DC amps = AC rms amps in terms of cable heating. This would be conservative, since ac ampacity will actually be slightly less than dc ampacity due to skin effect and higher ac resistance.
In terms of voltage drop, inductance is not an issue with dc, only resistance.
As a practical matter, we sized dc circuits just like ac circuits. Unless you have a lot of it and are trying to economize, that approach should work.
But since DC loads are often critical loads operating at 125VDC or 48VDC off a battery, voltage drop may be more of an issue. That breaker trip coil may only draw 4 amps but a #14 awg wire could have too much voltage drop to reliably operate the coil if the breaker is a ways out in the switchyard.
