - each way feeding 2 outlets independently wired.  The 24 circuits are split across the 4 cables so 6 circuits in each cable.  Design current for each is indeed based on one LED fixture per circuit so is 0.62A (I originally thought the pairing was done on the bars but it isn't).  So maybe we should apply grouping factors for 24 circuits - table 4C1 doesn't go up that high though....


With such a low design current, arguably grouping can be disregarded as-per note 9 to 4C1, however whichever way I look at this practically we are so far on the right side of the border that this is all nonsense!


Thanks again,


Jason.

The fact that certain types of cables are mentioned in the regs does not mean you cannot use others, if you know what you are doing. This is now a well trod path for you...

The same applies to the de-rating. Just because there is not a table for  some particular installation method or arrangement of very many circuits, does not mean it cannot be made to work safely and well, if you have designed it properly.


The actual requirement  is that at full load it does not reach a copper temperature exceeding 70C in the most heated core (which may not be the one with most current, it may be the one with most hot neighbours.)

Given it is in actually wiring in place, and no longer a paper design, you could just do that. There are stickers that indicate a 'high tide' temperature that we often fit inside kit we think the customer is abusing - and if we open it up and the sticker has over heated, we know they have. These also have a place as part of inspections to see if sub mains overheat and so on, and are a lot cheaper than hiring logging equipment for months.

The other approach is to say what is the total current limit - for example a house may have a 60A company fuse, so even if there are 3 30 amp rings, a 40A cooker and shower cable, there is no need to consider them all under load at once when looking at cable grouping in the bundle going to the fuseboard - only combinations that add to less than the company fuse can occur for significant lengths of time.

 

But here, I think you have a lot of margin - consider that figure of  current is 30% means the heating is 0.3*0.3 or 0.09. of the full load heating, so that 40C rise from 30 to 70 becomes a 4 degree rise from 30 to errr, well about 34 degrees actually. So the assumption that can be ignored stems from that. It is not a safe assumption when you get cylindrical bundles of cables ten cables in radius,  but that is rare.

Given the total copper resistance and the currents, the dissipation in watts per meter of length can be deduced, and close behind that  and knowing the surface area for cooling is the core temp.

For the individual cables, look at the datesheets.


Mike.

Thanks Mike, you saved me some typing! Another way you could look at this Jason is as follows:

As the design current is low, put all the current in one of the cables, 4*0.62A (I may have misunderstood the exact current, but if a bit higher the method is fine. This is 6 circuits of 2.48 amps in one cable. I am assuming the cables for this example are 1.5mm² cores. Now to table 4C1. 6 circuits have a group factor of 0.57, and the cable rating is probably about 10A (I have no manufacturers data), so 5.7A per circuit is safe, in fact, you have a good margin if someone plugs in a multiple lead of some kind. The manufacturer will give a circuit rating with all cores loaded too I expect, I have been very pessimistic. Now you see that even if one cable carries all the current it is adequately rated, and as spread across 3 cables will be even safer, as the dissipation ability and surface area will be greater.


Feel free to ring

Regards

David

So expanding on these thought trains - the conductor operating temp for this cable is 90 degrees - so we have even more margin there.....


An equivalent approach on the calcuations could be - assume that each 19-core multicore cable is fully loaded - 6 circuits in each so a total of 3.72A in each multicore.  Individual cores are 1.5mm2 so the total load is effetively carried across 9mm2 in each cable, and then we have 4 cables.  If we took a tabulated rating for a 6mm2 cable from 4F2A for flexible cables that gives us 55A.  Grouping factor for 4 cables from 4C1 is 0.8 for Method E so should be good for 44A total - a good factor of 10 away from the load current in this case.


Is this a valid approach?


Jason.

Certainly Jason, and as you see there is loads of margin, but then anyone who has done this will have put 6 PAR64 2kW lamps on similar cable without problems for years!