Multimeter traces home wiring

-October 13, 2011

Last week, I moved into a circa 1900 house after extensive renovations. We gutted the entire first floor and while there were no walls, I had the contractor’s electrician run twelve Ethernet cables from around the house to a closet. He also ran phone wires and cable TV from around the house to the closet. One of each cable runs to the basement where a Verizon technician connected the phone to the outside, then installed a terminal strip in the closet and connected the remaining phone cables together. The phones work everywhere. When we get cable installed, a technician will again be able to connect all the cables together in the closet. The cable in the basement will make the connection to the outside. That will let me lose three ugly satellite dishes from the front of the house.

You can’t, however, connect data cables together. You need to know where each one runs and connect them to an Ethernet switch/router. Unfortunately, the electrician didn’t document the data cable endpoint locations and he didn’t terminate the cables with RJ-45 connectors. The photo below shows a blue electrical junction box and an orange box that now contains an RJ-45 for data (blue cable) and an RJ-11 for phone connections.

  Wall containing power and data wires

To connect the data cables, I bought some RJ-45s and a crimping tool. Next, I had to find an Ethernet wiring diagram. You can find many of them online (here’s one) , but there are two wiring standards, TIA/EIA 568A and 568B. I opened one of the outlet wall plates that held an RJ-45 receptacle and found that the electrician used the 568B wiring. I began attaching the RJ-45 plugs to the cables in the closet using that wiring.

With internet access up and running, I used a wireless router with two laptops to check e-mail, but I needed a wired connection for the desktop PC. To get the desktop running, I ran a 25-ft. Ethernet cable from the desktop PC to the DSL router, which has four Ethernet ports. One port connects to a wireless router, which also has four wired ports. That gives me seven usable ports.

Three of the Ethernet cables run to receptacles in the home office, which is adjacent to the wiring closet. Wanting to get the desktop connected through the wall, I randomly started connecting the cables to the router ports and eventually found one that went to the wall plate near the desktop PC. Now I had confidence that I’d properly attached the RJ-45s. But, that port is close to the wired closet and it was easy to run back and forth after connecting each cable to the router. But, other outlets are scattered over three floors. There had to be a better way.

I asked former colleagues what to do and Jon Titus had the answer: Make a wired loop and use a multimeter set to measure resistance.

The next day, I bought an RJ-45 receptacle and connected wires to pins 4 and 5. Then I cut one connector from a known good Ethernet cable and shorted the blue and white/blue wires that were connected to pins 4 and 5. I then had to go around to each wall plate just once and connect the cables, one at a time, to the test receptacle wired to the meter. The photo below shows the receptacle attached to the shorted cable. In reality, the two were connected through each of the installed cables. A quick ohms measurement told me when I had found the cable in the closet that connected to the shorted cable.

  Ethernet cable finder with DMM

I’ve identified and documented eight of the twelve cables. Of the remaining four, one goes to the basement and the others go to wall receptacles located behind furniture and I don’t need them now anyway. The electrician ran cable to any place where I might want to put a TV so there are more receptacles that I’ll ever need.

Ethernet ports are available in the office, den, living room, bedrooms, and a second small office off the kitchen. Having wires in the wall provides better security than using wireless without having visible wires around the house. You just need to know where the cables go.

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