[Top] [All Lists]

[TowerTalk] Schedule 40 PVC

To: <>
Subject: [TowerTalk] Schedule 40 PVC
From: ("Dick Green".)
Date: Tue, 16 Sep 1997 10:54:50 -0400
>1. If you put all the cables through the individual pipe sections BEFORE
>you glue them together, you eliminate a lot of trouble.

Generally, this isn't a good idea for long runs with multiple cables. First,
it's extremely difficult to keep the cables from twisting around each other,
especially when installing sweeps or bends. This makes it impossible to
remove a single cable later. Second, working with exposed cables in the
trench results in them getting caked with dirt that will add a lot of
friction if the bundle has to be removed later. Third, it's very hard to
keep the PVC cement off the cables, further complicating their removal
(perhaps making it impossible.) I suspect that this is one reason why some
on this reflector discourage cementing of the joints. Fourth, it's a lot of
extra work to walk to the end of the cable run, slip a long heavy piece of
PVC over it (e.g., 4" conduit), and slide it over the cable bundle to the
next joint without nicking or twisting the cable.

Professional installers use gentle sweeps to take the conduit up to grade
(these are not "elbows" like the ones you find at the hardware store -- they
typically have a 16" radius.) In frost-prone areas, they use expansion
couplings where the conduit emerges above grade to prevent the pipe from
pushing or yanking on the terminations. They also use termination boxes or
"LB's" (narrow rectangular boxes with a removable plate that allows access
for pulling -- these are designed for mounting to the side of a house.) The
joints are glued with PVC cement (since they do it just like plumbers do it
on your sewer drain pipe, I doubt there will be any leakage there.) Leakage
is probably further limited by the use of flared pipe instead of separate
couplings. I have a feeling that they generally do not drill drain holes in
the conduit, perhaps because they use weatherproof termination boxes which
minimize condensation and leakage (at least this is true in my area.) After
the conduit is in place and terminated, they make a "mouse" out of plastic
and tape, tie nylon twine to it, and suck it through the conduit with a
vacuum cleaner. The twine is use to pull a rope and the rope is used to pull
the cables. A special lubricant is often used on the cables to ease their
way through the conduit.

Before I knew all this, and decided on a professional installation for three
250' runs of conduit to my antenna site, I installed a 25-foot length of
conduit from a cement telescope pier to the house. The conduit carried an AC
power line, a 25-wire parallel printer cable for a digital CCD camera, and a
six-wire RS232 cable for operating the telescope (yes, I now know it's a
code violation to run AC with other cables.) I used standard white schedule
40 PVC with separate couplings. There was a termination box at the telescope
(the PVC actually ran through the slab to it.) I assembled the pipe around
the cables and found it nearly impossible to keep them from getting badly
twisted and dirty. I made the further mistake of using two 90 degree elbows
and one 120 degree elbow. I think I can forget ever removing these cables or
replacing them. Lucky (?) for me, my wife is lobbying for an addition that
will wind up right on top of the telescope pier, so I'll probably get a
chance to redo this one correctly.

73, Dick, WC1M

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>