My comments embedded:
> -----Original Message-----
> From: Alan NV8A [mailto:nv8a@att.net]
> Sent: Wednesday, September 13, 2006 9:36 PM
> To: towertalk reflector
> Subject: [TowerTalk] Conduit for cables to the tower?
>
> I have questions about the best way to get cables (coax, AC, and
> control) from the shack to the tower.
>
> Here are the things I have read or observed and am trying to
> take into account. Please correct me if I am wrong on any point.
>
> 1. Low-voltage wiring (e.g., for rotor and other control
> functions) cannot be run in the same conduit as regular power wiring.
Our local code says separate conduits must be used.
> 2. All cables should run in grounded metal conduit for
> effective lightning protection, according to Polyphaser.
I can't imagine using metal conduit. I used PVC. Not the white schedule 40
plumbing stuff, but the grey stuff specifically designed for electrical
work. You should be able to get it at your local electrical supply house.
> 3. The bends in the conduit for a single run of cable may not
> exceed 360 degrees or it becomes too difficult to pull the
> wires through.
Yeah, I'll say. Others have addressed this question.
> 4. EMT is not buriable. This suggests that RMC must be used,
> but can it be bent? Or can liquid-tight flexible metallic
> conduit be buried, and does it provide sufficient protection?
> Is there a means of grounding the metal in such conduit?
I wouldn't use metal conduit. PVC is sufficient. That's what they used to
run AC from the meter to my house. Ground any cable shields at each end, and
use lightning suppressors on all leads.
> It looks as though I will need three separate runs of
> conduit: one for the feedlines, one for power for the
> crank-up, one for the control cables and rotator power.
My shack is 265' from the tower. I have three runs of electrical-grade PVC
buried four feet deep. I found out years later that local code only requires
conduit to be buried three feet deep. This varies from town to town, and I
think it's based on the depth of the frost line. Anyway, I don't think the
extra digging would have made a huge difference cost-wise. The big expense
was dealing with several seams of ledge along the way. That required
bringing in an excavator with a huge jack hammer mounted on it to bust up
the rock. I live in a rural area, and 10 years ago there was no such machine
available locally and it had to be brought in from a neighboring state.
There was also extra cost for an excavator to remove the busted up ledge.
The general rule of thumb is that the conduit should be twice the diameter
of the cable you are going to pull. One of my runs is 4" conduit for the RF
feedlines, though I only put in two runs of standard LMR400 cable (1/2"
each, very flexible.) My plan was to replace those with two runs of 7/8"
hardline, but I've never gotten around to it. Still on my list of things to
do. There was a bit of a misunderstanding on this between me and the tower
rigger who advised me on the conduit. He thought I planned to run 1/2"
hardline, while I was expecting to use 7/8". When he found out what I wanted
to do, he expressed doubt I could pull two runs of 7/8" hardline through 4"
conduit. I don't agree, for several reasons. There are only two 90-degree
sweeps at each end and a very gradual bend in the middle of less than 90
degrees (I'm not sure, but I don't think a sweep was used -- the contractor
just shifted the direction of the trench very gradually over a long
stretch.) After seeing someone pull 1-5/8" hardline through a 3" conduit, I
think my job won't be so bad. It'll probably require a lot of cable lube,
however. You can get that at the local electrical supply house, too. I'll
probably have to use a winch or vehicle to pull the cable, too.
The other two runs of conduit are 2 1/2" and 1", respectively. The 2 1/2"
conduit carries five runs of low-voltage control leads for the tower, rotors
and switches. This conduit and the 4" conduit go to a large metal utility
box mounted on a pressure-treated wood panel next to the tower. There's an
identical panel at the shack end of the run. The 1" conduit contains four
runs of #8 THHN for the AC. It goes into a separate metal AC subpanel
mounted on the wood panel. Given the length, I ran 220VAC to the subpanel,
and used 110VAC breakers to feed the crankup tower and a handy outdoor AC
receptacle also mounted on the wood panel.
All conduit runs are terminated with expansion couplings to deal with the
frost heaves that are common in our area.
Here's a rough cost breakdown:
Trench digging, coduit installation: $1,725
Conduit materials 1,247
Conduit terminations (sweeps, couplings) 299
John Deere hammer to remove ledge 863
Labor for hammer operator 288
Equipement time for machine to remove ledge 736
Moving charge for hammer 403
Total $5,561
Mind you, this was 10 years ago. I'm sure it would cost more now.
I have an extensive ground system at the tower consisting of 4 sets of 3
ground rods (12 total) bonded to the four anchor bolts supporting the tower
(it's tubular.) The rods are connected with 1/0 stranded wire, and all
joints are cadwelded. All leads are connected to Polyphaser lightning
suppressors at both ends. I used surge suppressor AC breakers in the
subpanel next to the tower, and MOVs on the AC leads at the house end.
Side note: When planning my new 110' guyed tower, I considered putting in AC
as a convenience (not required for this tower, but nice for testing rotors,
SteppIRs, switches, etc.) The building inspector told me I could
shallow-bury AC if I pour concrete over it. I couldn't see this as practical
or cost-effective over a 220 foot run, so I decided not to run AC to the
tower. Having had my fill of conduit and ledge, and having never pulled
anything new through the conduit to the crankup tower in the 10 years I've
had it, I've decided to shallow-bury all my hardline and control lines to
the new tower (two runs of 1-5/8" hardline, which are sort of like metal
conduit anyway, and about a dozen runs of multiconductor control cable.) I'm
planning on having a guy with a ditch witch dig a trench 12"-18" deep for
the runs. Hopefully, I won't regret skipping conduit this time. It'll surely
be a lot cheaper than what I did with the crankup tower.
73, Dick WC1M
> The conduit run for 120V power looks like needing six
> 90-degree bends (two too many), and the other cables eight
> (double the permitted number).
>
> How have others handled their cabling?
>
> 73
>
> Alan NV8A
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|