What kind of network cable to run underground?

February 28, 2007 6:37 AM

I want to network 2 buildings about 15m apart with CAT6 cable through an underground 100mm PVC conduit. What could go wrong?

We will upgrade our telephone system soon. This will entail connecting the two buildings with said conduit at a depth of 1m. The smaller building is currently connected to our network via wireless bridge. This presents the perfect opportunity to connect the network with a physical cable and eliminate some stability problems and increase transfer speed. I plan on using CAT6 because of the extra shielding and outer jacket that seems to be a bit thicker than usual. Will this suffice? If it is laying safely within the dry conduit, do I need to use a special kind of cable? What if condensation builds up in the pipe after a while? I know there are some cables rated for underground use but I am unsure if this is only for cable being directly buried in the dirt a few feet below the surface or if this includes conduit runs as well. Also, if there are telephone cables running alongside the CAT6, will it still be safe from interference?

We did consider using fiber but it was a bit cost prohibitive and we need to have a connection up and running sooner than we can get technicians out here to do it. Maybe we'll do it later. The conduit will be laid with pull wires so we can pull additional cable in the future if we need to.

posted by chillmost to computers & internet (13 comments total)

Who's laying out the telephone system? Did you ask them what grade of cable they're going to use?

Are you just running one ethernet cable? I'd say run it and if you have any problems pull it and run it again. But I think you'll be OK.
posted by RustyBrooks at 6:41 AM on February 28, 2007

Be sure to check the grounds / earths at either building. I went to a school with a split site and they had to run fiber between the two sites as there was a earth difference between the sides that would have resulted in current flowing. This could potentially damage the switches at either end of the link.
posted by gi_wrighty at 6:54 AM on February 28, 2007

What gi_wrighty said. Fiber isn't *impossible* to do yourself, but you do need to have the proper termination tools, and they aren't cheap. If you don't *know* how to pull fiber, don't.

The easiest way to check the ground is to pull a wire, attach it to ground in building #1, then meter the current between that wire and ground in building #2. I'm not going to give you the exact procedure -- you either know it, it which case, you can do it, or you don't, in which case, you need to call an electrician to check it.

Secondly, you may need to call an electrician to make the run away -- code may require it. Cable type needs to be check -- will it require plenum cable (translation, not PVC insulated, which releases chlorine in a fire.) This depends on code, and if it's not up to code, various bad things can happen, like occupancy permits being revoked, or insurance not paying out if there is a fire.

If there's a messenger line in the conduit, the pull will be easy. The easiest way to piss installers off is to use the last messenger line and *not* pull a new one along with the wire. So, you'll actually pull your cable *and* a new messenger
line with the old.

If someone has been an asshole and done just that, you'll either need fish tape long enough to run from hither-to-yon, or a sacrificial cable to act as the messenger.

There's an old rule I have -- NEVER PULL ONE CABLE. If you need one, pull two. At worst, the unused one will be a fancy messenger line, but pulling cable is a pain, and expensive in time. You can pull two cables at the same time almost as easily as one.

The pull should be easy. Try to avoid cable lubricants unless you have lots of bends in the conduit. 100mm is a large conduit, if the conduit is mostly empty, the pull itself should be trivial.
posted by eriko at 7:19 AM on February 28, 2007 [2 favorites has favorites]

The "official" network operations group did this between two building where I work (a big university/medcenter), but they used stainless steel pipe since it was all above ground. It's been working flawlessly for two years so far.
posted by smallerdemon at 7:23 AM on February 28, 2007

FWIW, I shared an Internet connection with a neighbor for about 8 years. We were connected by a plain Cat-5 cable buried shallowly in the dirt. No conduit pipe or anything. Never gave us any trouble. Sure, this was the ghetto approach, but it worked.
posted by adamrice at 7:33 AM on February 28, 2007

I would second(third?) pulling two cat6. We just pulled 200ft of cat3 (100pair) and 2 cat6 lines that work perfectly. Those two will not affect one another from a interference standpoint.

good luck!
posted by ronmexico at 7:38 AM on February 28, 2007

Be sure to check the grounds / earths at either building. I went to a school with a split site and they had to run fiber between the two sites as there was a earth difference between the sides that would have resulted in current flowing.

This comes up a lot, so I thought I would do a little work..

Most common ethernet devices (self link) are transformer isolated. Here is the data sheet for the isolation transformer on the pictured NIC. I couldn't find data for the transformers on the Switch, but no doubt the specs are similar. I have at least one ethernet device that does not have an isolation transformer, but that is very rare. All this means that there should be no grounding issues when connecting devices or buildings. There is no DC path connecting normal ethernet devices.

Technically, there may be some high frequency grounding issues due to capacitive coupling, or extremely high voltage issues due to dielectric breakdown. Electrical engineering is a very subtle business in this way, and one is constantly surprised. Here is a previous question about sharing ethernet with your neighbour.

Finally, I'm not an ethernet expert. Please correct me, but only if you have evidence.
posted by Chuckles at 8:36 AM on February 28, 2007

Not sure if you've planned on this already, but be sure to have the cable there and ready, when the people come to install the phone wiring. You really don't want to try to pull the wiring through after the phone wires are already in there. There may be space in the conduit, but it's just more of a pain (you have a lot more friction when you're pulling wire through a conduit that already has stuff in it). It's a lot easier to get ALL the wires that you want to pull, and pull them through all at once, than do them individually. At least, such has been my experience.

And I would pull twice as much cable as you think you're going to need. If you need one CAT6, pull two. Heck, might as well pull 3 if you can afford it and there's space in the conduit -- STP wiring is darned useful. Cable is cheap compared to the labor involved in installing it. And with two or three, when you need more speed later, provided both cables are still working, you can use them in parallel with correctly configured routers at either end.

Oh, and if you can get the installers to do it (and there's still a substantial amount of room left in the conduit) ask them to pull some cord and leave it in the conduit, along with the wiring. So that when you're done, there's a piece of cord running through the conduit, in addition to everything else. (This was SOP with the electricians I used to work with, but might not happen unless you ask.) It makes it much easier to put another cable in later.
posted by Kadin2048 at 8:40 AM on February 28, 2007

If there's no pull string in your PVC pipe yet, and the pipe run is too long for your available fish tape, you can also use a shop vac (with its hose on the exhaust outlet) to blow fishing line through surprisingly long pipe runs.

Use a cheap plastic funnel (Wal-Mart auto parts) as a shop vac hose adapter on the starting end of the PVC pipe. Insert the funnel in the PVC pipe, and drill a small hole (1/8" or less) in the narrow neck of the funnel, just outside of the end of the pipe. If the hole's in the wrong spot, just tape over it and redrill.

Remove the funnel and thread fishing line (10 lb. test or more: Wal-Mart sporting goods) through the hole in the funnel neck. Tie a wadded plastic grocery bag (your Wal-Mart bag) onto the free end of the fishing line. The wadded bag should fit snugly inside the PVC pipe to make an air-tight seal, but not so tightly that it can't move (you should be able to pull the wad back out without much trouble.)

Insert the wadded bag with attached fishing line into the PVC pipe, then insert the funnel. Make sure the fishing line can move freely through the hole in the funnel neck (put the line reel on the shaft of a screwdriver.) Stick the vacuum hose into the wide end of the funnel, turn on the shop vac, and fire away.

If all goes well, the bagged end of the fishing line should pop out of the other end of the PVC pipe run in a few seconds. Use the fishing line to pull a proper pull string though the run.
posted by cenoxo at 9:25 AM on February 28, 2007

We ran a cable underground between our shop and house. A single Cat5 cable and a satellite cable are in the 125ft (38 meter) conduit. We are actually running telephone signal on the unused pairs in the Ethernet cable. It has been buried for about four years with no issues.

Yes, it is nice having TV, Television and Internet in the shop...

I don't know if this helps you, but I thought a little more info wouldn't hurt...
posted by Leenie at 9:25 AM on February 28, 2007

Chuckles is correct that ethernet devices provide galvanic isolation using transformers. Ground loops are not an issue because the transformers provide isolation.

However lightning is a real risk for any cable connecting buildings, even buried underground. Typical ethernet transformers are only rated at 1500 Vrms. Transformers for telephone lines are typically rated at least 2500 Vrms. A lightning strike anywhere in the area could blow up the switches on either end. Depending on your locale and the frequency of thunderstorms, this may not be a major concern. I would recommend that you insert surge suppressors in line on both ends to provide more protection.
posted by JackFlash at 11:19 AM on February 28, 2007

Without reiterating a hard-learned lesson involving capillary action, a construction site, a couple of pints of water and a fried Cisco blade - pretend that someday water will be introduced into the cables and plan accordingly.

You're on the right track re: condensation, but think in terms of 'gallons' instead of 'drops'. I believe there's Cat5e/6 with some sort of liqui-gel inside to protect against this sort of thing.
posted by bhance at 1:42 PM on February 28, 2007

We ran some antenna wire underground, and used some conduit to bring it in the house. If it's 'sealed,' stuff like rats getting in isn't such a big issue, but cold coming in is. We used some "Great Stuff" or whatever it is to seal the ends. The big issue, I'd think, would be where it interfaced with the buildings, and how well-sealed it was.

I have no experience in the area, but I seem to recall that there's normal, cheap twisted pair wire, and then there's the ungodly-expensive Plenum, which is fireproof. Far-fetched scenario, but I can imagine a fire in one building, and the fire spreading via the conduit. It's things like this that make me think you should get a professional.

As others point out, lightning issues and the like exist, too.

Honestly, you probably won't have big issues short-term, but there's lots that could, especially long-term.
posted by fogster at 2:48 PM on February 28, 2007