If my project uses 30A DC, what wire gauge should I be using?
#10 AWG is code compliant for 30 amp circuits.
The gauge of wire you use depends on:
- how much voltage drop is acceptable. You can use a voltage drop calculator to estimate this. Voltage drop depends on the voltage, current, length of wire, and resistance of wire (i.e. the wire gauge, material, construction).
- how much heat is acceptable. This depends partly on the specific application, and also the insulation on the wire. Running current through a wire causes some heating due to electrical resistance, and a smaller wire will heat up more. That's not necessarily a problem, unless the heating is excessive and creates a fire or melting risk.
(These two points are really just two ways of looking at the same phenomenon: loss of electric power due to heat. But they often have different ramifications or solutions so I think it is helpful to consider them separately.)
Since you haven't provided any information about the voltage, length of wire involved, application, or insulation, it is not possible to give a reasonable number.
The wiring standards that are used in building construction are based on common assumptions about voltage, acceptable losses, typical temperature ranges, etc. They may or may not be applicable in your situation.
P.S. - I don't know what project you are working on but make sure it is properly protected from over-current conditions. Most "household" breakers, switches, etc. are not rated for DC. DC fuses are cheap and easy to use... make sure the fuse is always the weakest link in the entire project.
We're on DIY.stackexchange, so the context here is house repair. For architectural wiring, 10 AWG is required by code. However on projects, UL deems 12 gauge acceptable in short runs inside equipment. Check the guidelines.
You may want to go higher still if you are going long distances and don't want to lose a lot of your voltage to Ohm's Law. In some cases this is required.
Edit: See also, this, which is not official but gives you a sense of the difference between "chassis" and "pole line transmission": http://www.powerstream.com/Wire_Size.htm