Multiple cables in one channel?


Postby MidAtlantian » Mon Aug 06, 2012 10:34 pm

When running 2.5mm or 4.0mm cables through a channel cut into stone walls - there will be capping and plaster over them - is there any limit to how many cables can run in one channel? i need to run from the consumer unit to a ring main AND a spur.

Also, when cables are running on the surface though a boiler cupboard, do they have to be in conduit?

Many thanks,
MidAtlantian
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Postby ericmark » Tue Aug 07, 2012 10:52 am

I would not want to cut channels in stone walls I would be looking at mineral insulated cable or maybe the newer Ali-tube cable.

I do not really see point of capping. Although it protects cable from plasters trowel the plastic stuff flexes so much it really makes plastering hard and the metal stuff falls foul of the regulations concerning walls with metal in them and protection methods.

Running cables where the ambulant temperature is higher than normal does require some de-rating of the cables. So I would look at thermosetting (90 degC) rather than thermoplastic (70 degC) cables. Or even with the stone walls mineral insulated cables.

I have with bare stone walls taken out some mortar pushed in the mineral cables and re-mortared so even with bare stone walls the cables are not seen. Because this cable can run hotter a 1.5mm sq is rated around 27 amp where with twin and earth even 2.5mm sq is only 22 amp approx of course as how installed will alter the current carrying capacity. Using thinner cable you do have to watch the loop impedance of course.

The regulations do say how to rate cable and give all the de-rating factors. However in the main one does not go to the limits and often one realises it does not require careful calculation. I did in college use the formula given in the regulations and work it out. The chance of error was high with so many powers of numbers being used and I would normally use excel so I could check first with known figures so I could highlight mistakes in formula used before using figures for exercise.

Since leaving college in the main I just add a bit and go up a cable size rather than sit there with calculator.

Warning making off mineral insulated cable glands requires some skill and I would not consider this cable for DIY use. The Ali-tube cable to BS 8436 Guardian, Earthshield, Flexishield, or Afumex is far easier to use. The diameter of ali-tube is greater than mineral insulated and the larger sizes are hard to buy in short lengths. There is also 624B Twin & Earth LSZH Cable 300/500V BS7211 with is 90 degs rather than 624Y Twin & Earth PVC Cable BS6004 A05VVH4-U which is 70 degs using LSZH Cable may be the answer.
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Postby MidAtlantian » Thu Aug 09, 2012 10:22 pm

Dear EricMark,

Many thanks for your response.

I thought that running "standard" twin & earth cables channeled into plaster on stone was standard fare in older houses! In fact, I would not expect the ring main to carry more than 5 amps, with brief periods ( a few minutes) of double that.

I thought that capping gave you some real protection since most masonry bits don't do well on steel, so that even unearthed, it is of value. In addition it gives you a sometimes clearer indication when using metal/current detectors to scan the wall before drilling. And if the cables are within 150 mm of the ceiling, capping is not required, and so it is just an extra precaution. Am I missing something?

The total length of the loop will be about 20 meters, so impedance should not be a problem.

But you've raised a lot of issues that I admit I am not familiar with: derating among them, and temperature considerations, although it makes sense that a cable buried in cement and plaster cannot dissipate much heat. Would it be better to lay out the three cables side by side in the channel?

Cheers,
MidAtlantian
MidAtlantian
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Joined: Mon Aug 06, 2012 10:22 pm


Postby ericmark » Sun Aug 12, 2012 4:25 pm

I will admit I have gone to jobs weighed it up and taken a chance and fitted stuff without testing first. I have also been caught out from time to time.

There is nothing to say you must use either metal or plastic capping. Unless the capping complies with BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436 then it does not help as far as protection goes. And although you can get capping which complies it's very heavy stuff not normal run of mill.

The problem is 522.6.8 Irrespective of the depth of the cable from a surface of the wall or partitions, in an installation not intended to be under the supervision of a skilled or instructed person. a cable concealed in a wall or partition the internal construction of which [b]includes metallic part[/b], other than metallic fixings such as nails, screws and the like, shall:

And it lists a loads of ways to comply. The use of metallic conduit puts the cable into the 522.6.8 and instead of making it easier it really makes it harder to comply.

Now in that "OR" list it includes using a 30ma RCD and all new sockets should have a 30ma RCD anyway. You don't give too many details and even if you did I would not want to say 100% what you are permitted to do it's too easy to miss something.
ericmark
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Joined: Fri Oct 02, 2009 8:49 pm
Location: Mold, North Wales.


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