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[TowerTalk] Conduit for cables to the tower?

Subject: [TowerTalk] Conduit for cables to the tower?
From: Nick Pair <>
Date: Thu, 14 Sep 2006 15:04:01 -0700 (PDT)
List-post: <>
Hi All,
  The NEC generally doesn't require more than 24 inches cover from the top of 
the conduit or direct burial wires in all areas with a lot of shallower 
exceptions listed. So if you have 24 inches cover, you have all circumstances 
taken care of.( NEC article 300)
  You may run different voltage circuits in the same conduit if the voltage 
rating of the insulation is the same. There are two large exceptions to this 
rule. One is that almost all modern 120/240 rated wire is 600 volt insulation 
and most control type wiring is 300 volt insulation,with 600 volt hard to find. 
Second is that if the low voltage is supplied with a current limited class 2 or 
class 3 transformer or supply then they must be separated with a additional 
barrier besides the cable jacket. (i.e. a partition or plastic conduit, both 
rigid or flexible ) This means that you can get flexible nonmetallic tubing and 
pull that in with your power circuits, then pulling the control through the 
flexible inner raceway. I won't go into tray type of installations because they 
are special conditions that I've never seen in amature stations.( Although I've 
never seen all those super stations out there)
  You could also buy your conduit in continuous rolls by special order and 
eliminate the infiltration of water. (Which can be controlled or eliminated in 
properly installed and designed systems) I"I've used lots of this in plow in 
type of installations.
  You could just use a UF type insulated wire for the power and just throw it 
in the ditch. Size it for less than 5% voltage drop and put it on a GFCI feed 
and you will have the most economical solution for a crank-up motor or 
convience outlet.
  I don't see the need for GRC conduit but do see the need for a grounding wire 
to tye the tower system to the SPG shack ground and service entrance ground. 
This too can be thrown in the ditch and not pulled through the conduit system 
in either insulated or uninsulated conductor. The addition of GRC risers on 
each end is optional but will effect some choking on current flow that may be 
beneficial to limiting lightning flow. Of course these risers must be bonded to 
  You may be to eliminate some of the bends with creative digging of the 
trench, If very large radius bends are used (20 feet or more), you can turn 
without creating too much drag on conductors. In any case use large sweep 
elbows for your 90's.
  To use a vacuum to get a pull string in raceway, you tye a light string to a 
plastic bag that is partly air filled to make it the approximate conduit I.D. 
and insert opposite end of raceway from vacuum. Take care to spool out string 
as it can reach high speed quickly and tangle or burn fingers. You can also 
purchase a conduit piston (mouse) from wholesale electrical distributors in 
sizes for your conduits.

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