Re: ethernet and AC in conduit
- From: Jeff Liebermann <jeffl@xxxxxxxxxx>
- Date: Sat, 03 Jan 2009 19:08:10 -0800
On Fri, 2 Jan 2009 12:12:13 -0800 (PST), seaweedsl
<seaweedsteve@xxxxxxxxx> wrote:
I know it's not recommended or allowed by the NEC , but I'm wondering
if anybody knows how likely I am to have problems if I run ethernet
and AC (120 volt) in the same conduit for 20 meters....
Other than it being in violation of NEC (National Electrical Code), it
will probably work if you have some spacing between the AC wires and
the ethernet cables, and if the AC wires are parallel, as in Romex.
Loose wires in conduit are going to be a problem.
The failure mode is 60 Hz pickup by the ethernet transceiver. Most of
the 60Hz will be rejected by the intentionally lousy frequency
response of the typical ethernet transformer. The CAT5 cable and
ethernet input circuitry also has very good common mode rejection,
which is mostly what's keeping stray signals out of your ethernet
transceiver. Under ideal conditions, the amount of AC pickup by the
CAT5 wiring should be identical in each wire and therefore cancel.
That's fine if there is some distance between the AC wiring and the
CAT5. Past about 4 wire diameters, the pickup on each wire is
essentially identical and your ethernet will work as advertised.
However, when the AC wire are at different distances from the CAT5,
you're going to get different signal levels on each CAT5 wire. Since
they're different levels, the common mode rejection of the input
xformer can't get rid of the crap.
The worst case situation happens when the wires are right next to each
other. Although the wire spacing is small, the differences in wire
spacing is large. Therefore the induced voltages are large.
Realistically, it takes a considerable length of parallel run AC and
CAT5 before there will be any noticeable problems. My guess(tm) is
that the problems start at about 25 meters with loose wires. Romex
will probably work over a longer distance.
The real danger is safety from electrocution and induced power line
glitches. I'm sure you put the convenience of sharing the conduit,
over having your insurance company invalidate your fire insurance and
the potential risk of electrocution. Therefore, I won't suggest that
perhaps the NFPA has a clue and that your safety might be worth
considering.
--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.
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