Excellent advice on conduits, Bill. I agree with almost everything. Here are
1. Water in conduit
I have read many, many posts here about drilling holes in conduit to let the
water escape. My conduits do not have holes drilled in them, do not have
gravel drainage under the terminations, and I have never, ever had a drop of
moisture in them during the 10 years since they've been installed.
I believe the drainage issue depends on your location, the conduit material,
how the conduit sections are connected, the type of termination, and/or how
deeply the conduit is buried. Mine were installed by a building contractor
and terminated by an electrician, using schedule 40 conduit (the gray stuff
used for electrical work, not the white stuff used for plumbing.) We also
have professionally installed conduit for our electrical service, and
there's never been any moisture in that pipe, either.
I have three runs of conduit: 4", 2.5" and 1". The runs are about 265' long
and are buried 4 feet deep in rocky soil (in some spots they run through
ledge that had to be pounded out with a jackhammer.) The conduits slope down
a very steep hill. The 4" and 2.5" runs are terminated at both ends in a
watertight metal NEMA box with a gasket on the door. The 1" run, which
carries 220VAC, is terminated at the tower end in an outdoor subpanel with a
door that's not watertight but doesn't appear to leak. At the tower
(downhill) end, most cables exit the NEMA box through coax bulkhead fittings
or watertight bushings, and silicone sealant is used where the ground cable
enters (I did all that work myself.) At the house end, the cables exit the
NEMA box through holes drilled in the back of the box and through the sill
of the house. The 1" run is terminated with an L-fitting through a hole in
the side of the house.
Now, I don't know if there's a small layer of water puddled in the sweeps or
along the length of the conduit because I can't see those places and haven't
pulled any cables or added any. I've looked down the pipes with a flashlight
to the start of the sweeps, but see no moisture there. My conduit runs down
a very steep hill, so I would expect to be able to see moisture at the end
of the downhill sweep. If there was an appreciable amount of water, I would
expect it to fill the pipes right up into the NEMA boxes, but that's never
happened over the 10 years the conduits have been installed, and we've had
many, many soakings. We often have several feet of snow cover in the winter,
which totally soaks the ground when it melts.
Our rocky soil drains well, though there are certainly swampy areas in the
flat (we don't have much flat ground!) The water table is far deeper than
the conduit. We have a relatively soft layer of rocky soil for the first two
feet, then a much harder layer down to four feet. It's not clay, but is so
packed and so rocky that it's tough to dig through. But I think it drains
The PVC conduit was sealed very well. If you carefully follow the
instructions for using PVC cement, I don't see how it can leak. You're
supposed to put a thick layer on both ends, then insert and turn 1/4 turn.
There's so much cement, and it gets spread so effectively, that it should
remain watertight. Heck, most places around here use PVC pipe for water and
septic drainage, so if there was a significant risk of leakage the plumbers
would never be able to keep up with it. OK, most of those are fixed indoor
pipes, but the septic drains do run underground to the tanks.
The only way I can see a conduit leaking is if it flexes a lot. Around here,
the conduit needs to be buried under the frost line to avoid this. That
would be at least 3 feet deep here. Another reason for burying conduit deep
is to avoid invasion from tree roots. Roots seem to be able to penetrate PVC
conduit joints, but I've only seen that when there's food or water inside
the pipe, as there would be for water and septic pipes (the roots seem to
You must use expansion joints at both ends of a long conduit run. It's
certainly possible that repeated expansion/contraction without a way to
relieve the pressure could cause the cement to fail.
Finally, the termination makes a difference. If your conduit just rises out
of the ground into a 180-degree curve with cables exiting downward, and your
location frequently has high humidity, then I think you're going to get a
lot of condensation in the pipe over time. It's certainly possible that
moisture can enter the pipe from inside the house. In my case, the basement
is quite dry -- especially when we run heat and air conditioning. I'd be
real concerned about open conduit entry/exit in a place like Barbados!
However, you can seal up those points with silicone sealant, at the cost of
making it a lot more difficult to add/subtract cables.
My advice is to check out how professional contractors bury electrical
cables in your area, and try to mimic exactly what they do. I would be
willing to bet that drilling drainage holes in electrical conduit is not
code anywhere in the US.
2. Messenger line
I like this idea a lot. I have a messenger line, but I pulled it with the
cables, so it's probably twisted up with them and probably useless. I'll
probably have to pull all the cables and re-pull in order to add any cables.
Luckily, I installed a bunch of spare cables just in case. My messenger line
isn't twice as long as the conduit. I figured I could splice another line
onto either end.
Only thing about the messenger line is that I don't see how it can help you
more than once. If you use the messenger line to pull more cables and more
messenger line, the latter will get twisted up with the new cables.
3. Ground wire
Similar to Bill, I put a run of 1/0 stranded ground wire in the bottom of my
conduit trench. It connects the tower ground system and the single-point
ground at the shack end. I used uninsulated wire on the theory that this
long run of wire is essentially a radial or giant horizontal ground rod that
may aid in dissipating a lightning strike. However, I later read literature
stating that from a lightning standpoint, the grounds will not "see" each
other (i.e., be at the same voltage potential) if separated by more than 75
feet. Any strike energy would be dissipated well before that. So, I probably
would have been better off burying a 50-foot wire at each end or not burying
any wire at all. OTOH, Bill's run is short enough that a ground wire makes
sense. Given today's copper prices, I wouldn't even think about burying 265'
of 1/0 again. In fact, I didn't do that for the 225' run to my new tower. I
figured that if the extensive tower ground system didn't dissipate all the
energy, the 2" diameter copper outer conductors in the two runs of 1-5/8"
heliax would present a much lower impedance path to ground than any copper
wire or ribbon I could bury alongside them!
I agree with trying to lay the cables flat before pulling, but I don't think
it would help all that much with a long run like mine. Long cables are going
to twist around each other, no matter what you do.
5. Conduit size
I agree that using less than 3" conduit may be asking for trouble, but it
depends on what you plan to put in there. The rule of thumb for pulling is
that the conduit must be at least twice the diameter of the cables. So, if
you have a 2" bundle of cables, you need 4" conduit. But there's more to it.
I originally sized my 4" conduit to eventually accommodate two runs of 7/8"
heliax. But the number of angles is another consideration. I've got the two
90 degree sweeps. That should be OK, but the conduit takes an almost
90-degree turn between the house and tower. I'm not sure, but there's
probably a sweep in the middle somewhere. I don't know the angle on it.
Anyway, that could make it extremely difficult to pull two runs of heliax.
I've had two runs of low-loss coax in the 4" conduit for 10 years. At some
point I'll either bite the bullet and try pulling the heliax, or use
something line LMR-900.
For long runs with stiff cables and/or lots of angles, you can use special
pulling grease sold in electrical supply stores. You slather it on the
cables as you feed them in.
7. Conduit or not?
When installing my new tower, I elected not to use conduit for the 225' run.
I was not willing to bear the expense of digging another deep trench and
laying a lot of conduit. I was concerned that there would be ledge between
the shack and tower (turned out there wasn't), and there's a 90-degree turn
in the middle. So, I had a 2-foot deep trench cut and direct buried all the
cables. The trenching was done with a tractor-mounted ditch witch, which was
cable of making the cut more shallow if we ran into ledge. I went that deep
mainly to avoid critters like moles from getting to the cables, though I've
not had any problem with the many shallow-buried cables I've installed here
over the years. The main problem has always been cables laying on the ground
-- the critters always chew throgh those.
I installed two runs of 1-5/8" heliax, and there was no way I was going to
pull that stuff through conduit! I used direct-bury cables for as many of
the control wires as I could. Unfortunately, I couldn't do that for all of
the cables, most notably three 16-conductor twister-pair SteppIR cables.
However, I've had lots of shallow-buried (<6" deep) cables here for many
years, including a SteppIR cable for 2 1/2 years, and have never seen any
evidence of unusual wear when I've pulled them up. I know it's a gamble, but
I feel comfortable with it.
You can see how I installed the cables on my website at www.wc1m.com, under
73, Dick WC1M
> -----Original Message----
> From: Bill Ogden [mailto:firstname.lastname@example.org]
> Sent: Thursday, June 07, 2007 10:28 AM
> To: email@example.com
> Subject: Re: [TowerTalk] buried conduit
> On a much more basic level than some of the other replies - and for my
> simple, straight 40-foot run:
> 1. Be certain to use "sweeps" rather than "90 angle" fittings. The
> provide a more gentle curve.
> 2. Be very certain you have drainage from everywhere in the pipe. Have
> slope and/or drill holes in the bottom and set it on a little gravel.
> plastic over it when filling in the trench so the gravel is not packed
> dirt. Dig a little dry well under the ends if you depend on sloping it.
> conduit will eventually fill with water, no matter how you seal it.
> Use a
> good level to determine slope; do not try to eyeball it. (I ignored
> advice and had to dig up my conduit and set it again.)
> 3. Be certain to leave a good "messenger" rope in the conduit, tied at
> ends, and at least twice as long as the conduit. I found it better to
> mix this with the initial cable feed to avoid having it intertwined
> the initial cables. I placed tension on the permanent messenger cable
> hold it against the top of the conduit) while feeding the initial
> through. This allowed the messenger line to run freely after the
> cables were in place. I used a 1/4-inch dacron line for the messenger.
> 4. I ran two #12 insulated wires outside the conduit (and mostly under
> to server as an additional link for the common ground just outside the
> house. This was probably not needed, but it seemed a good idea at the
> 5. I have 180-degree fittings at both ends, to keep rain out. These are
> glued on. I remove them when pulling cables, although the cables (after
> emerging from the conduit) run through these fittings.
> 6. Before feeding cables, lay them side by side in an orderly way and
> to feed them into the conduit this way. This will make it much easier
> pull one of them later. If the cables twist around each other inside
> conduit, pulling one becomes very difficult.
> 7. I have three coax and three control cables in a 3" PVC pipe. This
> a small bundle lying in the pipe. Do not even think about using
> smaller than 3" pipe..
> 8. The top of my conduit is only about 6 inches deep in some places and
> averages about 10 inches deep. Not very deep. (I dug it by hand, in
> that is hard clay and shale. I am a bit lazy, also.) This seems deep
> for grass to grow normally on top.
> These are trivial points, but might help with a first-time buried
> Bill - W2WO
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