What could be causing strange voltage readings?

Question

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?

Answer 1

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.

Answer 2

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.