AS an electrician, the only thing I will comment is that most jurisdictions
will require you to get an electrical permit and it will be by NEC code, it
will be a GFI protected outlet. Whether you place it at the beginning and
protect the outlet as a load, or you place it at the end, is a matter of
one's own preference. Also, by code you will have to bury your conductors,
whether or not you use underground conduit or cable. Off the top of my
head, I think it is 18 inches without going to find a book. Local codes will
supersede national codes. Each locality may have their own "standard". Or
simple ask your local inspector. One trick I have found that works well,
because the inspector always normally wants to see that the cable has been
buried deep enough, is to use a two inch piece of conduit, whatever size you
happen to have around, usually two inches works good. So when it is done,
you can then just pull the pipe out and bury it. A couple of other tips: is
be sure to label the circuit panel and caulk around the conduit where it
leaves your building. I cannot tell you how many failures I have seen on
these two simple points. 73's and everyone be safe. Gene K2QWD
----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Nick Pair
Sent: Thursday, September 14, 2006 5:04 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Conduit for cables to the tower?
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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