[TowerTalk] Conduit for cables to the tower?

[Daron J. Wilson]

> > 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.

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.

> 
> >
> > 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.  Of course, you want to be sure it is
bonded as it comes out of the ground. 

> >
> > 3. The bends in the conduit for a single run of cable may not exceed 360
> > degrees or it becomes too difficult to pull the wires through.
> 
> 360 is a good rule of thumb to try to adhere to.  (That's to keep the
> cable from being crushed against the conduit sidewalls during long pulls
> with many turns, NOT to make pulling easier on you.)  In your case, where
> you have several runs with more than 4 ninety-degree turns, you should use
> intermediate pullboxes in the conduit runs.  With these boxes you pull the
> cables through and lay them above ground.   Then you start another run of
> 4 90's by pulling into the pull boxes and on to the next box or to the
> termination.  The boxes come with lids that are fastened in place after
> the cables have been placed.

180 degrees is much more reasonable for sensitive wiring.  We don't allow
more than 180 degrees for fiber for example.  If I were pulling multiple
runs of heliax or some more rigid type of feedline, I would keep it to 180
degrees or less.  Save yourself and get a good quality lubricant, not
"yellow 77" which hardens to a wax after pulling.  Several manufacturers
make good lubricant that doesn't set up after use.  We use a product by
polywater that is fantastic for fiber pulls, due to it's nature we refer to
it as 'Dragon Drool'.  Use lots, save your back and the wire.


> >
> > 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.

> >
> > It looks as though I will need three separate runs of conduit: one for
> > the feedlines, one for power for the crank-up, one for the control
> > cables and rotator power.
> 
> If your rotator control power is less than 50 Volts (likely), then you MAY
> run these wires in the same raceway (conduit in your case) as the coax
> cables and control cables, per NEC.  You can $ave one conduit run that
> way.
> 

See response above.  You can combine the coax, control, rotator wires in the
same conduit depending on the rating of the wire insulation.

> >
> > The conduit run for 120V power looks like needing six 90-degree bends
> > (two too many), and the other cables eight (double the permitted
> number).
> 
> See my response to your #3, above.  You can purchase pullboxes wherever
> you buy the PVC conduit.

Pull boxes or conduit "C" boxes would be ideal. I prefer the "C" boxes due
to the watertight nature.  If you use PVC, and you buy pullboxes, you end up
drilling the box and you need something like a Meyers hub to get a
waterproof solution.

> 
> >
> > How have others handled their cabling?
> 
> I have one fifty foot long run of 3 inch PVC conduit from my shack to the
> tower.  I should have installed a four incher.  Too late now.
> 

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.

73
N7HQR