Twin and Earth cabling

Hi,

I'm using twin and earth cables for a office building refurb. In the BS 7671 regs i can only see cable current carrying capacities for these cables in thermal ceilings and walls, this will not be the structure for the office building internally. How can i size the twin and earth cables?  I.e are there alternative tables?  I hope you can answer soon.

Regards

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  • I believe i've seen on a few occasions T&E bundled and installed in basket. How would this installation cause the cables to deform? Is it just theoretical situations you are considering?  

    I understand that there is a problem with basket and the higher "category" data cables - as these need the pretty precise spacing of the twisted pairs to be maintained to perform correctly (hence things like the X shaped extrusions in the middle) . The normal installation recommendations even say to avoid ordinary ty-wrap type cable ties as a even a single overtightened one can significantly impact top data speeds (we're meant to use velcro ties instead). A good sized bunch of data cables can be heavy enough to similarly crush the lower cables on the bars of a cable basket - hence the mats.

    I'd be a lot less worried about 50Hz power cables where a small amount of distortion won't affect performance or longevity. Maybe if it was a very large bunch of cables the weight could be enough to damage the lower cables - but such bunches of mains cables must be pretty rate - if only from a grouping factor considerations.

       - Andy.

  • The trouble with basket is that the cables are bundled, so the rating becomes quite low quite quickly with quantity. OK for data but power cables, much more tricky. This cable distortion thing is largely imaginary, the effects of compressing cat 6 etc is increased crosstalk, but until this is operating at 100GHz a single cable tie width will make an almost imperceptible difference, and the impedance discontinuity is very short and very small. But then the people who write this stuff have probably never measured any effect properly with TDR and network analyser!

  • OK for data

    Not always ...  PoE ...

    I'd be a lot less worried about 50Hz power cables where a small amount of distortion won't affect performance or longevity

    Agreed in general. But that doesn't mean the manufacturer would if a problem were to occur.

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