I am slicing into knob and tube wiring in my basement. Over the years I have done a lot of wiring but in this case I am baffled. I placed a J-box for the splice of new romex 12-2 wire very close to where the K&T were coming down from the floor above and the ran new 12-2 wire to the panel box cutting out the old wire as I went. I used a comination of staples and drilled holes through the wood to run the wire. When the final hook up to the box was done and I flipped the breaker on it tripped back off. Thinking that I may have the wires at the J box crossed I went and reversed them. The breaker still tripped. I checked the insulation of the K&T entering the J box and it is fine. When the splice is not connected the breaker does not trip. There is two basement lights that also tie into this circuit and they both work fine. I also moved the breaker down one slot to see if maybe the phase may have been wrong.
Any thought to this problem?

Mark

Any time you mess with K&T - you should replace the whole run. The insulation is extremely brittle, and most suceptible to breaking. I would expect that if you check with an ohm meter, that you've developed a short in the run you've tied back into.

One other note - what size breaker are you running on that old K&T? You've said you're hooking 12 ga into it; I'd figure you're running 20A breakers. That old K&T is probably 14 ga, meaning at most you should be 15A.

Be very careful what you are getting into, tieing into that old wire - and be extremely careful that you don't overlad that stuff.

In my house, I took it all out - finding burns and overheated wires all the way through where the PO had "somebody" that "upgraded" the elctrical...