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I recently moved into a house built in 2015, and I’ve been trying to see how practical it is to wire parts of the house for Ethernet.

In the basement, there’s a bundle of 9 Cat5e cables. 6 of them have blue jackets and 3 of them have white jackets. They all run up into the ceiling, but most of the basement is finished so I can’t really see where they go past that.

There aren’t any RJ45 plugs in the house, but I did track down some of the cables:

  • Two of the cables (white jacket) are sticking out of exterior walls by the front and back doors, presumably for PoE IP cameras
  • Two of the cables (blue jacket) are wired to phone jacks.

That leaves 5 cables that I have absolutely no idea about. I’ve checked closets and everywhere I can think of, but I can’t find anything.

There are a handful of unused motion sensors mounted around the house, but those seem to be connected to a separate alarm panel in the basement with different wires.

Any ideas for where these cables might be running to? I’d imagine they weren’t just left dangling in the walls, but at this point I’m not sure.

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  • probably left dangling in the walls
    – jsotola
    Sep 30, 2021 at 4:22

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I'm making the following suggestions in the form of questions, but they're hypothetical -- not expecting a comment that answers them!

Does one of the white cables go to a doorbell, or to a place where a doorbell transformer might get added? (or, is there other wiring you know serves the doorbell role?)

Does one of the blue cables go outside to the place where the telephone demarc is (or could be, if you had phone service hooked up)?

Is there any prep for a TV to be mounted on a wall, perhaps somewhere in a family room, or above a fireplace, or in a master bedroom? Do any of your mystery cables show up unterminated in those areas, possibly behind a blank cover plate?

Was the home built by an owner-builder (cables could go any random place) or by a home-building company (likely to be some intent in their placement)? In the latter case, if there are similar-looking homes in the neighborhood, you might ask a neighbor if their house has a bundle of cables too and whether they've figured out some that you haven't.

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    Look for blank cover plates and open them up (giving adequate care in case they are mains power, but that level of care is easy to manage, IMHO) is a good bet. As is the Telephone Demarc.
    – Ecnerwal
    Sep 30, 2021 at 13:29
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    Remember to look for blank plates on ceilings - that would be a bit of "good design for wireless access points" if some went there; also look in the attic if there is an attic to look in.
    – Ecnerwal
    Sep 30, 2021 at 13:46
  • It turns out that they were left dangling behind empty-ish wall plates. The house has ceiling speakers in a few rooms, and each set of speakers has a two gang wall plate with a volume knob / transformer in one slot and nothing in the other. Lo and behold, each of those plates had an unterminated Cat5 cable hanging behind it. I have no idea what they’re there for, but I was able to extend them down to the floor and add new plates for them. Thanks!
    – Matt K
    Jan 1 at 20:52
  • @MattK The setup you describe means that the cables were intended to be control and/or audio input cables for the speakers using a "whole house audio routing" system. Most of those systems that I've seen use CAT 5 cable to connect controls (and inputs/outputs) in each room with the main control unit (example).
    – Moshe Katz
    Feb 11 at 4:25
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I would shut off the mains power and energize each cat5 cable in turn. There are several kinds of through-wall scanners / detectors that will find the live cable conductors, so just follow them through the wall/ceiling until you find the end of the cable.

There are also tracers that you can use, where you clamp one device on to the conductors at the visible end and then probe through the wall to find the cable. However be careful with these because there are a lot of inexpensive ones that don’t work well.

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