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Old 05-24-2010, 11:39 PM   #1
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Knob and tube wiring


How can you tell which wire is hot in old knobe and tube wiring?

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Old 05-25-2010, 12:17 AM   #2
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Knob and tube wiring


Unfortunately you cannot tell except by doing voltage-to-neutral measurements. You would need to string a long 12 to 16 gauge wire from a known neutral terminal, say at a panel, to the place where you are working and doing tests.

With wiring of that vintage, a switch does not always identify the hot wire. Back then, switches were sometimes put in the neutral going up to a light.

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Old 05-25-2010, 01:16 AM   #3
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Knob and tube wiring


How can my non-contact electrical tester tell me correctly which wire has current? Under $12 from Lowe's, used it for 10 years, accurate even on k&t for the hot one?

Be safe, Gary
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Old 05-25-2010, 03:04 AM   #4
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Knob and tube wiring


I've been dealing with a lot of K&T lately.

Here's what I've found:

My non-contact voltage detector can pretty reliably determine which wire is the hot, provided there are no loads on the circuit. Otherwise, the voltage detector (I call it my "idiot stick") can occasionally pick up the current returning on the neutral through the load, weather it's a light bulb, TV, whatever.

The fairly easy way I use most of the time is to plug a long extension cord into a known-good outlet, and carry the female end around with me while I work. That way, you have a reference hot, neutral, and ground handy to check against unknown wiring with your voltmeter. This is the only reliable way to detect reversed polarity on an ungrounded outlet.

The best, and my personal favorite method to determine which wire is the hot, is to turn the power off, snip the wires as far back in the wall as I can reach, and fish in some new romex back to the circuit's origin. Then you know for sure
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Old 05-25-2010, 05:14 PM   #5
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Knob and tube wiring


Quote:
My non-contact voltage detector can pretty reliably determine which wire is the hot, provided there are no loads on the circuit. Otherwise, the voltage detector (I call it my "idiot stick") can occasionally pick up the current returning on the neutral through the load, weather it's a light bulb, TV, whatever.
If you buy a real tic tracer (mine is a Greenlee like shown below) for about $80-$90 at most supply houses it is adjustable and can block out a lot of false readings! It is also audible so you can hear as well as see and can detect from 24v to 1000v fairly accurately. The $10 ones like most homeowners and some electricians have are only good for personal safety. The more expensive one is a great help in identifying circuits for things like panel schedules.



A trick I have learned to aid in false readings is to hold the insulation of the wire you are testing! If it is truly live it will still register, if it is just picking up the field from another wire it will no longer register. Try it!

Another trick for helping you trace circuits is to take a standard adapter that goes from a three prong outlet to a 2-prong outlet pictured below and drill out the ground hole so the tester will stick inside and you can plug it in to an outlet and go to the panel and turn off breakers until you hear the audible alert change from a scream to a beep. Great for working alone!



Funny story: I used to do a lot of work at a resource recovery plant that took in trash, separated the metals and glass and then burned what was left to heat the boilers that ran two 30 megawatt turbine generators! Anyway when I first started there one of my first tasks was troubleshooting a problem with some light poles on the roof of a tall cooling tower. Working with a helper with radios trying to find the circuit feeding the lights I held my tic tracer on the wire while my helper went through all the breakers. After he had gone through all the circuits twice without finding the right one I stood up and pulled the tester off the wire and as I did I immediately noticed my tester was still screaming. That's when it hit hit me, I was right next to the 240KV powerlines!

You can't use standard tic tracers in power plants!
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Last edited by sparks1up; 05-25-2010 at 06:32 PM.
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Old 05-25-2010, 05:40 PM   #6
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Knob and tube wiring


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Originally Posted by sparks1up View Post
A trick I have learned to aid in false readings is to hold the insulation of the wire you are testing! If it is truly live it will still register, if it is just picking up the field from another wire it will no longer register. Try it!
Must be that your body capacitance to ground
is much greater than the cable interconductor capacitance [~100 picofarads per foot of cable]
so a phantom voltage is reduced almost to zero.

You must have been standing on something conductive and grounded, like concrete or metal?

That's a pretty good trick and should work with regular voltmeters. No more incand. bulb needed to discriminate between phantom and 'real' voltages. A 'real' voltage shouldn't change in the least when the insulation is held onto.

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Old 05-25-2010, 06:32 PM   #7
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Knob and tube wiring


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You must have been standing on something conductive and grounded, like concrete or metal?
Standing on the floor!
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Old 05-25-2010, 10:56 PM   #8
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Knob and tube wiring


Mine is nice with the adjustable strength dial feature. Turned down to tick on hot while other wires are 1/8" away. k&t ticks only on the hot. Romex hot ticks two or three times faster than neutral. Turned up shows hot wires in wall when cutting holes from outside through siding (Hardie, T1-11, osb, etc.) For a DIY'er, I recommend one, so much better than a standard ticker, non-contact. All good info, well, most of it.

Why would anyone suggest something as dangerous as listed? They are not funny when someone tries what a few of you suggest and they die; this is a DIY site, to HELP others.
It does sort out the more professional ones for the readers, though…….
.

Be safe, Gary
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Old 05-26-2010, 07:15 AM   #9
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Why would anyone suggest something as dangerous as listed? They are not funny when someone tries what a few of you suggest and they die; this is a DIY site, to HELP others. It does sort out the more professional ones for the readers, though…….
What are you referring to? If you are talking about my jump test which was clearly a joke...I deleted it ...just for you!

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