Steve,
How many twists are acceptable in a wire nut? What length do you strip the wires in a wire nut? How much strength to separate a wire nut?
As stated earlier insulation is a different issue altogether. You're flogging a dead horse.
I'm amazed that this forum is looking for loopholes as opposed to the exact letter of the law. If you went into a box and did a bunch of Fig.21s and Fig.22s, would your AHJ approve it?
The difference is that wire nuts are UL tested and approved, and the instructions printed on the packaging- easy to do, easy to comply with, and easy to check. While the solder joint has has no official approval beyond what the AHJ is willing to allow. One author's instruction on how to make a western union joint isn't the same thing as NFPA endorsing it. As listings cost money, nobody is going to get a DIY soldered splice method UL-approval, so the status quo remains. Only exception I can think of would be official US Government mil-specs and such, which are widely recognized as an acceptable listing authority, and do go into detail on methods like this.
At the end of the day, though... Why? Why even bother? There's no reason to do a solder splice in this day and age. It takes more time, takes a higher level of skill, requires insulation to be applied separately, and is difficult to reverse. There's more reason for NFPA to explicitely ban its use than to allow it. (Thanks for the graphics, btw, it's great to see stuff like that!!)
