At 09:35 PM 9/13/2006, Daron J. Wilson wrote:
> > > 1. Low-voltage wiring (e.g., for rotor and other control functions)
> > > cannot be run in the same conduit as regular power wiring.
> > Electrical power circuits MAY not be run (not permitted) along with
> > control wiring and coaxial cable, per National Electric Code. It's also a
> > good idea.
>Not exactly. We run control cables and AC power cables in the same pipe
>when needed, it is not a code violation provided the insulation rating on
>the wire is correct. For example, if you use shielded tray cable it is
>rated at 600volts, you can run 120vac on one cable and 24vdc data on the
>other one without violating the code. Should you want to cut it even
>closer, I'd have to dig in the book to confirm this, but I believe the
>lowest insulation rating on any wire in the same conduit has to be double
>the highest voltage used. For example, 120vac on THHN wire or romex can be
>run in the same conduit with Cat5 wire because the Cat5 is CM or CMP rated,
>a rating which requires an insulation voltage of 300volts, which is more
>than double the 120vac on the other wire.
Actually, it's not the conduit that causes problems, code wise, it's
where the cables terminate in the box. You have to have a one of
those funky boxes with a divider in it, unless it's just a pull box
with no break in the cable. The moment you strip the insulation off
one of those wires and connect it to another wire, you've just lost
the insulation rating exception.
Local jurisdictions vary on how they regulate this, but I've had to
improvise a horrible thing with two boxes and a condolet T (which
essentially served as a "no-splice inside" pull box) to make the
inspector happy. At least we didn't have to trench a new conduit.
>Now, I wouldn't do it unless I had to for inductive coupling reasons, and
>safety reasons, but to say that electrical power and control cannot be in
>the same conduit is technically incorrect. When we do it for underground or
>in cable tray industrial work, all the control wiring is shielded tray cable
>rated at 600volts.
An interesting point... Cable trays are slightly different than
enclosed raceways. I've not done so much with trays (at least as far
as code compliance), but I suspect that once you pull the cable some
reasonable distance out of the tray (towards the load or supply), it
turns into an independent run, NEC wise. You'd have to go into an
enclosed box for any splices on line voltage, but I can see just
bringing the cable down into a punchdown block for low
voltage. Hmm.. I'll have to go look at the codes and
interpretations. That's the cool thing about these lists. You learn
something new every day.
> > >
> > > 2. All cables should run in grounded metal conduit for effective
> > > lightning protection, according to Polyphaser.
> > Great thought but difficult to implement, especially on a Ham budget.
> > Most installations I've seen, or read about here, use PVC conduit and it
> > works just fine, including for my own tower.
>Polyphaser is a great product, and their references are great. However, PVC
>is fine for this purpose. If you really want to spend the money on metal
>conduit, you would be using rigid conduit or a coated rigid conduit which is
>a real pain to put in the ground.
Especially if you really are serious about keeping the coating
intact. Did it once, never again.
> Of course, you want to be sure it is
>bonded as it comes out of the ground.
> > >
> > > 4. EMT is not buriable. This suggests that RMC must be used, but can it
> > > be bent? Or can liquid-tight flexible metallic conduit be buried, and
> > > does it provide sufficient protection? Is there a means of grounding the
> > > metal in such conduit?
> > Choose PVC for your conduit material and eliminate these particular
> > problems.
>As I said above you can use Rigid, or coated rigid pipe. It certainly can
>be bent, if you are using something larger than 1.5" your last name better
>be Sampson or you'll need a mechanical bender. You can buy various radius
>bends already bent and threaded. Choose the largest radius you can find to
>get the most 'gentle' sweep, easier to pull. Let's say you get wild and
>bury 3" rigid conduit, I'd pull a ground wire in it as well and bond it at
>the entrance and exit of the conduit. Stuff in the ground tends to corrode.
>I'm not sure if liquid tight flex is rated for direct burial, but I suspect
>not. As to 'providing protection', it will protect your wire against the
>elements, but the flexible metal jacket should not be depended on for a
>grounding conductor, again, run your own ground inside.
And besides, the real mechanical protection comes from the soil over
the conduit (or concrete). My usual thing is to do the trench, drop
the conduit in, then lay concrete bricks, or big rocks, or scrap from
previous demolished slab, then backfill a bit of dirt, then one of
those "there's a wire buried under this ribbon" tapes, then finish
filling the trench. If the guy digging the hole for a new bush misses
the bright orange ribbon, his shovel is most likely going to hit the
bricks with a "clank" before he severs the conduit. Mind you, this
won't stop the pneumatic hammer or backhoe, or even the
overenthusiastic pickaxe, but it certainly helps with run of the mill
landscape guys digging trenches for sprinkler lines, etc.
>At commercial installations we run 3 or 4" PVC, sometimes several of them,
>occasionally a 6" from the equipment room to the base of the tower. Each
>wire has a ground kit on it where it leaves the PVC conduit and is bonded to
>a buss bar on the base of the tower. Wherever it connects to the antenna,
>it has a ground kit on it there as well and is bonded to the tower. When it
>comes up inside the building, guess what, ground kit and bonded to the
>building ground at a buss bar then they hit a lightning arrestor (such as
>the polyphasers) before going to the radio equipment.
>Personally, not knowing your installation, I would run 3/4" PVC for the
>power, a 2" for the control wiring (nice if you have some oddball cable with
>a plug on it you want to pull through, and lots of room to do so) and a
>large enough conduit to run twice as much wire as you anticipate. Keep in
>mind that conduit fills up real fast, you shouldn't be filling the conduit
>more than 40% or so. My personal preference would be a conduit sized for
>all the stuff I was planning on putting in, and one the same size as a
>spare. It's cheap at the time you are planting it.
I agree.. the cost is in digging the darn trench and backfilling, not
in buying PVC conduit. You can just stub up the spares above ground
and plug them. Don't even need the pull boxes or anything
else. Anything to avoid having to dig up the ground again (or worse,
having to saw, jackhammer, and patch the concrete slab over the top).
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