New romex to light switch with knob and tube
If you feed your light with the hot from a new (Romex) cable, you must use the same cable for the light's neutral.
If hot goes into a switch first, the neutral must accompany it. Then hot and neutral continue on to the light.
Admittedly many K&T installations have single conductors running hither and yon. For example hot runs along the exterior wall with branches to receptacles and lights, and neutral runs down the middle of the house accepting the branches from those receptacles and lights. The loose hot and neutral ends left behind when you run a brand new Romex must be taped off and enclosed in accessible outlet boxes or junction boxes until such time as you identify and cut off the other end of any given segment of K&T at which time both ends of that segment may be buried in the wall with no boxes. A loose K&T wire can be taped and curled up inside the box where a new Romex was just strung to replace it. Beware, you may not bury two loose ends in the wall when you are really abandoning a K&T tee with the third end still connected somewhere.
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Last edited by AllanJ; 08-24-2010 at 06:22 PM.