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I was asked to install two wall sconces. The old ones were removed but there was only one feed wire (hot from the switch). This is an old craftsman house in california,

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  • Were the old ones working? There's literally only one conductor? There's no ground or neutral at all? Electrical current requires two conductors. – JPhi1618 Nov 16 '15 at 16:01
  • How old is this house? – ThreePhaseEel Nov 17 '15 at 2:01
  • i realize all of this. perhaps the box is grounded and they were using this as the neutral. The house is an old craftsman, probably from the 40's at least. – generickey Nov 17 '15 at 16:55
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I think you answered your own question if there is only one wire to the fixture then the metal box must be completing the circuit through a ground wire attached to the box possibly fastened under the internal clamp. Highly illegal according to any version of the code in modern times.

It is impossible to have a circuit without a completed path. Either through a second conductor or some conductive material such as the earth.

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Possibly you found only 1 wire but the other is still around. Knob/tube wiring split the conductors, usually the hot and neutral conductors would travel completely different paths to fixtures. You might just not be looking in the right area. In any case, K&T is not code anymore (for many reasons, including because it's really old and insulation will be degraded), so you'll have to replace it.

You probably know this, but you wouldn't have the neutral going to the switch anyways.

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  • Existing K&T installs are still Code-compliant -- its just that they get goofed up in other ways by ignorami. See NEC 394.10 for details. – ThreePhaseEel Nov 17 '15 at 1:57
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    Knob and tube doesn't have any insulation. It uses bare wires. The ceramic tubes act as the insulation. As ThreePhaseEel said it is still code compliant for existing historic homes in most areas. – ArchonOSX Dec 16 '15 at 22:01

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