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In a few days, we will be having a small tower set up so that we can have internet connection in our rural home. We need to trench a cat-5 cable from the tower to our house but we also want to, in the same trench, run a power line out there so we are able to go farther with electrical items such as weed-eaters. What I wish to know is, how far down should the power cable be from the cat-5 as to not interfere with data signals? Both cables will be in separate conduits.

Prowler1000
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  • What kind of tower is this? I think there's a limit to the number feet that something like cat5 can be run, regardless of nearby power lines. Are you sure they won't be bringing fiber or cable to the house? – BrownRedHawk Aug 11 '16 at 15:26
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    If it's metallic conduit, that will give you good isolation even if it's right next to the CAT-5, but even without metallic conduit, CAT-5 has built-in noise isolation, so it should be fine laying next to the power cable. But, if this were my house, since you'll have power at the tower anyway, I'd run fiber and use a media converter on each end to convert back to cat-5. That will prevent stray voltages/lightning from making it back to your network equipment. – Johnny Aug 11 '16 at 15:28
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    Just use shielded flavor of cat5 (STP or FTP instead of UTP) - if you can't afford fiber – Agent_L Aug 11 '16 at 15:29
  • @Johnny We don't have a whole lot of money for that. The company is supplying the cat-5. They aren't metal (I think they are just a type of rubber). – Prowler1000 Aug 11 '16 at 15:31
  • @BrownRedHawk It's an absolute maximum of 50 feet. – Prowler1000 Aug 11 '16 at 15:32
  • @Prowler1000 Beware, shielded cat5 is quite expensive, especially compared to a cable provided by the company for "free". Over longer distance fiber can be cheaper, because only endpoints are expensive, the fiber itself is almost as cheap as the sand it was made from. – Agent_L Aug 11 '16 at 15:36
  • What size power cable would you want to put in the trench with the cat5? – mmathis Aug 11 '16 at 15:36
  • @mmathis I'm not sure, we have an electrician coming to do that. All I know is they are in two separate conduits (which we have) and that the electrical is below the cat-5. – Prowler1000 Aug 11 '16 at 15:38
  • As far as National Electrical Code is concerned, the two cables must be separated by a permanent barrier. NEC only seems to be concerned with preventing short-circuits between the cables, not interference on the communication cable. – Tester101 Aug 11 '16 at 15:41
  • @Agent_L We have the cat-5 already and it is UTP. Do you feel there will be interference even if the there is nothing plugged into the other end of the power line? – Prowler1000 Aug 11 '16 at 15:44
  • @Prowler1000 No, I believe even worst kind of such installation will mostly work. It may be unable to reach full 1000Mb/s sometimes or will lose some packets every time something big is switched on or off but it will mostly work. Just not as perfect as it should. But, there is chance you'll never notice any problems. – Agent_L Aug 11 '16 at 15:50
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    @mmathis I saw that but it was for inside walls. I am wanting to trench cables. – Prowler1000 Aug 11 '16 at 15:52
  • @Agent_L Alright. We won't be using the internet if there are tools plugged into the other end of the power line as we will be working. Thank you. – Prowler1000 Aug 11 '16 at 15:53
  • Does anyone know if this is a new common practice? From what I've read there is a limit to length of ethernet runs without a powered switch/relay. Does anyone know if this is a new standard versus cable, fiber or just DSL over existing telephone? I'd imagine most people are a minimum of 50' from overhead utilities. – BrownRedHawk Aug 11 '16 at 16:15
  • @BrownRedHawk GigE specs show 100m (~330 feet). Anything beyond that and you need to worry about amplifying the signal. – mmathis Aug 11 '16 at 16:24
  • @Prowler1000 What's the length of the run from your service provider/utility to your first device in the home? – BrownRedHawk Aug 11 '16 at 16:26
  • @BrownRedHawk I don't know. But it is an absolute maximum of 30 meters. I don't know how long the cable is. – Prowler1000 Aug 11 '16 at 16:30
  • This is NOT a duplicate of that. The exterior conduit case is covered by an entirely different standard (from an entirely different organization) than the interior cables in the walls case. – Ecnerwal Aug 12 '16 at 13:28

2 Answers2

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There is, in fact, an applicable standard.

National Electrical Safety Code (NESC), Section 320 (B) (2),

Separations Between Supply and Communication Conduit Systems: Conduit systems to be occupied by communication conductors shall be separated from conduit systems to be used for supply systems by 75 mm (3 in.) of concrete, 100 mm (4 in.) of masonry, or 300 mm (12 in.) of well-tamped earth.

Only one of those is practical and affordable for a project on a budget, so I bolded it. You can be 12" apart at the same level, 12" above (power deeper, communication shallower) or a diagonal that works.

Type of conduit makes no difference - only the material separating the conduits changes the spacing required.

Ecnerwal
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  • Is that spacing for safety only or for interference as well? – mmathis Aug 11 '16 at 19:20
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    @mmathis: The design of CAT5+ cable (plus ethernet wiring standards) is such that it inherently rejects interference. The separation has more to do with lightning and static protection than signal pollution. – wallyk Aug 11 '16 at 19:22
  • @wallyk The design (twisted pairs) does not reject external interference (like that from a nearby power cable). This is why, after all, it's suggested to not run data lines next to power, and if you do need to cross them, they should be crossed at right angles. – mmathis Aug 11 '16 at 19:30
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    @mmathis: Yes, it will inject signals, but in common mode only. The transceivers at the end only look at differential signals across the pair and ignore common mode information. – wallyk Aug 11 '16 at 19:32
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    It is an electrical safety item (first and foremost) and a maintenance item secondarily. If a power cable fails and shorts, burning an arc through its conduit, the intent is that the communications conduit should not be affected. The "side-by-side" orientation is preferred (but not required) such that one of the conduits could be dug up without disturbing the other. – Ecnerwal Aug 11 '16 at 21:38
  • The NEC has similar language, however, it only applies to service entrance cables. There doesn't seem to be any specifications for wiring within the building. I'm not familiar with NESC, so I'm not sure how "Supply" is defined. – Tester101 Aug 13 '16 at 12:44
  • NESC definitions section "2. electric supply lines.* Those conductors used to transmit electric energy and their necessary supporting or containing structures.Signal lines of more than 400 V are always supply lines within the meaning of the rules, and those of less than 400 V may be considered as supply lines, if so run and operated throughout.*" – Tester101 Aug 13 '16 at 12:57
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Possible duplicate of Can I run CAT5/6 cables parallel to electrical cables?, although that was more concerned with in-wall installation rather than buried. The general consensus from that thread seemed to be about 16" spacing (one wall cavity between studs) for typical #12 wire (servicing 20A circuits), though of course more distance is better. Assuming your power cable will be #12 or smaller (i.e., you're just planning to put a couple outlets over there, and no pool equipment or an AC condenser or something), that would be a good guide to use as well. Depending on what the conduit is made of, you could probably go a bit closer together to avoid more digging...but not closer than 12" (as per @Ecnerwal's answer)

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mmathis
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