Re: Cat 5 for power

[Nigel Orr <nigel@xxxxxxx>]

At 09:46 18/10/00 +0000, you wrote:
>Can't seem to find this anywhere but what is the maximun
>power that can be safely drawn at 12v throught a Cat5 pair?

I've guessed in the past at a couple of hundred mA.  I use cat5 cables for
CCTV cameras, 20-30m runs, no problems, not measured the voltage drop or
current consumption, don't have any cat5 to hand here to measure the
resistance.

Bear in mind that it's _very_ unlikely that you'll get dangerous heating at
these powers, the most likely outcome if there is any problem is that the
voltage drop in the cable would be too great so the equipment you are
powering gets upset.

>openers. Typical power consumption would be around 250mA.

Is that continuous or peak?  You could have a local capacitor or
trickle-charged NiCad at each location, with a resistor in series with the
remote 12V supply to limit it's contribution to the current.  That would
work well with 'peaky' loads, like curtain openers, but wouldn't help with
cameras etc which draw fairly constant current.

At 250mA, and assuming 10-20m runs, I'd _guess_ that it would be fine, but
run a separate line from the supply to each appliance, so no sections of
cable are carrying more than that.

>Is this safe to do this

Should be safe, as long as it works.  I'll assume the voltage at the
terminals stays above 6V, or the equipment doesn't work.  If that's the
case, and it needs 250mA, the power loss in the cable would be 1.5W, which
should be fairly evenly spread along its length.  That's not likely to melt
anything, and is more or less 'worst case'.

You could 'double up' with another cat5 pair (but don't use 2 wires in 1
pair for 12V, 2 for 0V, use 1 wire in each pair for 12V, 1 for 0V), which
would reduce the losses in the cable, and the heat produced.

>or should I look at mains power and
>relays at all points. The other alternative would be to run
>a 12v "main" aound the appropriate areas to hang devices off.

If you're doing that, probably best to make it 15V and regulate it locally
to 12V where necessary, so it can cope with cable losses.

In summary, it's fine to try it- if it works, you can measure the voltage
drop (in Volts) along the cable and the current (in Amps) being drawn and
multiply the numbers to see how much heat (in Watts) the cable is going to
be dissipating.

Off the top of my head, 1.5mm^2 mains cable drops around 100mV per Amp
drawn, per metre of length.  That would be 2V drop for a 20m run at 1A, so
you could run that as a separate supply if the cat5 is inadequate for a
particular application.

As usual, I'll add the caveat that if this is a commercial application, not
your own home, it's better to fit the thicker cable or arrange local mains
supplies than 'suck it and see'- it depends who is paying for your time!

Nigel


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