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I’ve got around 40 Ethernet cables bunched together. They’re sticking out of a wall in a basement room. Their ends are cut, so there are no connectors on them.

All of them lead to endpoints elsewhere in a large building. Two of those endpoints I have access to. At those points, the cables end in female RJ45 sockets.

I would like to find out which cables in the basement lead to those two endpoints. Once I know this, I’d like to connect those two cables to establish a connection between the two aforementioned endpoints elsewhere in the building.

I have several devices I could connect to one or both of the two endpoints, including laptops and a router.

I do not currently own or have easy access to a multimeter or any even more sophisticated tools. I do have batteries and a phase tester.

I’m looking for an easy way to identify the right cables without having to strip the isolation off countless cables or even connecting RJ45 connectors to the cables one by one until I hit gold. I’d like to avoid purchasing tools I might never need again.

It occurred to me that applying an electric current to the endpoints of the two cables and then checking on which of the cables in the basement I can detect the current with a phase tester might work. However, I am unsure about how to safely (!) do this.

Any help would be much appreciated.

I found this question to be asking for something similar, but the solutions offered seem to not transfer to my issue

2 Answers 2

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Quick and dirty:

Make a connector connecting the pairs to a battery and a low power light and put in one of the two sockets.

You need a helper with a walkie talkie or smartphones…

Then take each cable end in turn and put into a conductive solution. You can test this first to make sure it will work.

There is a variation of this about a high rise with 3 black wires on the bottom floor and the top that need to be labelled 1, 2 and 3. You have a multimeter. The question is how many trips are needed in the lift to label all 3. And the answer is one round trip.

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  • Wont the solution corrode the connectors?
    – Willk
    Aug 14, 2021 at 21:25
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    This answer has merit for ease of use. After the cables have been identified, use alcohol to clean off the affected cables.
    – fred_dot_u
    Aug 14, 2021 at 21:28
  • Lovely idea, thanks! Any suggestions for the kind of light bulb (fridge light?), battery (AA?), and solution (water?) to use? I’ve got 99% isopropyl to clean the cables afterwards, but I could also just cut off a bit after testing them - right? There’s about 15m / 45ft of cable lying around uselessly in the basement, so shortening them by an inch is not gonna bean issue. Aug 16, 2021 at 8:13
  • Am I correct in assuming that I would just connect the two sides of the battery and the two pins of the light bulb to one random pin each of the 8 pins of the RJ45 socket? My knowledge of elementary physics is substandard... Aug 16, 2021 at 8:16
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Tug the side with connectors and watch to see which ones move

Electricity with the volts and the ohms and all of that. How about just tugging on them?

Recruit a helper. Have her stand on the side with the connectors. We have cell phones now which makes this sort of thing so much easier. Call or video her.

Now have her pull one of the lines with connectors. See which of the cut ones moves. She needs to pull gently because you don't want it to disappear into the wall. Yell at her to quit pulling because it is going to disappear into the wall. She reminds you that you don't have to yell because you have the cell phones on; apologize and note that old ways die hard. Mark that cable A on both sides with a tape label. Pull it back out because it almost disappeared into the wall.

Now the next one. Mark that one B.

You could do all the rest while you are set up. Even though they dont have connectors maybe it will be useful to know later on and it is not much extra work. For example after A accidentally disappeared into the wall you could pull it all the way free on your assistant's side then use C to pull it back along the path that it used to take.

Take assistant out for smoothies.

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  • So if this “large building” means runs of 20 metres then that ciuld be a lot if tugging… especially if the cables are clipped to trunking or trapped in sharp bends…
    – Solar Mike
    Aug 14, 2021 at 21:39
  • @SolarMike - you are right that if it is securely attached along the way tugging one end might not do anything on the other. If not attached but there is a lot of slack you truly might have a lot of tugging but it would still work. Maybe before recruiting your assistant pull on one of the cut ones and see how much play you have.
    – Willk
    Aug 14, 2021 at 21:49
  • probably because I have used these methods before…
    – Solar Mike
    Aug 14, 2021 at 21:59
  • Props for the entertaining read :D Not going to work though, for the exact reasons Solar Mike suggested. Aug 16, 2021 at 8:08

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