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Old 12-29-2010, 10:04 AM   #1
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Very Basic Questions... Need Help


Hi there - I'm confused over some basics of electricity. Using a multimeter I don't understand how hot to ground gives me 120V - but hot to common gives me no voltage... as in how does a light bulb connected to the two light up? Or if hot to common gives no voltage reading then how does hooking an outlet to just the two wires do anything?

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Old 12-29-2010, 10:13 AM   #2
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Hot to ground and hot to neutral should both give you 120 v reading.

What wires and where are you testing?

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Old 12-29-2010, 10:16 AM   #3
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Nope - hot to neutral reads nothing. My home is 1950's and uses the armored BX style wiring where the armoring is ground... so if hot to neutral reads nothing what would I look for?
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Old 12-29-2010, 10:20 AM   #4
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You've given us no clue. What is it you're trying to troubleshoot or accomplish?
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Old 12-29-2010, 10:28 AM   #5
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I'm not sure what you need is all... I have two separate runs of bx in my living room that went to old outlets - the outlets were painted over so I never even tried them. I'm now doing a small remodel and putting in new outlets. In testing I find that reading the voltage between hot and common - black/white in my case shows 0 for the voltage. Hot to the ground (armoring on the bx) reads 120V.
So, this is where I am lost.
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Old 12-29-2010, 10:33 AM   #6
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Making sure we're on the same page, this is what you got:
http://homerenovations.about.com/od/.../artbxwire.htm

If you get voltage hot to the ground (or sheathe of the cable), but not to neutral, then you have an open neutral. It could be anywhere along the run. It is probably insulated from the sheathe, so it is either simply not touching, or the insulation is not broken through. 1950's is going to make me guess aluminum wires. AL cracks very easily when it's old, I'd guess if it was, it'd be where it was moved or bent the most- the sheathe will look fine, but the wire can still be all messed up inside.

I know, the next thing is gonna be, 'But it worked!'. There's all sorts of ways it could have been wired for that to happen. Lots of people, when they get an open neutral, just wire to the ground. It'll work, and it's nice for electrocuting people, because now every metal part in your house is carrying current to some degree. That's why you never connect ground and neutral, ever (except one spot at the service entrance which should have been done by an electrician when it was built).

Edit- Oh, so it never worked ITFP? Yeah, probably an open neutral and you'll need all new wire. Sorry for the bad news.
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Old 12-29-2010, 10:35 AM   #7
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The neutral must be disconnected some where upline. Are you checking both/all receps? It's possible a splice may be disconnected in one of the upstream boxes. If the circuit has been abandoned it's very strange that the hot is not disconnected.
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Old 12-29-2010, 10:39 AM   #8
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A broken or loose connection between the point you are measuring from back to the breaker/fuse panel, including receptacle and switch boxes along the way. I try to find the middle point of the circuit, and see if there is 120v hot to neutral there. If so, then the problem lies between the middle point and the end (where you originally checked), if not, the problem lies between the middle and the breaker/fuse panel.
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Old 12-29-2010, 10:40 AM   #9
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Assuming that your multimeter is working correctly, the only possible conclusion if you read zero from hot to neutral is that the neutral is not connected to ground. In a modern system, you have a hot wire which is at 120 volts with respect to ground, a neutral which is the "grounded" conductor, and should be at zero potential (zero voltage), and an independent equipment ground (the green ground wire) which is also grounded at the main disconnect and is at zero potential.

In a modern system, both the neutral and the ground wire are grounded at the main disconnect. This means that they are both a zero potential, so a multimeter should read 120 volts between a hot leg and either the neutral or the ground wire. If the neutral is disconnected and is therefore not grounded, you will read zero voltage between the neutral and the hot. If the BX cable is grounded, you will read 120 volts between the hot and the cable, which sounds like your situation.

As to why the light works, it is possible that the current path is from the hot through the light and returning to ground via the ground wire, which is dangerous and not code compliant. The ground should only carry current in the event of a fault. You need to trace the neutral and see if it actually connects to ground at the box. If not, fix the problem. And then find out why the ground is carrying current, it should not under normal conditions.
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Old 12-29-2010, 11:02 AM   #10
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Also seen from time to time is having the switch cut the current via the neutral which is also against today's code. "Switch on" the light and see if hot to neutral now gives a 120 volt reading.
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Old 12-29-2010, 11:05 AM   #11
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It is like the BX pictured except there are only two wires in the line - a black and a white - and they are copper not aluminum.

I guess I will try and trace the wire - just a pain as it goes up into the attic and from there I don't know... I'll have to remove floor boards to find out.

Also, I wasn't saying there's a light that works - I just didn't get how it could if there was no voltage reading between hot and neutral - and I now understand it wouldn't.

Last edited by dmen; 12-29-2010 at 11:08 AM.
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Old 12-29-2010, 11:18 AM   #12
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It's possibly disconnects somewhere, as the others have said. However, before you pull floor boards, try everything else first.

1) Trace the wire as good as you can, and guess where it comes out. If it's disconnected, it's going to be in some box, and those generally aren't going to be under the floorboards. This could fix the whole problem if you find the right box and hook the white back up.

2) If it really is a broken wire somewhere in the middle, you need new wire unless you find the exact spot it's broken and install a juntion box to make a splice. At this point, it's easier (more than likely) to just run a new wire. You'll need to pull up *all* of the floorboards to find & fix, just one or two to replace, maybe even none depending on how the house is built.

There's a few tricks for running the wire if you need new. If it comes to that, I'll fill you in, but the main thing is where it is and how the house is built. The outlets are on the first floor, and the hot goes up into attic? The wires come from the fuse panel, to the attic

(More later gotta suddenly go)
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Old 12-29-2010, 11:43 AM   #13
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The one I tried to trace goes into the attic and then out towards an outside wall... I couldn't trace it further. I've already found one junction box in the attic, under a floor board yesterday... which leads me to believe there might be more. Of course, it tongue and groove flooring so it doesn't just pull up either.
I'm now stuck as I need to put in the outlet boxes and electrical in order to proceed with my project - and I can't do that.
I'm having an electrician friend stop today and hopefully he can provide some advice.
This is frustrating.
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Old 12-29-2010, 11:56 AM   #14
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Is this a receptacle you're testing or a light fixture, or a switch? If it's a receptacle, how are you physically measuring the voltage? The best way I know to trace the circuit is to remove the receptacle and test between the two wires - Don't rely on probing the receptacle slots.

If your testing at a light fixture, take the fixture down, and measure directly on the wires (black to white)

Keep us posted.
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Old 12-29-2010, 01:10 PM   #15
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You said that the old receptacles were "painted over". Not sure what that means, exactly, but consider this; a house built in the 50's had considerably less receptacles than one built today, simply because we did not have as many things that required electricity in those days. My point is that for someone to have abandoned two receptacles in a 50's vintage house, you may very well have something more than a "simple fix". I would try chasing the neutral back, through the attic and all, to see if you can find where it is damaged, disconnected, etc., but it might be a good idea to get an electrician in to look at it, because you could be seeing a symptom, rather than a problem.

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