Someone I know is doing some lighting in their basement, and there’s a junction box in the ceiling from the prior owner, and they’re unsure of what it is exactly. We are going to try to trace it etc., but I’m also wondering about testing it with an appliance. Touch one wire to one prong of the appliance and the other wire to the other prong, and see if it works, and then test out which circuit it’s connected to and so on.
Question is whether there’s any reason not to do this, whether safety (obviously we would have to avoid touching the bare wires) or that it simply won’t work.
If they’re “doing some lighting” without basic electrician’s tools and knowledge they’re idiots who deserve the shock & the fire they get.
I started to explain what to do, but decided against it. Trying to make fool-proof instructions is hard when you don’t know the level of knowledge, of anti-knowledge, or of common sense in your audience.
We’ll, I don’t think they’ll deserve it, but yeah, if you can’t figure it out or know how to test it, you need an electrician, and this question suggests to me that you don’t understand the danger involved. I’m not going to give more advice than, get an electrician.
How will doing that help you find out anything about the rest of the circuit? I agree with the others, you need to find someone who knows what they are doing.
The idea is not to find out anything about the rest of the circuit. The idea is to find out whether they can attach a light fixture to the wires in that junction box, which wall switch it might be connected to, which circuit breaker controls it, and so on. ISTM the simplest way to test that would be to connect something to the wires and see if it goes on, and if it does, to see what makes it go off. (Could theoretically use a mutimeter, but it might be simpler to grab a nearby appliance, especially since these people don’t own a multimeter and I would have to bring mine.)
[It’s in a basement which has the framing for a drop-ceiling in some places - they suspect that the previous owner had partially finished it at some point and then taken it apart, and this may have been an actual light fixture at some point. But they don’t know.]
There are better ways. I have a circuit breaker tracing device that can either be plugged into a receptacle or clipped to bare wires. Then you go to the breaker panel with the receiver and sweep it across the breakers to find where the beeping is loudest. Flip that breaker and if the beeping stops, you got the right breaker.
Once you know the circuit, do you actually know there’s a wall switch involved? Probably 80% of junction boxes in a basement have nothing to do with anything in the basement.
There are better ways. I have a circuit breaker tracing device that can either be plugged into a receptacle or clipped to bare wires. Then you go to the breaker panel with the receiver and sweep it across the breakers to find where the beeping is loudest. Flip that breaker and if the beeping stops, you got the right breaker.
That sounds like it might be a better way if you happen to own that device. If you don’t, and are faced with a choice of running out to buy a device that you will likely never use again or just flipping a few switches, I think the latter is a better way.
Once you know the circuit, do you actually know there’s a wall switch involved? Probably 80% of junction boxes in a basement have nothing to do with anything in the basement.
Very possible. So then there’s an 80% chance that you will flip the wall switches and nothing will happen and you won’t be able to use the wires in that junction box and will be forced to run new wires. But there would also be a 20% chance that one of the wall switches shuts off power to that junction box, and then you saved yourself a lot of work running wires. Seems worthwhile.
You say you will likely never use it again. Probably true if you’re going to die in the next six months. If you plan on living longer, you’ll likely find yourself using it again one of these days.
Electricians use them all the time. They can fail to detect voltage, or detect it from the wrong wire, but most of the time they work right and you start with one of those before taking things apart or stripping wires.
There’s a bit of a Catch-22 here. If OP is planning to test circuits in a damp basement by touching appliance plugs to live wires, his life expectancy is too short to make it worth spending the few dollars on a safe voltage tester that would extend his life expectancy.
I’ll just make one serious common sense point here that I’ve even seen professional electricians skip.
Whether using a $10 voltage tester or a professional multimeter, check it against a positive control - a known live circuit - each time you start using it to ensure that it’s working right.
Can you elaborate more on some of the things which can go wrong?
I get that if you touch live wires you can get a shock (or worse, though highly unlikely, I believe), and I assume if you connect the hot and neutral wires to each other you can short the circuit.
I like non-contact voltage detectors, but I don’t fully trust them, either. I always want to see them light up and beep when they are near a known-live wire, before I believe that an unknown wire is dead.
I would strongly recommend a meter over a non-contact voltage detector. You can get by with a $15 cheapie from Walmart. You don’t need a fancy meter.
You need to know what a switch loop is, and what the implications are for voltage in both of the most common types (so you need to know what the two most common types are). You need to know how 3 way lights work and what the implications are for voltage in the wires for all of the different switch combinations. You need to know how to use a voltmeter. You need to know the differences in house grounding from an older house (say 1960s-ish) to a modern house, and what the different implications of that are with respect to common voltage and grounding of the electrical box that you are looking at.
If you don’t know all of these at a bare minimum, then you have absolutely no business poking around in house wiring.
Find someone who is qualified to do this and also make sure that this person thinks that touching wires to the prongs of an appliance to test something is a really bad idea.
The worst case scenario in this is the house burning down or people dead. But you know, other than, it’s fine.
(Speaking as an electrical engineer with over 3 decades of experience)