If I remember correct current flows on the surface of the wire thus multiple strand wire has a higher current carrying capacity than a solid strand wire of the same gauge. So when you solder two multiple strand wires of the same gauge together how much does that affect its current carrying capacity?
Ms

Based on Cerrowire charts, you need to go to AWG8 before your comment regarding ampacity is true. What are you trying to accomplish.

Hey Mike, your question can go a few different ways; as far as the electrical code is concerned I can't find any required reduction of ampacity with soldered connections.
That being said, if you intend to do the soldering yourself I would never use it for it's maximum rating. Personally I was never very good at that...
If the question is theoretical maybe some web searching would provide answers from wire manufacturers that you may find more useful.