[TowerTalk] Conduit for cables to the tower?

[Nick Pair]

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 ground.
   
  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.
   
  Nick
  WB7PEK
   
   
   
   
   

 				
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