I have to enclose some open air splices in knob and tube wiring that ties to romex. I will be going to look at it tomorrow, but I need some ideas how to get the old stuff into a j-box. Usually k and t is ripped out and replaced, but these people are selling and do not want to spend much.
I haven't seen the splices, but if they are original to the wiring, I think they are safer if left alone. I personally would refuse the job if the customer wouldn't allow a rewire of the circuit. Make sure your liability policy is in good order.
Extensions from an existing K&T branch circuit have a history of being normal wiring practice. Depending on local ordinance, they may still be legal to create. The NEC still describes how to do it without a j-box.
300.16 includes "fitting" as well as "box". If the open air splice you are looking at has a "monkey face" transition fitting on the end of the NM cable sheath, then the splices to the existing K&T are permitted without a box exclosing them.
I haven't seen a real monkeyface for sale in a very long time, either. This image shows one that terminates the armor sheath on BX cable. The basic function is that of a weatherhead, that is, end the raceway or cable and let the individual conductors run on in open air.
If one were to transition now from NM to K&T, using contemporary material, I'd suggest using a threaded ½" weatherhead with a ½" threaded romex clamp.
I do not know if the below stuff is _listed_ for the application, though the catalog claims 'UL recognized', which I belive applies to components going into apparatus as opposed to finished products. Is this stuff be suitable as the 'sleeving' described? If so, it is pretty easy to get. We use it in our lab for low voltage motor terminations.
Over at EC&M, there is a question and answer about this very topic. It is from 2001 and is under the 1999 NEC, but the answer is still accurate, although the code citation numbers shifted some under the 2002 NEC. The NM to K&T splice is the third question, the question at the bottom of the page.
[quote}TUBING, FLEXIBLE NONMETALLIC (YCTR) These listings include woven flexible nonmetallic tubing (fiber loom) in trade sizes 7/32 to 2-1/2 in. incl. for installation as mechanical protection in accordance with Articles 320 and 324 of the National Electrical Code. The basic standard used to investigate products in this category isUL3, ‘‘Flexible Nonmetallic Tubing’’. The Listing Mark of Underwriters Laboratories Inc. on the product is the only method provided by UL to identify products manufactured under its Listing and Follow-Up Service. The Listing Mark for these products includes the UL symbol (as illustrated in the Introduction of this Directory) together with the word "LISTED," a control number, and the product name "Nonmetallic Flexible Tubing".[/quote]