RE: Low voltage / CAT5

["David Pullen" <dpullen@xxxxxxxxxxxxxxxx>]

John

I can't answer this question definitively but it is something that I
have struggled to find (only considering CAT5 here). I have used CAT5 to
power relays and LED lights. I think this may be a good question for
some sort of reference sheet on Automated Home although I am not sure if
it would qualify as a FAQ since I haven't seen it asked or answered
before!

I started off by looking at Power Over Ethernet specs (standard IEEE
802.3af).

Voltage between 44V and 57V
Maximal current 550mA
Typical current 10mA-350mA
Maximal power 15.4W

My understanding is that PoE uses two pairs so we have some indication
of maximum power although this is using a higher supply voltage that
allows lower currents and therefore less power loss.

The site www.powerstream.com provides information on maximum current for
power transmission and chassis wiring (See section on wire gauge and
current limits). This suggests maximum current of 3.5A for chassis
wiring (applies to wiring in air not a bundle) and 0.577A for Power
transmission for 24AWG wire. These figures are supposed to be very
conservative. The page also points out that these figures are rule of
thumb which may indicate why it is so difficult to find a definitive
answer.

Just out of interest if you double the cross-sectional area the wire
gauge decreases by 3. This would suggest two cores of CAT5 is equivalent
to 21AWG. In terms of power loss and current capacity this may not be
strictly correct especially if there are higher frequencies involved
(not a problem if it is strictly DC) but it is probably okay for rule of
thumb!

Information on ePanorama.net
(www.epanorama.net/documents/wiring/wire_resistance.html) indicates
"Ampacity" of 1.3 for 24AWG copper wire.

There appear to be two important effects that are taken into
consideration when calculating maximum current: voltage drop and heating
effect. The figures for voltage drop are generally such that the heating
effect would not be significant so voltage drop figures can be used to
calculate maximum current. However, guidelines seem to cover a wide
range. I have seen recommendations ranging from 2-6%.

Voltage drop can be calculated on the PowerStream page or you can use
the formula

V(drop) per hundred feet paired run = 0.2 * I(load) * 1.26^(AWG-10)

Unfortunately I have found that many of the formulae in this area
haven't been touched by metric units!

Using this formula, a run of 10m (pair of cores out and back), and a
supply of 12V suggests a maximum current of 0.4A for CAT5. If you double
up the cores (equivalent of 21AWG) then you should be able to get up to
about 0.8A.

Having said all this there are two further problems:

1	Most of the information on this topic is really focussed on
higher power applications eg mains or automotive wiring.

2	None of this takes into account twisted pairs.

Twisted pairs will have some effect but I do not think it will be very
significant (hopefully someone more knowledgeable will add to this). The
figures I have used do cover 24AWG so hopefully we are okay and all
figures are quoted as being very conservative anyway.

So what does it all mean? Well I have decided that I am happy to carry
about 0.4A per pair for a 10m run (I am using the voltage drop
calculation with 6%). I am also setting a maximum of 0.6A per pair even
for shorter runs that would allow higher currents with the 6% limit. I
think it would probably be safe to go above this but I want to make sure
that there are no problems.

I hope this helps and that others will add to this with their
experiences.

BR
David




Date: Tue, 24 Aug 2004 13:02:02 +0100
From: "Benfield, John (ITD)" <john.benfield@xxxxxxx>
Subject: Low voltage / CAT5

I'm looking to install a number of devices (CCTV & curtains) that will
require 12vdc. I would like to mount the wall warts somewhere else,
maybe
even centrally. Not being much of an electrician, is it possible to
workout
the amps (@ 12vdc) that either normal CAT5 or eight core alarm wire will
take?

JB