[TowerTalk] Pulling coax through PVC pipe
grants2 at pacbell.net
Wed Jan 31 13:49:21 EST 2018
I agree, "different strokes" but to understand the principles, the
Carrier Psychometric Chart was pretty helpful to me as it sorts out Rh
from dew point and shows "lbs of moisture per pound of dry air". It was
developed for HVAC uses.
The variables for conduit are its (ground) temperature and air flow
water content. I think that occupied buildings almost always have lower
Rh than outside - heated in winter, AC in summer. That doesn't mean the
inside air dew point is lower than that inside the conduit, which
determines if there will be condensation. If temps inside the conduit
are higher than dew point, no condensation and evaporation of water that
is present. When reversed there is condensation, eg frozen ground and
warm outside temps. Venting to outside is another set of differential
temperatures and water content circumstances.
So IMO, with varying seasons, lagging ground temps and variable inside
(or vented) temps and moisture levels, it seems almost certain to get
condensation at times in the conduit (extreme desert climates
excepted). Sealing may help if done at the right time, but hermetic
seals that prevent in/out air flow are really hard. Broadcast stations
sometimes purge air dielectric coax with calibrated dry gases, bottled
N2 or dry air from a refrigerated dryer or desiccant system or both.
Then dew points are known to be in well into negative F temps (eg
-40). Possible, but a bit over the top for hams.
So my approach, since in conduit condensation is highly likely, is to
always use direct burial cables in conduit with no connectors or breaks
in cables. The polyethylene (PE) covered RG's or Heliax are my choice
as are PE sheathed control cables and UF rated power cables (which go in
the trench for AC, not inside the conduit for me). And water tight
conduits with glued joints. Generally schedule 40 PVC but SDR35 is a
good and cheaper choice for large diameters. Drain pipe IMO doesn't
make water tight seals with the gaskets. Why let in ground water and
dirt? I've seen the water flowing out of conduit (and out of coax!) as
you had when there is a hole in the conduit.
This is an interesting topic, maybe a few hundred threads about it on
TT, but probably 97% of water in coax is due to poor connector sealing.
On 1/31/2018 5:54 AM, Patrick Greenlee wrote:
> Different strokes... Some, like Chick, vent underground conduit to the
> surface air, others put drain holes in the underside, some try to seal
> it up (with or without desiccant), others purge with "dry" air
> intermittently or warm dry air continuously, others may get good
> results doing nothing extra, and so on and so forth A N D most say
> "it works for me." There probably is no single best answer, certainly
> with the wide variety of climates. In some places if you tried drain
> holes the ground water would flood your conduit. Some places venting
> to surface air will allow condensation in the vent pipes as temps and
> RH go through the diurnal cycle adding water to your buried conduit
> over time.
> If the conduit is sealed well one of the few ways for moisture to gain
> entry is via poorly sealed coax. Moisture can egress via the shield
> of improperly sealed coax. I did nothing "heroic", just installed my
> various PVC conduits with plenty glue, a twist, and hold fully engaged
> for 30 sec to get a proper glued joint. When going back to a
> previously wired conduit to add more wires I never detected any
> moisture. I guess I'm lucky to be fortunate enough to have benighn
> conditions that don't require special amelioration.
> One glaring oops in my conduit experiences is the last 4 inch S&D run
> which is a minor embarrassment in that during backfilling I got lazy
> and too hurried in backfilling and ended up with rocks and large hard
> clods in the backfill against the conduit which when driven over
> damaged the conduit. How do I know the conduit was damaged? Ground
> water entered the conduit which was on a slope which made a
> considerable head of pressure and over flowed the terminal end of the
> conduit which was 2 ft above my barn floor. Water poured out onto the
> slab at a rate of several gallons per minute. I used a cordless 4 1/2
> inch angle grinder to cut a relief hole just above grade outside of
> the building so the water stays out side the building.
> Now what to do to salvage the situation, preferably without replacing
> the conduit. Local big box stores sell rolls of 1" flexible
> irrigation pipe rated at 100PSI for $12. I pulled my wires and cables
> into some of these, sealed the ends super water tight and then pulled
> those through the flooded conduit.
> My cables are still in the PVC conduit and so protected from the evil
> cable munching critters we have that delight in chewing on direct bury
> cables A N D they are protected from water up to 100 PSI (not likely
> to be a problem.) The best part was not digging up and replacing the
> conduit. A side benefit is that additional wires could be placed
> inside another run of 1 inch irrigation tubing and easily pulled along
> current cables as the irrigation tubing gives you a smooth
> non-snagging pulling experience.
> Hopefully none of you TowerTalkers will need to use my remedy but
> there it is, just in case.
> Patrick NJ5G
> On 1/30/2018 4:02 PM, Chick Allen via TowerTalk wrote:
>> One thing to consider that will mitigate the "wet location" issue is
>> to vent the conduit with some vents installed along the conduit.
>> This is done using a simple pvc "tee". If you're using 2" pvc, just
>> connect the lengths at several different points with a 2"x2"x2" tee,
>> reduce the vertical tee to something smaller, like 1" or 3/4" and
>> bring that about one foot above the ground. There you can add a
>> sweep el that will prevent moisture from entering the conduit while
>> allowing condensation to evaporate. Plan your above ground vents
>> around a fence post or some other above ground object so it won't get
>> broken as easily. Otherwise, a short piece of treated 4"x4" will
>> provide a stable support. Works for me.
>> 73 de Chick / NW3Y
>> -----Original Message-----
>> From: Shane Youhouse <kd6vxi at gmail.com>
>> To: towertalk <towertalk at contesting.com>
>> Sent: Tue, Jan 30, 2018 2:34 pm
>> Subject: [TowerTalk] Pulling coax through PVC pipe
>> I wouldn't be so quick to run ANY romex through PVC when it's
>> buried. It's
>> against code.
>> Article 300 of the NEC states any buried pipe is considered a wet
>> *300.5 Underground Installations.*
>> *(B) Wet Locations.* The interior of enclosures or raceways installed
>> underground shall be considered to be a wet location. Insulated
>> and cables installed in these enclosures or raceways in underground
>> installations shall be listed for use in wet locations and shall comply
>> with 310.10(C). Any connections or splices in an underground
>> shall be approved for wet locations.
>> Then, article 334 implicitly states you may NOT use romex (NM) in damp
>> *334.12 Uses Not Permitted.*
>> *(B) Types NM and NMS*. Types NM and NMS cables shall not be used
>> under the
>> following conditions or in the following locations:
>> (4) In wet or damp locations
>> As such, you are MUCH better off running single runs of wire in
>> addition to
>> a heavy ground to keep SPG intact
>> There are a LOT more rules regarding NM style wiring, but this is the
>> Also, it's not always wrong to mix 120 volt and low voltage. As long as
>> you use THHN or other 600 volt insulation and all wiring is class 1,
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