12/2 or 10/2 -- 220 general purpose outlets

[Steve Meliza]

10/2 is expensive and stiff compared to 12/2 and for a 120V circuit you'll never have a need for more than 12/2 unless you think you'll some day re-purpose the circuit for 240V then it may be worth considering 10/2. Your choice also depends on accessibility, if the wire is buried in the wall then a little up front expense and hassle may be worth it. If the circuit is to be 240V from the get go then you may as well install 10/2, even if you use a 20A breaker and outlet.

In my case I've run all the wire through conduit so any changes to the wiring is easy and it was cheaper to buy 500' spools of #12 THHN rather than mix #12 with a little #10 "just in case". I also found that making up something like 60 splices and wiring 20+ outlets with #12 wire was a pain enough that I can't imagine doing it all with #10 wire. Not to mention some of what I wanted to do would have required up-sizing my EMT to 3/4" and installing junction box extension rings to meet fill requirements. As it is I have enough fill space to up-size one circuit in each conduit run to #10 if needed.

Regarding using NM 12/2 or 10/2 for 240V, around here it is common practice to wrap black tape around the white wire to indicate it is no longer a neutral wire.