I have knob and tube wiring for large portions of my 1950's house, including to my bathroom. Currently, I have hot and neutral coming in from the knob and tube wiring and a separate ground wire coming in from somewhere(we haven't had any electrical work done since 2014, so I don't know how that was permissible. I can't seem to trace the wire in the wall). This powers one 20A labeled GFCI (without a separate prong for a 20A plug?) and a light. The circuit is on a 20A breaker. I would like to add a second outlet by adding some NM cable to the load terminals from the GFCI (and the existing ground connection).
My jurisdiction follows the Residential Code of Ohio. Would adding this outlet be safe, and would it be permissible under code? I'm afraid of violating restrictions on extending circuits involving grandfathered wiring.
Here is a photo of the GFCI box. The nm cable goes out of the box to a switch and light. The power comes in via the old knob and tube wire on the side.
Here is a photo of the knob and tube wiring that feeds this. No idea about the NM that supplies the ground.
Best Answer
First off, get yourself a new GFCI with screw terminals and do not use the back-stabs... those are trouble waiting to happen.
When you are adding the new receptacle down stream of the GFCI, it is not an extension of the old WIRING, it is an extension of the CIRCUIT. No restrictions there, so even if it is K&T (I also question that), you are fine.
The potentially problematic issue if it IS K&T is if it is also an MWBC (Multi Wire Branch Circuit), i.e. one where the neutral is shared by one or more other hot circuits. When you do that with a GFCI, it will cause nuisance tripping. But the fact that you HAVE a GFCI in there now, and we assume it has been working, would indicate it is not an MWBC.