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I am renovating a shed which is going to require doing at least a 100' trench for power from the house. I also want internet via a hard wire out to a a WAP in the shed. My question:

To avoid EM interference, do I need to run a separate trench just for the network cable?

My intuition tells me dropping even a shielded cable in with the 60-100A power line will not end well. Maybe a wireless approach would be more cost effective?

Updated 10/13: thanks for all this great information...let me clarify the use case here. The shed/cottage is to be for peloton, yoga, tv; like a "she shed," but it's for "us" :) I walked off the likely path to get conduit from router in the basement to the shed and it is more like 200 feet. So I am thinking this is like $50 of cat5e stuff, $200 for fiber, or there are these long range wifi things I have seen on amazon that look like $150-200 or so. I would not want to spend much more than that.

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    "Faster than wired using consumer gear" [citation needed], @dandavis
    – FreeMan
    Oct 13 '20 at 10:24
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    @dandavis TBH, I've not been in the WiFi marketplace for quite some time, so this is rather surprising to me. At the risk of wandering too far off-topic, I wonder, though, if a 6000Mbps WiFi router isn't wandering into the 10G wired territory in terms of pricing and residential/commercial application. Also, I do know of many folks who run 10G at home between their media servers and streaming devices. (I personally think it's overkill, but it's their $$$...)
    – FreeMan
    Oct 13 '20 at 14:26
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    Since this is 2020, you may want to go ahead and use Cat 6 instead of Cat 5. What data do you expect to be pulling out there at the shed? At any rate, wired will be far more reliable than hoping for a decent WiFi signal 100ft away, not to mention far faster, and 100ft of even Cat 6A cable is far cheaper than a WiFi 6 router.
    – TylerH
    Oct 13 '20 at 18:59
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    @dandavis: Your claims that a new wifi router outperforms gigabit, in real-world situations, is complete BS. Wired ethernet isn't subject to interference, saturated channels if you are in a populated area, and a host of other problems wifi can encounter. When already digging a trench, there is no easier time to lay an additional conduit for what the future will bring, and it costs very little to do so. Not doing so is insanity Oct 13 '20 at 19:34
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    @dandavis WiFi vendor speed claims fell victim to marketing shenanigans years ago. Claim-inflating tricks: Assume use of 160 MHz channel (there are only two; both use DFS; in practice it's uncommon to achieve outside a lab). They may sum the bandwidth on both bands (but no single client can use both bands concurrently) and may sum the bandwidth available through all 4 or more streams (but there are no clients that can use more than 2 streams). Then the AP is fed by Gbit Ethernet anyway.. wifi will often keep up with Gbit Ethernet but it won't outperform like "6000 Mbps!!" leads one to believe.
    – Greg Hill
    Oct 13 '20 at 19:37
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Same trench is fine. Use conduit, you may want to upgrade it later without re-trenching. Keep some space between the conduits, vertically, horizontally, or both.

Interference between 60 Hz power and 100+MHz ethernet is wildly overestimated by numerous people. Ethernet encoding is designed to ignore noise, and ethernet cabling is deign to reject noise so it does not get picked up, and the "noise" of a powerline is very, very much slower than what goes on in ethernet.

Still, the better bet for "what goes in your conduit" is fiber optics, but that's mostly because it provides total electrical isolation for your network equipment at both ends. It also happens to be immune to any electrical noise along the way, but that's far less likely to be a practical problem than you imagine, while side effects of thunderstorms are far more likely to be a practical problem with a wired connection.

Whatever cable you place should be "wet-rated" - for whatever reason, direct burial cable seems to be the most common wet-rated variant seen in ethernet, but put it in a conduit anyway. Outdoor conduits are defined as (and usually are) wet locations.

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    Yeah, the NEC requires primary protectors on outdoor network/telecom runs to deal with those thunderstorm side effects... Oct 13 '20 at 2:16
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    if you're running parallel conduits I'd suggest stacking them vertically with networking on top. It may mean digging deeper, but any future errant shovel/back hoe will be more likely to only breach the conduit which doesn't have the potential to kill anyone whose tool just made contact with the wires within. Oct 13 '20 at 18:54
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    Regarding lightning strikes, check out The 8-Bit Guy's video regarding his experience. Starts at 5:55: youtu.be/Ev0PL892zSE?t=355
    – Eilon
    Oct 13 '20 at 23:21
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    @DanIsFiddlingByFirelight Separate conduits, vertically stacked in the same trench. Use a metal conduit for the network cable and now you have both shielding and protection from damaging the 100A cable.
    – Mast
    Oct 14 '20 at 6:39
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    There should be white indicator tape above the comms cable in any case. Black outdoor Cat5 cable is a little hard to spot. The case for fibre is more removing the issues of earth differential and lightning protection rather than avoiding EMI. You should definitely use conduit, and for shared trenches there's particular indicator tape and signage so that people don't kill themselves believing they're already removed the conduit without realising there's another service underneath.
    – vk5tu
    Oct 14 '20 at 14:20
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Definitely go conduit here

I would lay two conduits (one for mains, one for data/telecom) if I were in your shoes; this way, you don't have to dig anything up later to upgrade it. 1" is adequate for most telecom cables you'd want to run; however, while a second 1" will accommodate smaller mains feeders (up to 60A, or perhaps a bit more), I would recommend using 1.5" instead if you are interested in having a full panel at the shed with a feeder to it instead of a simple MWBC+disconnect box configuration. That way, you will have enough room in the future for a 100A or 125A feeder that's ample for a full-blown shop, ADU, or other such power-hungry use in the future. Don't forget to leave pull-strings in conduits you aren't going to use right away!

Fiber is also a very good idea, because you'll need some primary protection otherwise

The other thing you'll want to do is get a couple of small switches with SFP support (or media converters, if you can't find suitable switches), a pair of SFP transceivers with duplex LC connectors on them and some preterminated direct bury/wet location duplex fiber patch cord (LC/LC) of the appropriate length for your run (don't forget to leave length for the stub-ups at each end!); this way, you won't have to deal with the problems inherent with running copper telecoms lines outside. If you do insist on using copper for this application, you'll need to fit UL497 listed primary protectors at both ends of the run to satisfy 2017 NEC 800.90(A) regarding interbuilding circuits that are exposed to lightning, as that's the main threat to your Ethernet run, not mains interference.

This requirement, unfortunately, is not easy to find parts for when it comes to datacom circuits. ITW/Linx is the only vendor I've been able to find carrying any (no affiliation with them), and their CAT6-75-110 model would be the best suited for your application I reckon. You'll need one for each end of the link in order to protect both buildings from lightning or power cross induced voltages. Furthermore, you'll need to connect their grounding points to the intersystem building termination or grounding electrode system for their respective buildings with a 10AWG minimum bare copper ground wire in order to meet installation and NEC bonding requirements. This also means the shed will need to have a grounding electrode system (2 8' rods 8' apart, connected to the disconnect or subpanel with 8AWG minimum bare copper), even if it is not otherwise required to have one by Code.

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    Usually it's less expensive to get a pair of small switches with SFP slots and SFPs than to buy "media converters" - probably because there's more market for 4-8 port switches than (effectively) 2 port "switches."
    – Ecnerwal
    Oct 13 '20 at 10:52
  • Absolutely conduit, with at least one run of draw-cord installed already. It should be an Internal Diameter big enough to pass a pre-terminated fibre cable end like a duplex LC/LC connector, or if OP might need older kit then a SC/SC duplex plug might have to fit.
    – Criggie
    Oct 13 '20 at 23:55
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    @Criggie -- 1" seems to be big enough for LC/LC preterms (at least according to a distributor source I found) Oct 13 '20 at 23:58
  • can you tell me precisely what I am looking for in terms of the fiber here? So I need a switch on each end that I can patch to my verizon router and to an access point on the other end...and I need the fiber itself. Assume this is a 200-foot run and at best I have gigbit access; actually only 300Mbps now from verizon. What is the most cost-effective set of devices I need to buy here? Nov 2 '20 at 17:56
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Once you've done all the hard work of digging the trench, dropping an oversized conduit for the power is a cheap upgrade, as is dropping a separate conduit for the low-voltage wiring. Conduit, while it will cost a bit more to install, is very cheap insurance against any future upgrades/repairs, as it's pretty dead simple to pull replacement wire/cable through the conduit as opposed to re-trenching to direct bury new cable.

Whether it's required or not, there's not much reason not to, and it certainly won't hurt. Though, I'm not sure that PVC conduit will do much to stop the EM, the construction of both the mains voltage and low-voltage cabling will do a fair bit to reduce/eliminate the interference.

Just be sure to use CATx cabling appropriately rated for outdoor/wet area use. As I've been reliably informed, buried conduit will fill with water. It's guaranteed enough that you should just go ahead and fill it immediately upon installation to get it over with... ;)

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I don't think you'll have EMI problems with your CAT5e because of the voltage-diffential-signaling nature of modern Ethernet. Any induced current from the electrical feeder to the CAT5 should not cause problems.

However, you may want to use a conduit instead of a direct-burial CAT5 cable. For 100' the conduit will be inexpensive and allow you to replace the CAT5 with optical fiber or other types of data cabling in the future, if desired. If it were my project I'd consider conduit for the electrical feeder as well; same reasoning, plus in some areas, your trench can be shallower, depending on if you use metal or PVC conduit.

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    Note that if they do run copper Ethernet, they'll have to put primary protectors in at each end of the run, most likely. Oct 13 '20 at 2:13
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    From experience - CAT5, laid alongside with power cabling, feeding industrial equipment (including motors and various machine tools etc), did create network problems. I have no knowledge about exact cable types and layout, but separating network cables did wonders :)
    – Arvo
    Oct 13 '20 at 9:44
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Ethernet is supposed to be EM tolerant, but if it's really a concern, there are other options. Running a second line through another ditch is asking for maintenance problems down the road.

Option 1 Run your power line in shielded cable or conduit. Then your Ethernet line can lay in close proximity without worry of EM interferrence. Note: If using shielded cable, make sure the shielding is properly grounded on one end.

Option 2 Use the newly existing power line to do Ethernet Over Power (EOP), not to be confused with Power Over Ethernet (POE).

Ethernet Over Power (EOP) converter example

Option 3 Run fiber between your buildings. Fiber optical cable is completely impervious to EM, as well as the deleterious effects of ground water.

Ethernet-Fiber converter example

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    Powerline Ethernet adapters are definitely the easiest solution here, but with a caveat. They may not work if the shed has its own separate breaker box or subpanel. The units I've used in the past could talk to other circuits on the same panel, but couldn't see devices connected to subpanels.
    – bta
    Oct 15 '20 at 1:43
  • Powerline Ethernet will also produce a whole new problem: your powernetwork is now a big antenna radiating garbage all over the place.
    – Mast
    Oct 15 '20 at 7:03
  • @Mast In-home power networks are already large antennas. Before the era of digital television, I recall there being some products that would even allow you to use your home power network as exactly that, a giant antenna for your TV! At any rate, EOP works on a similar principle, minute differentials--too small to be of any concern for plugged-in appliances--are created by the transmitting side of the network and interpreted by the receiving side. Even if additional EMI is generated by this process, it's so small that it would require lab-grade equipment to discern it.
    – Jim Fell
    Oct 15 '20 at 13:41
  • @JimFell You'd think that, yet I've seen plenty of stories of people finding out differently. There's unnotched garbage on the market (yes, even by the prime brands) that's simply too disgusting to have in house. A HAM operator can detect you from 2 miles out.
    – Mast
    Oct 15 '20 at 14:28
  • Fiber optic cable that is NOT outdoor/wet rated is NOT "immune to the deleterious effects of water." I happened to get an opportunity to chat with a fiber company engineer (as opposed to the average sales-type) because I was inquiring about fiber when he happened to be coming to the place next door about a "warranty claim" which was firmly denied as they had run indoor type fiber through outdoor (wet) conduits.
    – Ecnerwal
    Oct 15 '20 at 17:06

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