Cesar, I've never had to use this sort of wiring and at this point I suspect that you may be the board's leading expert on it. Here's some white 14ga that came up. You didn't mention distance, but if you're going under the carpet by a relatively direct route, it might seem that even 14ga wouldn't be necessary(there seems to be some 15ga flat, for some reason), using the usual wire gauge suggestion table.

-----------------------------------

Enjoy the music, not the equipment.