[mythtv-users] OT: Wiring a new construction home for A/V, Ethernet, etc

Allen Edwards allen.p.edwards at gmail.com
Wed Dec 3 16:05:53 UTC 2008


> I would not suggest doing it yourself unless you are significantly more
> skilled than the average Do-It-Yourselfer, if for no other reason than the
> fact that having a licensed electrician on the job tends to make things go
> MUCH easier with the building inspector (and maybe the G.C.).

He is only talking about stringing the wire, not connecting it up.
Should not be a problem especially if done under the supervision of
the electrician.


> They do not need to -- and pending how extensive the other associated
> equipment in your A/V system is (or might become), probably should not --
> be on the same circuit.

I have a projector, a 1800 watt sub wolfer, a Class A power amp, and a
surround amp, a computer, a preamp, DVD player, and more running on
one circuit.  If you really needed several circuits like you said, you
would also need an air conditioner in the winter in that room.  I am
assuming a new modern LCD projector and they just don't draw that much
current.  About like a couple of light bulbs.

You are correct in that it is important that you be on one phase.  It
is possible that would be enough, if you could guarantee that nobody
ever added a circuit and changed the phase of the existing breakers in
the process.  That is a big assumption, in my opinion.  Pull a breaker
out and plug it in the next slot and it is on a different circuit just
like that.


>  > The other issue is grounds.  You want everything grounded to the same
>  > reference point.
>
> Agreed, but it goes beyond that.  More importantly, you also want
> equivalent paths to ground for each branch circuit (at least those which
> will be used for any part of the A/V system).

In your own words, this is just plain wrong.  There is no current in
the ground wire.  It just doesn't matter how long they are.  It also
doesn't matter if they are short or long unless they get struck by
lightening and in that case I don't think it matters but for a
different reason (like it doesn't matter the size of the shot gun that
shoots you in the face).

>
>  > This goes for the TV antenna,
>
> That is pretty much impossible to achieve in practice, in part because of
> code issues.  In every jurisdiction I'm familiar with, a ground block is
> required on the exterior of the building in the immediate vicinity of where
> the downlead enters the structure; and a dedicated ground MUST be run from
> this block by the most direct path possible, even if that means (as it
> sometimes does) a separate copper rod in the dirt.
>
> The good news is, this really doesn't matter.  A properly designed antenna
> system will NOT forward-pass anything even close to DC.  (If you're running
> a mast-mounted preamp, its power supply unit (which is preumably mounted
> inside the building) will inject DC into the downlead to power the amp
> circuits; but that's a "one-way only" proposition.)  So as long as the AC
> outlets that any active components (such as preamps, distribution amps,
> etc.) are plugged into are *also* properly grounded (preferably by
> home-running) and on the same phase as all the other A/V gear, you're home
> free.
>

Again, this is just plain wrong.  You are talking about the center
conductor not passing DC, which may or may not be true. I am talking
about the ground.  Everything will pass the ground uninterrupted even
for DC unless you build something to block it.  I have done that where
I needed to run a coax to another room that was not grounded to the
same reference.  You may have an issue with code as you say, I don't
know.  I know my direct tv installers didn't ground anything so when I
added the ground block I ran it to my panel.  My TV mast is also
grounded there but I installed it myself.  I was unaware you need a
permit or an inspection to put up a TV antenna, btw.


>
> Ultimately, yes; tho' a proper copper cold water pipe that is buried under
> the frost line for at least several feet is just as good (and will meet
> code in most places).

Won't meet code around here.  The problem is that someone may put in a
coupler that isolates the ground.  This can happen if someone drives
something into the ground and puts a hole in the pipe.  Then they put
one of those screw on couplers on to fix the leak.  That is exactly
what happened to me when my Rotor-Router pierced the water pipe when
looking for the sever line.  I replaced everything later when I found
out these are not permanent repairs but stuff can happen.  So, it is
code around here to have a ground rod.

> It's not so much a matter of the single (ultimate) ground point, per se.
> After all, even with conventional residential wiring, that's generally what
> you get (as described above).  It's really a matter of dissimilar paths to
> ground between one branch circuit and the next.  *That* is what produces
> the hum; and it's exactly what you get with the typical hodge-podge
> daisy-chain wiring that most hack electricians are trained to do.

I don't know why you are saying that the path length matters.  I just
doesn't.  The hum is produced by voltage differences in the grounds in
the two pieces of gear that are then hooked together.  There are
currents such that different grounds (in the real ground) can be
several volts apart.  If everything is brought to a single ground rod,
they will all be at the same ground voltage.  Now, if the wires going
to this ground rod are different lengths, they will have different
resistances.  The voltage at the equipment will be equal to that
resistance times the current through the ground wire, which is zero.
If that current isn't zero, you have a ground fault, which is a very
bad thing.  Since, zero times anything is zero, the length of the
grounds just doesn't matter.

Bottom line, you can home run them if you want but then you have to be
very careful about what goes to what phase of the 220 feed.  Run one
circuit for all your AV equipment and you are guaranteed to be fine.
The exception of course is if you really have a setup that draws more
current than that, you might need more feeds.  However, I used to work
in an electronics lab.  We would have spectrum analyzers, signal
generators, power supplies, all kinds of equipment on.  Lots of power.
 We ran one circuit per person, which was considered a lot, but that
was fine.  We drew so much power that the line voltage was a square
wave because everything was drawing current out of the peaks of the
sine wave.  I don't think you will see that kind of load from any AV
equipment in a home.  Again, if you are, you will need an air
conditioner, not just air vents.

Allen


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