What's the right connector for connecting #12 solid copper to #14 stranded?
That must be one powerful fan to require a dedicated 20 amp line. WOW.  The silver wire you see is probably a tinned copper wire. (solder on the end of a stranded copper wire).  To connect a 12 and 14 AWG set of wires together, you should use a yellow wirenut. If in fact the tinned conductor is smaller than a 14 AWG, you could use an orange wirenut. If you look on the package of wirenuts, it will give you a chart of the number and size of conductors the specific color wirenut will work with.