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Author Topic: Burying coax in conduit??  (Read 4456 times)
W6PC
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Posts: 5




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« on: February 06, 2018, 04:35:14 AM »

I am looking at burying various types of coax and control lines out to my tower in conduit.  Planning to use the gray (Cantex) schedule 40 or 80 electrical type.  Will use various sizes from 1/2" for rotor / control cable up to 1.5" for coax.  As with any buried tube, condensation will build up over time, even though it is underground.  Does anyone have experience with controlling this?

One school of thought is to put drains, i.e. breaks in the conduit along the way to facilitate drainage.  Others say maintain watertight continuity for the entire length.

Assuming you can keep the water out at the entry and exit points, are there opinions about what works best? Is my condensation concern valid or .....a non-factor?  The main purpose here is keep the cables from direct earth contact and provide some element of protection from accidental contact while digging in the yard.

Are there any hams out there that do this kind of commercial cable / telecom work for a living?  Any opinions or experience are welcome. As always, thanks to those who might have meaningful advice.

de W6PC
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N4IAG
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« Reply #1 on: February 06, 2018, 05:45:33 AM »

As someone who worked in the telecom field, we used solid conduit, no gaps. It seems like gaps would cause problems with junk getting in or being unable to get a fish tape thru it later on. My advice would be airtight with no gaps, and adding a pull string along with the coax for possible future use.
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W8JX
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« Reply #2 on: February 06, 2018, 06:36:54 AM »

As someone who worked in the telecom field, we used solid conduit, no gaps. It seems like gaps would cause problems with junk getting in or being unable to get a fish tape thru it later on. My advice would be airtight with no gaps, and adding a pull string along with the coax for possible future use.

You could also fill it with Nitrogen and seal it as nitrogen lacks ability to absorb water vapor. But if you use mil spec 213 you can just simply bury it if soil is not rocky.
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K3GM
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« Reply #3 on: February 06, 2018, 11:02:25 AM »

I had 2 instances of 3 inch, buried conduit.  One was a continuous, unbroken run about 40' long, with the other run 165' long and broken in 3 places with Tee's and a short stub surrounded by a shallow pit filled with crushed stone.  Both had unsealed ends.  I pulled everything last fall in preparation to move.  The Heliax and control lines in the continuous run came out soaked, and slimy.  The broken run was bone dry like the day I put not in there.  Conduit breathes with temperature changes.  If you get a warm, humid day, the moisture will condense on the walls of the cool pipe.  Over time, the conduit will flood.  The extra materials to provide a means of removing the condensate is worth the time.
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K8AXW
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« Reply #4 on: February 06, 2018, 09:15:16 PM »

My first attempt to do this wound up bad!  The unbroken PVC filled up with condensate.....similar to GM's experience.

Since my run was downhill somewhat to the tower I simply bored a hole in the elbow at the tower vertical junction.  I dug a hole and filled it full of gravel and then covered it with dirt.

I had also installed a "T" fitting with a plug for inspection for water.  After the hole/gravel mod, no water showed in the inspection hole.

Also learned two more things.  After calculating the size PVC needed, double it!  Also, always pull through a very strong pull cord, or a piece of #12 or 14 wire. (I used #12 stranded wire with insulation - no worry about cord rotting)  If a new wire is pulled through make sure it includes another pull cord/wire.  OK, that's three lessons!


FWIW.
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KD8IIC
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« Reply #5 on: February 07, 2018, 06:09:37 AM »

  They make Direct Burial Coax you know... mine's been in ground since '08. Laid in a bed of sand just underneath sod about 2 inches . Trench cut with my lawn edger. Still in use daily without a failure yet.
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K8AXW
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« Reply #6 on: February 07, 2018, 08:00:27 AM »

IIC:  Correct!  If you want to bury one cable.....but Invariably one leads to two...to three....and in my case a run of wire as big as my wrist!  I finally had to dig a second ditch for another run of PVC pipe.  Digging a slit trench for each cable would get old pretty fast!
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WA8ZTZ
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« Reply #7 on: February 08, 2018, 02:31:19 PM »

Have never pulled conductors out of a continuous run of underground conduit (steel or PVC) without finding water inside the conduit.
You didn't say how long a run you are planning but for all but a very short run, IMHO, 1/2" conduit is too small. Especially with PVC as you are likely to create an internal ridge when you make up the joints if you use too much PVC cement.  It will be a PITA when you go to fish in the cable.  Go at least 3/4".  A poly pull string will not rot in a water filled conduit.   However a poly pull string may weld itself to a PVC elbow at the stub that emerges from the soil if pulled too rapidly.  For this reason, steel elbows are often used in a PVC run.  If you are planning on much future expansion, you may want to consider installing a spare conduit while the trench is open.  You would be amazed at how twisted things can get (especially a pull string) after a couple of pulls of additional cables.
Just some thoughts after doing this stuff for 50 yrs.  
« Last Edit: February 08, 2018, 02:34:19 PM by WA8ZTZ » Logged
K0UA
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« Reply #8 on: February 08, 2018, 03:11:39 PM »

Have never pulled conductors out of a continuous run of underground conduit (steel or PVC) without finding water inside the conduit.
You didn't say how long a run you are planning but for all but a very short run, IMHO, 1/2" conduit is too small. Especially with PVC as you are likely to create an internal ridge when you make up the joints if you use too much PVC cement.  It will be a PITA when you go to fish in the cable.  Go at least 3/4".  A poly pull string will not rot in a water filled conduit.   However a poly pull string may weld itself to a PVC elbow at the stub that emerges from the soil if pulled too rapidly.  For this reason, steel elbows are often used in a PVC run.  If you are planning on much future expansion, you may want to consider installing a spare conduit while the trench is open.  You would be amazed at how twisted things can get (especially a pull string) after a couple of pulls of additional cables.
Just some thoughts after doing this stuff for 50 yrs.  

Same here.  Having been in Telecom for many decades, ALL continuous conduits, have water in them eventually. Including ones totally inside buried in concrete, like in a basement. The cold ground/concrete will cause condensate.  Air has water in it, so if air can get in and it will, because of the built in air pump that a conduit is it will dump its water in the conduit. The constant temprature change will pump the air.  While many coax cables outer sheath may be no permeable, some control cables may not be.  Far better to have a drainage plan than being immersed in water constantly.  Direct burial will work fine, but may not be for everyone as some of us have burying rodents that like the taste of cable, and some of us have sharp rocks in the soil that can move during freeze thaw cycles.  So the conduit with drain pits sounds good to me.
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WZ7U
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« Reply #9 on: February 08, 2018, 05:18:00 PM »

Here ya go.
https://portland.craigslist.org/wsc/ele/d/ham-radio-coax-cable-new-500/6487255248.html
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K0UA
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« Reply #10 on: February 08, 2018, 05:23:17 PM »


Very good stuff, all of my new coax is Davis RF Buryflex.  I have some up for about 13 years now.  Good price on those too!
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WA8ZTZ
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Posts: 282




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« Reply #11 on: February 08, 2018, 05:34:03 PM »

Just a few more thoughts...  
Some guys have used plumbing 90s but don't even think about it unless you enjoy a struggle pulling in the cables and don't mind damaged insulation.  Use electrical elbows.
Approved wire pulling lubricant will greatly aid in pulling in the cables especially if you have a long run.  Pour some in the pipe at the feed end and soap up the cables real good as they are fed in.  This is at least a two man job, one to soap, feed, push the cables and one to pull.  Be sure to use plenty large enough conduit.  Consult the conduit fill tables in the NEC for guidance.  Of course, make the conduit run as straight and with as few bends as possible.
Wire pulling is an art, could write a whole book about it but hopefully these few tips will help you, good luck with your installation.  
« Last Edit: February 08, 2018, 05:39:40 PM by WA8ZTZ » Logged
KM4AH
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« Reply #12 on: February 08, 2018, 07:30:51 PM »

I have an 80 foot telephone pole 250 feet from my house with nothing but an 80 meter dipole which is where I operate most of the time.
I put Buryflex or whatever it was at the time and an empty run of 2 inch P&C flex with a nylon pull tape in a two foot trench.
After about 15 years the sun had damaged the jacket on the direct buried coax.
I actually ran ladder line through it out of curiosity and used it for a couple of years. Not sure how much loss but I'm sure it was there.
I used the ladder line to pull LMR 400 through it a few years ago.
I'm sure it has some water in it.
Been about 30 years and never any coax problems in the conduit itself.
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K8AXW
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« Reply #13 on: February 08, 2018, 10:03:48 PM »

ZTZ made a great point....one which I missed on my first run of PVC pipe.  That stupid move caused me to remember a lot of cuss words that I thought I had forgotten.

The second run I used 90 degree SWEEP elbows used in electrical runs. 
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AA4HA
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« Reply #14 on: February 09, 2018, 06:23:40 AM »

Something fun to do; put a Tee on the conduit at one end and every year or so rent one of those gigantic trailer mounted commercial air compressors. Remove the weatherstop from the far end of the conduit and blow high volumes of compressed air in to one side of the tee.

Instant geyser.

Just do not do that on a conduit where the other end stubs up under a switchgear cabinet where there is a 480 volt three phase bus. I had that done and there was a loud "BOOM" and the power went out in part of a refinery. People were not happy.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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