Determining Hot vs Neutral in Sheath (Cloth) Wiring without power feeding it

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Sep 20, 2016
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I have a mix of newer Romex wire and some original sheath wiring (house was built in 1950). I took down the drop ceiling in the basement and there were multiple wires that were not run properly through the joists; it was just laid on top of the drop ceiling "insulation/tiles". So, I'm trying to wrangle a mess and run all the wire through the joists like they should be. Luckily, some of the older sheath wiring would go to a junction box, and to another junction box and then to another. Where I am able to access the start and end of a particular section of sheath wire, I am replacing it with Romex (and elimiating as many "stops" in the junction boxes as I can). I do have some questions though:

There are a few junction boxes that I will have to leave the original sheath wire, because it runs up the wall to the main floor or upstairs and I can't access the wire to replace it. I paid attention to how the wires were hooked up in the junction box and took pictures with my phone. I want to double check myself when I hook everything back up. Obviously, Romex is easy, black is hot, white is neutral. When the newer Romex wire is feeding (incoming power) the older sheath wire, I want to be sure I hook it up correctly hot/hot and neutral/neutral. How would I make sure I know which is which on the sheath wiring when there is no power feeding it (can't use a multi-meter on a non-live wire, right?)

If I feed the sheath wiring with incoming power from the Romex, could I use an outlet tester on an outlet that is on that sheath wire circuit, past the junction box to check myself?

Or, before I hook up everything together in the junction box, could I connect the Romex feeding the power, the sheath and an outlet right there, just as a test? In other words, wire nut the Romex incoming power, the sheath wire and the outlet together and check it with an outlet tester. Would that double check me?

I should probably invest in a good multi-meter. Is there an easier way to check that I am overlooking? FYI, none of the older sheath wire is marked on the outside; I am going to mark it with black and white electrical tape at each connection point when I test it to be sure.


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Question: when you refer to "sheath wiring" do you mean knob and tube wiring which are individual wires?
Pics would be helpful.
Thanks for the replies. Here is a picture of what I am referring to. Sheath (old) wiring on the left. New Romex on the right. Both wires on the Sheath are the same color (when you peel the cloth insulation back,there is nothing to differentiate the wires like modern Romex has with black and white). I cannot tell which one is Hot or Neutral. It is going to be wire-nutted together with the Romex and the Romex is carrying the incoming power to the box, so I want to make sure I don't hook up the sheath backwards.


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What you are referring too a sheathed cable, is in practice, an original version of romex, were suppliers were using excess stock in the composition, prior to white conductors became available.

To determine hot and neutral they would energies one of the conductors, then mark it with a series of notches, similar to the same practice used to identify conductors in knob-n-tube systems.

There a couple of ways to I.D. the hot, with a bulb and socket and with a VOM, which is
recommended, and it helps to have or to establish a known ground.
Thanks snoonyb. Is a VOM a multi meter? Also, instead of a bulb and socket, if I used a standard outlet and used an outlet tester, wouldn't that also tell me if it was correct or the Hot/Neutral were reversed? If I use a bulb and socket, how could I tell if I got the Hot/Neutral reveresed?

Also, if I connect the Sheath cable, the incoming power (Romex) and an bulb or outlet all together, would I get an accurate reading on the bulb or outlet, because both the bulb/outlet and the sheath would both be connected to the same incoming power supply?

I don't have a way to test the sheath after connecting it to the incoming power, becuase that particular sheathing goes into a brick wall with to two light switches. Would the lights operate correctly if it was reversed?

Sorry for the basic questions, I am still in learning mode, but I am being diligent and making sure I do it right.
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Yes, a VOM (VOLT/OHM METER) is a multi-meter and If you have an existing 2 slot recep. you can use a 3 prong recep. tester, with a 3 prong to 2 prong adapter to test.

The bulb and socket was mentioned if you were dealing with unconnected conductors and didn't have a VOM, plus a known ground.

Depending upon how the light switches are connected, you'll likely blow a fuse or trip a breaker.

We're here to help, so keep asking.
This Will Work, V-
A) Turn off the power to the cable at the fuse or circuit breaker.

B) Remove any thing plugged into the circuit, including unscrewing light bulbs.

C) Verify by putting an Ohm meter or continuity meter across the two wires. Is should show infinite resistance. (No continuity)
If no continuity is shown, everything is unplugged.

D) Take the continuity tester or Ohm meter and touch one conductor with a probe. With the other probe, touch something well grounded.

One wire will show no continuity to ground. That is the wire connected to the fuse or circuit breaker.
One wire will show continuity to ground. That is the neutral. (White)

White-Out typing correction fluid works very well to color the older, NM cable conductors white.

Simple DIY Continuity Meter: See Drawing



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Thanks Paul. Very helpful! The Sheath in question is feeding two things, an overhead light and one outlet ( I mis-spoke earlier whan I said it was two light switches-that was something else I was thinking of). In lieu of doing your continuity test, could I also wire up the sheath to the Romex supplying the power and then test that outlet with my outlet tester? It would either show "Correct" or "Hot/Neu Reversed" right?

Also, I appreciate the recommendations on the non-contact testers. I have several, but those wouldn't do me any good if what I am trying to test has no power going to it presently, unless I am missing something?
In lieu of doing your continuity test, could I also wire up the sheath to the Romex supplying the power and then test that outlet with my outlet tester? It would either show "Correct" or "Hot/Neu Reversed" right?

Also, I appreciate the recommendations on the non-contact testers. I have several, but those wouldn't do me any good if what I am trying to test has no power going to it presently, unless I am missing something?
Your idea of using the plug-in tester probably will work as you described. But I've had occasional false readings of line/neutral on the cube testers. I think they pick up stray eddy currents from other neutrals & get confused.

Also note that some non-contact testers will alert on the neutral if something is energized in the circuit, so be sure to remove the light, etc. And, they sometimes also pick up stray eddy currents from other neutral or ground conductors.

You could also try a tone generator (tone tracer) if desired. You will disconnect from the circuit breaker and connect the tone generator to that conductor. Then go to the opposite end and use the probe to find out which wire causes the louder alert.

The test kit that Bud16415 linked will certainly come in handy for not only this job, but things that come up in the future.

If desired, you can add a Current Clamp to most any multimeter meter for measuring current draw in amperes. (Example pictured below)


Unsolicited Editorial If You Are Meter Shopping:
On Temu Com, you can buy a fairly good multimeter for about $5.00 - $7.00 U.S.. I always keep some in the truck because people constantly borrow stuff and it comes back dropped, driven over or soaking wet- if it comes back. The ones I've had have been surprisingly accurate and safe. (I HiPot the leads before use.)

Klein supplies good meters. Last I knew they were made for Klein by Ideal Industries. I have some for rough work conditions because they are rugged & if one gets destroyed or stolen, they are not expensive.

Ideal is a reliable and inexpensive brand.
Bachmann is very, very good.
Wavetek is pretty good, but not terribly rugged.
Extech has good electrical meters and k-type temperature meters.

Fluke is the Gold Standard. (In my opinion) My important meters get NTSC Standard calibrated twice a year. The Flukes never need adjusting.



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