Back in 1997, I installed a set of three conduits, 4", 2.5" and 1", running
265' down a fairly steep hill to a crankup tower and some other antennas.
The 4" conduit contains two coax runs (it was originally sized for hardline,
but I've never gotten around to replacing the coax.) The 2.5" run contains
all the low voltage control cables. The 1" conduit contains four #10 wires
carrying 220 VAC to a subpanel mounted near the tower. A 120VAC cirucuit
from the panel is for the crankup tower motor.
The conduits are buried 4' deep, as required by local code (they can be
shallower with a concrete cap, but I didn't know that at the time.) A length
of 1/0 copper wire runs along the bottom of the trench, under the conduits,
to connect the shack single point ground and tower ground systems together.
Probably not effective at that distance from a lightning perspective, but I
didn't know that at the time either. Luckily, copper was a lot cheaper in
1997. What wasn't cheap was hiring a special excavator with a jack hammer to
pound out the massive pieces of ledge we found along the way.
I used the standard gray PVC conduit rated for electrical applications. The
pipes were glued together using PVC cement, in the standard fashion, before
the cables were pulled. It would have been a mess trying to pull the
sections over the cables -- dirt and cement don't mix well. I tried that
once with a much shorter run to a telescope once, and decided it was a bad
The conduits terminate in 90-degree sweeps up to large weatherproof Hoffmnan
metal boxes at each end. An expansion joint was inserted between each sweep
and the box, to allow for the typical expansion and contraction we have
during the four seasons here in West Central NH.
After assembly, the trench was filled in -- i.e., before the cables were
pulled. Lightweight messenger strings were pulled through the assembled
conduits by attaching a small foam rubber cylinder (a "mouse"), sized to fit
the conduit, to the end of the string and pulling it through with a shop
vac. The messenger string was used to pull a strong but lightweight nylon
rope through each conduit. The ropes, in turn, were used to pull a bundle of
cables. Another rope was included with each bundle, though it likely is
twisted around the cables enough to not be useful. If that is the case,
adding another cable may require pulling the entire bundle out and
repulling. As I included spare cables in the coax and control conduits, that
hasn't been necessary (although all of the spares have been used!)
At the tower end, cables exit the box through holes drilled in the bottom.
In some cases, coax bulkhead adapters are used. In other cases, standard
cable clamps are used. In all cases, the exit holes are sealed with rubber
gaskets and silicone caulking. At the shack end, the box is mounted on the
side of the house. Two large holes were drilled in the back of the box and
through the siding/sill, leading to a closet containing a patch panel for
quick disconnect during lightning season. Every conductor is also connected
to a surge suppressor in both boxes, at the tower and on the side of the
I did not drill weep holes in the conduits and have never had any indication
of moisture in them. I'm we certainly have our share of humidity here,
especially in the summer, though it's not as bad as places closer to the
ocean. The soil is pretty rocky and tends not to retain water. Also, the
sealed boxes at both ends prevent humid air from entering the conduits. The
only place ambient air can enter is from the shack, which tends to have
relatively low humidity during the heating and cooling seasons.
Hope some of this is helpful.
73, Dick WC1M
From: Steve K7AWB [mailto:email@example.com]
Sent: Tuesday, April 05, 2011 6:15 PM
Subject: [TowerTalk] methods of using conduit in the ground for cables
As long as we are on the topic, which would be better?
1) secure the 4 inch conduit sections together in the trench and then fill
in the trench with dirt on top so the sections could not move when pulling
cables through them, or
2) keep the trench unfilled, but the 4 inch conduit sections put together,
and pull the cables through them without the dirt on top?
As I see it, once you put the dirt on top, there is no going back, but the
conduit does not move. On the other hand, conduit without weight on it will
tend to move as the cables as pulled through, especially when the "last"
cable is tried.
Comments from you who have done this.....
Nine Mile Falls, WA
TowerTalk mailing list