My wife bought some new wall sconces for the house and asked me to install them. I'm not an electrician (obviously) but this seemed like something I could handle. So I switched them out, following proper safety precautions, wiring the new ones analogously to the old ones, and they illuminated. Cool!

While we were at it, she says, lets put the old sconces in the hallway in these mysterious electrical boxes behind the hanging picture frames that are apparently wired but we've never used for anything. Sure, why not? Eight hours later I've partially reverse engineered the electrical schematics for the upstairs of my house, the new sconces aren't working, the original sconces aren't working, and a significant sub-circuit in my home no isn't getting any power. Let that be a lesson to you. If it's not broken, don't fix it.

So it turns out I've got an electrical box in my old (i.e. "has character") home and there are two cables coming into it (well probably one coming in and one coming out to be precise). The cables each contain two-conductors (I presumed a live and a neutral). For the purposes of discussion, let the conductors in the first cable be A and B, and let the conductors in the second cable be C and D. I believe, before I took things apart, that conductors A and C were wire-nutted together and conductors B and D were wire-nutted together in the junction box.

I couldn't be sure which of these wires were lives and which ones were neutrals, so I got out the multi-meter and took some measurements. I wasn't 100% sure what to expect. I mean sticking the two probes of the multimeter into a known working two prong (most of the house isn't grounded) electrical socket, something I advise against doing on a regular basis, I measured 120VAC (which is what I would have expected, being in the USA and all).

What I actually measured makes no sense to me - but what do I know... My question is, what are the implications of the following measurements taken in the electrical box?

  • Between A and B: ~18VAC
  • Between A and C: ~65VAC
  • Between A and D: ~65VAC
  • Between B and C: ~ 0VAC
  • Between B and D: ~65VAC
  • Between C and D: ~ 0VAC

I didn't measure these before I started this wiring odyssey unfortunately... but I can't make heads or tails out of these measurements - I don't see 120VAC between any two of them... what does that mean?

  • It sounds like you popped a breaker, or in your house possibly blew a fuse. Voltmeters are useless for this stuff since they are high impedance and will show you capacitive pickup instead of true driven voltage when a line is floating. This is what I think you are seeing. Either add a small deliberate load (a small 110V lightbulb for example) accross the voltmeter leads or use a real line tester. – Olin Lathrop Jan 2 '12 at 14:09
  • @OlinLathrop the house is on breakers, no fuses that I know of, none of them are tripped... 65VAC of capacitative pickup sounds like a stretch to me, but I could be wrong... when you say popped a breaker what do you mean? why not move your comment to an answer? – vicatcu Jan 2 '12 at 14:17
  • "Popped" means "tripped" or "open" in this context. 65V AC pickup into 10 MOhm is no stretch at all. I didn't write a answer since I'm just guessing. – Olin Lathrop Jan 2 '12 at 14:23
  • Call an electrician. I know everybody wants to do the work themselves, and nobody wants to pay out for a "simple" job. But sometimes it's just better to have a trained qualified person who has all the cool tools you don't, take a look at the problem. – Tester101 Jan 2 '12 at 23:21

Are the wires color coded? (they should be). Black is the "Hot" and White is neutral. If you had a three wire system the bare wire or a green whire would be the safety ground.

Next I would recommend that that there is a much better and safer way to debug your wiring. First turn off all power to your house by shutting off the main breaker. Since you do not know exactly where your wires go, that is the only way to guarantee that they are not hot.

Now use a wire tracing kit like this one from Triplett or something similar. That way there will only be low voltages on the wires and you will not risk electrocution or a fire.

  • One black and one white in each cable. A and C are black, B and D are white. But I didn't want to rely on color of wires to read meaning into my measurements... I appreciate your safety cautioning, but my meter is rated for up to 750VAC and I have a healthy respect for electricity :-) – vicatcu Jan 2 '12 at 13:59
  • This is not just a question of safety, if you read the manual for the referenced tracing kit you will see that it will let you trace the path of wires behind walls, something a volt meter can't. – JonnyBoats Jan 2 '12 at 14:31
  • Someone correct me if I am wrong, but one should never measure 65 V between any two white wires("Between A and D: ~65VAC"), this means there is a serious wiring fault. This could be a corroded connector (some aluminum wire perhaps?) or a loose connector somewhere. If there a high resistance causing this voltage drop it could lead to a fire; don't let this go. Find the cause and fix it. – JonnyBoats Jan 2 '12 at 14:35

Sounds like you might be missing a neutral. What do you get if you measure from the metal box to each wire? The box should return to ground which is the same potential as nuetral (but you can't use ground as a neutral!). Box not grounded? Or plastic box? Run a temporary wire to a water faucet as a ground to test with.

As Olin points out when using a high input impedance meter you can get strange readings because the meter can return through high resistance paths. As Jonny says, black is hot and white should be neutral. When doing house wiring, you need to expect anything because so many homeowners think they can do electrical work but don't know enough. A switch needs to interrupt the hot leg only, but I have seen it incorrectly wired interrupting the neutral leg (dangerous). You need to be certain as to where these wires land before you use them. You may try de-energizing the circuit and "ringing" out the wiring using a continuity test.

Try turning on nearby light switches, see if that gives you voltage (again, incorrectly switching the neutral). Open nearby outlets, see if any wires are dead-ended.

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