knob and tube cannot find source

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  #1  
Old 02-26-19, 01:46 PM
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knob and tube cannot find source

I am replacing all the knob and tube, which I thought was on 1 circuit.
Yet, when I come to this junction box, it still shows power on an ac detector even though the breaker is off. I know the ac detectors are notoriously unreliable but I cannot locate the source flow of these wires to be sure...everything's hidden in the walls.

If I put a tester across the wires I get 0V but I think because it's knob and tube, it might be like a switch loop or a shared neutral? There is also a cut KT wire in the ceiling, which could have been the return.
Is there anyway I can properly test from the hot wire to ground to see if it has any voltage? Should I get some spare cable and connect it to a known ground and test across that?
 
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Old 02-26-19, 04:09 PM
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Yet, when I come to this junction box, it still shows power on an ac detector even though the breaker is off.

If I put a tester across the wires I get 0V
Try testing voltage between one of the wire and known good neutral or ground. Plugging an extension cord to known working outlet will be the best way to achieve this. Neutral on the outlet is the one with longer slot.

Non-contact tester will can show a open, floating wire as hot if it run close to another wire with current flowing.
 
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Old 02-27-19, 05:04 AM
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I have found that it`s not when a wire is near but even when a cable crosses another cable further away, it gives false readings. Are the magnetic fields transferred somhow?
 
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Old 02-27-19, 05:08 AM
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Yes, anything conductive can act as an antenna which may pick up enough signal for a non-contact tester to alert.
 
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Old 02-27-19, 07:52 AM
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Is the recommended way now to take the source power to a switch first?
I will pull out the old KT, maybe use it as a fish to pull the new wire down the wall.
However, I have 6 switches all the way down the corridor to get power to and then another cable back over to the lights.
Is the best way to have 3 cables in each switch box?
One from source (or previous switch), one to the next switch, and then one to the lights?
 
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Old 02-28-19, 10:03 AM
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Is the best way to have 3 cables in each switch box?
That's acceptable, though can get a bit tight, especially if you ever want to add a dimmer or smart switch.

Would it be easier to run the power to each overhead light and use 14/3 down to each switchbox? That's probably the way I would do it if you have attic access. Plus, the ceiling boxes can likely be larger.
 
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Old 03-01-19, 04:37 AM
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Yes it is preferable to bring power to the switch first.

You can get a tone generating wire tracing kit (tool) to find out where that knob and tube wiring came from.
 
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