In a previous question, I had planned a Cat6 (Ethernet) cable run outside a home, adjacent to a coax run. I was advised against it, and I'm interested in taking that advice.

Now I'm considering running the cable inside an exterior wall. (I mean running it inside a wall which separates the house from the outdoors.) It will go from the basement to the 2nd floor.


  1. Once installed, is this "safe" in terms of lightning and surge considerations? Does the single wall actually add a significant amount of insulation in addition to the miles of air gap between the house and the clouds?

If it's a reasonable approach...

  1. Does it matter if the cable is shielded or not? If shielded, should the shield be grounded?

  2. What additional protections should I add? Like types and locations of surge protection, etc. I'd like to protect the equipment at both ends, but one is more valuable than the other.

  • You do have some risk with network cables that most don't consider. I had a friend whose entire PC was burnt through by a network surge. Fortunately any good UPS has a network port for this purpose, and APC even makes a cheap ($25, Amazon) surge protector that also does coax. – Arthur Kay Jun 28 '15 at 17:30
  • I would recommend fibre optics; as it can not conduct electricity thus no risk at all of surges from the outside. – Ashley Primo Jun 28 '15 at 20:11
  • @Jnrprimo, AFAIK, most companies don't run fiber on all vertical runs. How do they protect themselves? Never run inside outdoor walls? Other protection measures? – cp.engr Jun 28 '15 at 21:25
  • I would generally not advise running any kind of metal cable outside which connects to any electrical equipment you wish to keep. Even a small lightning surge will destroy Electronics such as a laptop or router. However I have seen these small devices in which claim to be (RJ45 Surge Protectors; which have a ground line built in.) ebay.co.uk/itm/like/… – Ashley Primo Jun 29 '15 at 0:29
  • @Jnrprimo, I'm not not talking about running it outside. I edited my question to hopefully clarify. – cp.engr Jun 29 '15 at 4:33

The standard would dictate you do not run a cable inside a wall without putting it into a pipe (conduit), however this is often done. You're talking of home installation here, not professional office so that is not uncommon to find.

Generally, by experience, you won't have issues with that, even with an exterior wall. Assuming your house has an operational lightning rod and that the ground cable is not right there next to your network cable.

However, also by experience, I have had many computer which had their network card fried through surges dissipating through the network cables running in the walls. They must have picked up the surge through contact with other metal structures in the wall or other wires since the lightning hit the side of the building (the lightning rods weren't operational).

If your cable goes outside the building, then it should be outdoor cable and have a surge protector at both ends which is well grounded. If it runs inside the walls, then you can take the risk. Might not follow the building code of your area. It might never cause problems.

  • What is the purpose of the conduit? (Besides convenience of future runs.) Why is it "standard"? And by which organizations? I don't think the home has a lightning rod. – cp.engr Jun 28 '15 at 17:07
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    Its not contact with other metal objects that is responsible for the surge - its the EMF generated by the strike acting on a long wire, basically an antenna - no direct strike is necessary. A metal conduit serves to block the EMF (generally); The exterior wall of a house provides no filtering worth mentioning. It is standard by many codes as a result extensive testing; and to protect the wire from idiots driving drywall screws :) – pete Jun 28 '15 at 17:53
  • What standard are you referring to? – Craig Jun 28 '15 at 20:05
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    @cp.engr the cable shield helps reduce low levels of interference, such as from florescent lighting or cell phones, but its not designed to prevent a surge from such a strong source like lightning – pete Jun 28 '15 at 21:26
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    @Craig NEC/NFPA 70 (electrical code) would be the most relevant, covering the type of wire, installation, location, protection, etc...but there are plenty others that cover explicit aspects of communication cables. – pete Jun 28 '15 at 23:34

Answer to questions is found in concepts introduced in elementary school science. Lightning seeks earth ground. An electrically shorter path to earth is via a wooden church steeple. Because wood is a better electrical conductor.

Why is an ethernet cable better protected by wooden (plywood) walls? Same reason.

Now that is a direct strike to the structure. What protects direct strikes to appliances? A direct strike to utility wires far down the street is a direct strike incoming to all household appliances.

To protect the wooden structure, earth a lightning rod. So that earth ground protects the structure. To protect internal appliances and ethernet cables, earth a 'whole house' protector. So that earth ground protects everything inside the building.

One a surge is permitted (by humans) to be inside, then that surge will hunt for earth ground destructively via appliances. Nothing inside the building will avert that destructive hunt. You must connect that surge (ie lightning) to earth BEFORE it can enter. For the same reason that Ben Franklin's lightning rod kept lightning from conducting to earth destructively via wood.

BTW, that is earth ground. Not safety ground provided by wall receptacles. If a surge is not earthed BEFORE entering, then nothing will avert the destructive hunt. As has been understood and demonstrated for over 100 years. Appliance damage indicates a human made a mistake.

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