RE: Re: Future proofing cabling to downlighters with C-BUS

["Andrew Walker \(CV/ETL\)" <andrew.walker@xxxxxxxxxxxx>]

Yes, I would agree with that.  5M is quite a long length when you are
talking about 200W, 12V.

The current would be 16.5A with 4 x 50W downlighters on 1 off 2.5mm T&E
if my maths serve me right.

-----Original Message-----
From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf
Of JonShaw
Sent: 20 March 2007 22:03
To: ukha_d@xxxxxxx
Subject: [ukha_d] Re: Future proofing cabling to downlighters with C-BUS

>If I wanted to move to dimmable CFL's in the future and providing that
the C-BUS hardware was available to do this, what cables do I need to
lay now.

I have already wired using 2.5 T&E in case I want to go LV for whatever
reason using a common tranny in the garage, but I understand dimmable
CFL's will require a 3rd wire, what is this for, heating element?

To play it safe now would it be advisable to lay some CAT5 along the
T&E? or should I be using 3 core cable?

<



I've been looking at kitchen wiring recently too and recalled this post
.



I understand that there are several "standards" for dimming CFLs
one of
which using a 3rd wire to offer a 1-10V signal to the unit to telling
how dimmed it should be.  Other standards do not use this but for future
proofing it cannot hurt.



Cable thickness: I used the volt drop calculator on TLC
http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

And shocked myself by finding that to run 4 50W 12V halogens off a
single transformer over 5m would require 10mm cable!!! (if I've
understood it
right!)

Similarly with 2.5mm cable you would only be able to run a single 50W
bulb :-/

This is why I think local transformers are so common for LV systems and
why mains halogen seems so good (to me)

The disadvantage of mains holgen seems to be start-up loads burning out
the dimmers.



Cold filament = low resistance fire 240V and as the filament warms the
current drawn can be enough to burnout the dimmer.  Hence dimmers often
get de-rated for this type of application.

>From discussions I have had with Karam@idratek their dimmers do contain
some overload protection but he still thinks they need to be derated to
75% rated load when using mains hologens.  He also recommends fitting a
suppressor/snubber across halogens to help combat this start-up surge.
This is well described in this guide which I think looks applicable to
any automated relay dimming system be it C-bus idratek etc
http://www.idratek.com/public/WiringNotes/Guidelines%20-%20Electrical%20
Swit
ching%20Considerations.pdf





HTH

Jon



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