394.56 Splices and Taps.
Splices shall be soldered unless approved splicing devices are used. In-line or strain splices shall not be used.
That counts out butt splices, or Wago type connectors....
This is a little article about crimp failures in appliances
, but for this type of crimp connector used in a wall, the results would be about the same = not 100% reliable, and dangerous IMO. Sure you are allowed, and in a few situations required to have irreversable connections in a few locations like in grounding, but those connections and conductors are much large, and to be done right require tons of pressure to mash the connector and wire together. I haven't seen any made well enough to trust in the wall of a home for wire sizes 14-10. Most are made for automotive and appliance purposes.
Some places/inspectors would allow you to use a wire-nut type connector. Others wont... But they too have moderate failure rates, and methods of failure (arching) that would make me think twice of using one concealed inside a wall. As it would be inaccessible behind a finish. If I use one on K&T, it'll be in a box...
But mechanical splices done right, with clean metal to metal contact, and sealed in solder, and cleaned again
, have been able to last without abuse or failure for over one hundred years! The only times I have seen ones that have failed, failed for reasons: dirty conductors, poorly or unsoldered connections, left over flux, or long term over-load of the circuit.
You can find more of my opinion on the matter here: http://www.markhellerelectric.com/wusplice.pdf
Some more here: http://electrical-contractor.net/ubb/Forum1/HTML/007216.html
Do a search in the general discussion area and you'll find many more.
[This message has been edited by e57 (edited 10-22-2006).]