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My home network setup is that I have a simple network switch fed by a cable modem in my utility/data room and then 6 RJ45 cables going out of it to different rooms of the house. I like to have wired network connection in every room. 5 out of 6 cables are accounted for positively and correspond to the other remote ends but one does not connect, which means when I mount female jacks on both ends I don't get connection.

Since the ordinary connectivity using regular connectors is failing, I would like to test each of the 8 constituent wires for continuity. My fears are that, since the cable runs through the attic, a rodent ate it or that the guy who installed the AC duct work may have accidentally cut it.

These wires are super skinny, I believe gauge 28 or thinner. Can I run some current using a AAA battery on one end on each wire and test it with a multimeter on the other?

P.S.: I didn't test this immediately after running the cable, which is really my bad and could have prevented it.

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  • RJ45 ethernet cable ... that's not really a thing ... RJ45 is a connector that is used on ethernet cables ... the cable is usually UTP (unshielded twisted pair) ... the cable is also referred to by category, such as CAT 5 cable
    – jsotola
    Apr 2, 2023 at 2:38
  • start by twisting the 8 conductors together at one end ... measure resistance between the 8 conductors at the other end ... that will detect any opens ... disconnect all conductors and measure again ... that will detect shorts
    – jsotola
    Apr 2, 2023 at 2:39

3 Answers 3

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10 MB or 100MB ethernet only require 4 conductors (wires). Cat 5 & 6 have 8 conductors, 1GB ethernet does require all 8 conductors.

Next: Although others have said differently, a cable tester is very handy and a huge time saver. Since product recommendations are off topic I won't make a recommendation, but just let me say I have some and they've been invaluable in trouble shooting, esp. when I help friends.

The prices for them vary WIDELY...from $10 on Amazon for really simple ones to over $1,000 for very sophisticated ones. I'd stay away from the $10 cheap ones, but you should be able to find one for about $40 or so that would more than meet your needs. My recommendation is to get the kind that have 2 parts: one that you plug into one end of the ethernet cable and another unit that actually does the testing that you plug into the other end of the cable. It'll tell you exactly what's wrong.

Trust me, they are worth it.

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28 Ga would be unusual. 24 is more typical...Unless you are inappropriately using patch cables as in-wall cables, rather than proper solid wire cable intended and marked for in-wall use.

Anyway, trivial to test - just connect a battery (I tend to use a 9V, but whatever you like) to two of the wires on one end (with the other end not connected to anything) and look for voltage on the other end between those two wires. No need to buy a network cable tester which someone will be along to suggest in a moment if you're just checking continuity and have a multimeter.

You can also short each pair at one end and check resistance from the other.

Likewise, you can make sure each pair is open (not shorted) and verify no continuity (infinite ohms) between all wires.

There are more subtle issues that a fancier network tester might be useful for, but worry about those after you do the basic tests, and really, running the cable anew will cost less than buying a fancy tester for "one cable in your house" (since the tester might well just show a problem that you'd still have to correct by running a new cable.) A cheap tester is no better than using a multimeter (other than speed) and can be worse (the indications of what's wrong can be unclear .vs. what you can see with an actual meter.)

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I think testing individual conductors for continuity is not a good use of time. If it doesn't work it doesn't work. Maye it's the ends so cut both ends back an inch if you can and replace the connectors. If it still doesn't work, abandon it. Run a new one.

Continuity doesn't mean it's good anyway. With ethernet the shielding and connectors all have to be intact and to spec. You can't test that with an ohmmeter. There are tools to test it with but you already know it doesn't work so just run a new one.

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  • I cannot run a new one because the drywall has been hung
    – amphibient
    Apr 1, 2023 at 15:57
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    You most certainly can. Drywall repair may be required, of course. Or you get your network to that room some other way.
    – Ecnerwal
    Apr 1, 2023 at 15:59
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    @jay613 i am almost religiously devoted to having wired internet in every room
    – amphibient
    Apr 1, 2023 at 17:01
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    Drywall is hung, but is the existing cable stapled down? Could you possibly use it to pull a new cable?
    – Huesmann
    Apr 1, 2023 at 19:25
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    When we built our house, our contractor said not to bother with cat-5 or cat-6 cabling bc everything was going wireless. I over-rode him on that and put POTS lines, Cable and Cat-5 to most rooms, really glad I did. It was a high quality WIFI router and not very far from the ROKO box. Can't beat hard wired both for performance and security. Apr 3, 2023 at 0:43

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