Switch polarity question (electrical)

[Ivan Vegvary]

Up in the attic. Have to bring hot wire to junction box with three legs going out. Each leg goes to ceiling light of different rooms. YES, I can power them up and the wall switch in each room turns on its respective light. BUT, what's an easy way to test that I don't have hot and neutral reversed. The old romex in the attic (two wire, cloth wrapped) is NOT color coded in any way.

Do I simply drag a neutral wire from a different circuit and probe the bulb socket in each room? Is there an easier way? My A.C. voltmeter obviously does not indicate polarity.

All replies appreciated. Sheetrock guy is coming tomorrow a.m. and I have to solve this today!

Ivan Vegvary

[dpb]

Ivan Vegvary wrote: ...

Every old cable I've seen is coded in some way -- how else could they have kept track originally, either?

Look for a stripe, a ridge, something that distinguishes the orientation of the cable itself.

--

[Nate Nagel]

better, drag a wire from a good ground (water pipe) and check voltage against that. Heck, if you're in the attic, why not just add supplemental grounds and/or pull new romex to replace the old cloth NM? If you can't pull the whole circuit, do what you can and then wire a supplemental ground from where you started repulling. Best would be to take it back to the breaker box but if you can't do that any ground point is acceptable per code (unless they've changed it recently.) Only thing that *wouldn't* be acceptable would be to repull some of a circuit so it would appear to be grounded but to not actually connect the ground wire to a real ground.

nate

[Jeff Wisnia]

I'll assume that the presence of two wire romex means that none of the outlet boxes themselves are grounded, so you can't use them to test against.

No warranty on this advice, but if you're using an "electronic" (digital) voltmeter, and are standing on something which insulates you from ground, then you can jsut touch one of the voltmeter leads with your hand and probe the shell and center contacts of the sockets with the other lead.

Your body capacitance will provide enough of a load to cause the voltmeter to indicate some voltage when it's lead is connected to the "hot" side iof the line. It probably won't read the full 120 volts, but enough so you can easily tell the difference.

There are commercial testers which will do what you want to do, if you want to spring a few bucks for one:

But, if you've already got a sensitive AC voltmeter, try my suggestion.

Jeff

[Ivan Vegvary]

THANKS JEFF!!!!!!!

[EXT]

Even if you are using a real cheap neon tester, this method works to find a hot wire, Just hold the end of one lead and touch the other to the wires, the neon bulb will light up dimly when you hit a hot wire. You won't feel a thing.

[Phisherman]

There are plug-in inexpensive testers to tell you from hot and neutral. Many just plug into an outlet, but I'm sure you can get one of those screw-in sockets that would allow you to plug the tester in.

[metspitzer]

They also make what is called a touch voltage tester. I don't know if you can get them at builder supplies but you can get them at an electrical supply.

[Jeff Wisnia]

I believ those only work if they are used on a circuit with a ground available. I don't see how they could work with one of those screw-in socket adapters.

Someone please correct me if I'm wrong, please.

Jeff

[John Grabowski]

I always have my little inductive tester with me to figure out which wire is hot. You just touch it to the insulation and it only gives an indication on the hot wire. It can also read through the outer jacket on Romex. I've used several brands over the years, but have found the Fluke model to give the least amount of false readings.

[Twayne]

It's not an actual polarity you want; remember, this is ac. The "polarity" applies to which is hot and which is neutra.

Arbitrarily assign one as Hot (H); use the wire to the hot connection on the fixture as H: Find an Earth Ground (receptacles are probably earthed). Then, check H - N 120V H - Earth 120V N - Earth 0V

If H - N = 0 then N - Earth should be 120V, meaning they are reversed. Switch the wires with each other.

Assumption: Neutral and Earth are ONLY joined back at the breaker box and nowhere else! Validate Earth connection by turning off all power (Mains). Use your ohmmeter to measure the resistance between Neutral and Earth. It should only be a few ohms, depending on the length of wire involved, or maybe

0.0. SWITCH YOUR METER OFF OHMS AS SOON AS DONE, so you don't accidentally try to measure voltage with it. The reason to turn off all power is that, should the polarity be wrong, you will fry your ohmmeter. I always check first for voltage and only after knowing it's not 120 V ac do I switch it to ohms. Then as soon as I finish ohmming thngs out, I put it back to measure ac voltage.

HTH

Twayne

[Twayne]

Well, some local codes have problems with things like that, but ... when it's that old you aren't likely to get much objection to improving the safety of the ckt if that's all you can afford or manage. I've even seen some knob & tube with earth grounds.

That's a good point; if the insulation is that old, beware the insulation falling apart in your hands and possibly already being missing from some parts of the wires. Especially if there have been any varmints running around there.

[Twayne]

Good suggestion; that also helps with the confusion sometimes that phantom voltages present.

[Twayne]

I can't speak for all of them, but mine works on two wire/no earth. In fact, missing earth is one of the light sequences, right along wiht a "missing" neutral. It can identify any of the 3 wires are missing. I suspect they all can, no? This is just a cheapie Radio Shack tester. GB is the only mfg ID I see on it.

It's also my second one; the first one grew legs apparently.

[terry]

That would be my quick suggestion also. Find a ground somewhere. connect one side of your meter or one side of light bulb to that. With lights switched on.

Any wire wire that has potential to that ground is most likely the live (today black) wire and is either a direct live from the supply fus/breaker or a switched live.

The wire that has zero or a very low potential to that ground (although even cheap digital multimeters can be deceiving) is most likely the neutral (white wire).

Don't mess around with readings of 30 or or some odd voltage on wires which are disconnected or switched off. In fact a light bulb is often a better tester than a meter that can pick up odd induced voltage from other circuits or wires!

[hrhofmann]

But the plug-in testers assume that you have a 3-wire outlet to work properly.

Bob H

[Gary H]

[snip]

Would that be the same as the $5 testers sold for holiday light strings?

[pheeh.zero]

Yessir!

[bud--]

If there is no ground it will tell you that.

But unless you have one I haven't seen, if there is no ground the tester won't tell you if there is a H-N reverse. That is basically what Jeff asked.

It won't tell you if there is a high resistance (useless) ground.

In an unusual case of N tied to G (which some idiots do when there is no ground available) and a H-N supply reverse it will indicate "normal".

[Twayne]

lol, not for long once the power is turned on!