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I am mapping out a path to run Cat6 thru my house. I know not to run Ethernet parallel to power and the Intartubes say to try to cross at right angles.

But how much interference will I get by crossing at right angles? Does it totally negate the interference or is the interference just not as severe?

The easiest path from my patch panel to the two most important wall jack locations would cross Romex three times at/near 90 degree angles. Once inside a wall cavity and twice in the attic.

Inside the wall cavity, the Romex runs "East/West" between wall studs. The Cat6 (12-18 total drops) would run "North/South" in the same cavity.

Half of these drops would come up from a 1" hole to the crawl space. The other half would be terminated in keystones on one side of the wall.

The Cat6 would then run up the cavity and out a 1.5" hole near the ceiling into a closet on the other side of the wall.

The cables would then be fed thru a hole in the ceiling into an unfinished attic. In the unfinished attic they would run across some joists, along a joist, then back down into another closet where the patch panel is

Along the attic path they cross two more Romex runs at right angles. The switches for the bathroom light and fan (2 Romex cables) and another Romex cable from a bedroom light switch. The bathroom switches and bed switch are maybe 5 feet apart

How much interference/network slowdown would I encounter?

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Darn near none. Even less if you do it right and run conduit so you can replace it when you can't live without Cat9, or 12 pair fiber, or whatever everyone will have in 5 years.

The whole point of the fancy-pants wire is that it is, in fact, resistant to interference pickup, in places a lot more challenging than your house. Though I hardly see the point of Cat6 - it can barely run 10Gbit, and Cat5e runs gigabit just fine (I have over half a mile of it (in sections less than 100meters, of course) connected to switches that report error rates, which are pretty much 0)

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    I agree about interference being a non-issue here, but I'm not sure that covering a house in conduit is a good investment. It's a lot harder to install effectively than CAT6 wiring and it's not clear if it will ever be useful. I doubt ethernet will go anywhere anytime soon. (Home AV cables seem to change a lot more frequently, so running conduit from a media closet to a TV makes a lot more sense, I think.) – Hank Jun 11 '15 at 22:20
  • Well ethernet may not go anywhere but it will get faster and that usually requires new cable. Still this is less of a concern than years ago. There aren't many applications which could use 10G or 100G ethernet. – Gerald Davis Jun 12 '15 at 15:05
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Wires that run perpendicular to each other will couple little or no signal between them, by basic physics. Parallel might be more of an issue, but as others have noted you'll probably never notice an occasional missed packet.

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None if done right. The entire reason for twisted pair cabling and differential signaling is to get a clean signal through a noisy environment.

A bad install is going to result in an inconsistent low performance link. The good news is it isn't rocket science so just take your time and go slow. Yes power lines can add noise and you want to cross at 90 degrees at that reduces the effective noise (after the effect of the differential signalling) but power lines aren't the only thing to watch out for.

Don't use too much force when pulling the cable. If it snags then fix that at the source and resist the urge to pull with all your might. The reason is that when you overstress the cable it can cause the wire pairs to deform so they are no longer properly twisted and that reduces the EMI protection. These are the worst type of problems because visibly the cable will look exactly the same as a good one. It also will reduce the crosstalk between wires in the cable. Each wire pair has a different twist rate so they block signals from each other but when you high force the lowest twist rate wires tighten up first and can become in sync with its neighbors. Short answer is just take your time, do it right the first time and the wire will probably outlive you. Use excessive force to just get it done and you may end up with a error prone link that makes your wonder why you just didn't go wifi.

Other things to watch out for. If you use solid wire (you should it is better than stranded for in-wall installs) make you the wire can't move. That means no dangling ends. No putting a RJ-45 jack on one end and have it hang out of the wall. Solid core is much less forgiving than stranded and the wires will eventually snap. So staple it down periodically and have it hard terminate into jacks or patch panels.

Be sure to make a loop of cable (usually I do a loop with 2" radius twice. That gives you about 1' of slack in case the ends need to be reterminated. Make sure to secure both ends of the loop so it doesn't move. Remember terminations may need to be replaced and the cable will be cut short each time (if it gets too short someone may decide to pull on it to try and get some slack). Lastly take your time to ensure the cable isn't damaged. A kink in TP cable can kill the EMI resistance. When stapling the wire in place be sure not to crush the cable. Use properly sized staples. They should prevent the wire from moving easily without crushing down on the cable. It also isn't that unusual for someone rushing to put a staple right through a cable (even pros do it) so take your time.

Also I would make a extra run to each drop. Even if you leave it unconnected it can be used as a backup or if you needs change (maybe you will run video over it in the future). It doesn't really add any extra cost or time to make an extra run at the same time.

If you are truly worried there is STP (shielded twisted pair) cable but it requires special terminations, generally is a pain to work with, and honestly would be overkill for your situation. If it makes you feel any better, data and electricity cross paths all the time in commercial buildings even in datacenters and STP is almost never used.

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