A 30-amp circuit needs 10-AWG conductors, which are contained in orange-sheathed NM.
Using a variety of wire gauges is crucial to safely managing the heat produced in the wire by the electricity as it flows, and each gauge is paired with an established maximum capacity for a fuse or circuit breaker. This pairing is important, because a circuit attached to a breaker that is sized improperly might serve as a gigantic heating element in your walls and give your fire department something to do for a few hours. You can always use a bigger wire than the code-required minimum, but you’ll pay for it. For example, a 50-ft. roll of 10/2 costs about $75, versus $46 for a roll of 12/2 and $32 for a roll of 14/2.
Most NM these days has a color-coded outer jacket: white for 14 AWG, yellow for 12 AWG, and orange for 10 AWG. This color-coding makes identification of new wiring easy, but it can lead to some confusion in older homes if you don’t pay close attention to the existing wiring. I often find DIY-style wiring that has tapped into 12-AWG/20-amp circuits (which was encased in a white outer jacket for a period of time) with new 14-AWG/15-amp wiring.
Other mistakes made with NM are the result of using it in an application for which it is neither designed nor approved. Some of the more common are installing it outdoors, leaving it exposed to sunlight, and running it underground. For these places, UF-B is usually the best cable to use. Similar in construction and configuration to NM, UF-B is UV-resistant and can be run outdoors as well as directly buried in some applications.