Still working on installing my exhaust fan. It required a dedicated 20A circuit so I ran #12 copper. The wiring from the unit appears to be #14 stranded. It's also silver colored but I don't know if that means it's aluminum. I need to connect one #12 wire to each of the #14s (so two wires per connector). What's the right connector to use for this?
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.
You can also use whichever push-in connector makes sense for your application. I haven't had as good luck using them with stranded wire, but they are "allowed" and they do work.
More information is available at http://www.idealindustries.com/prodDetail.do?prodId=in-sure&div=0&l1=push-in
You can properly use the push in connectors, from Ideal, if you are careful. Just twist the stranded wire clockwise to give it some solidness and push in same as solid. Technique and use described here on Ideal web site. http://www.idealind.com/media/pdfs/products/brochures/p-2854_in-sure_brochure.pdf Makes it very simple and quick.