IPC-2221A, calculation for current capacity of an external track :
I = .048 * DT(.44) * (W * Th)(.725)
Where I is current (amps), DT is temperature rise (°C), W is width of the trace (mil) and Th is thickness of the trace (mil). Internal traces should be derated by 50% (estimate) for the same degree of heating. Using the IPC formula, we generated Figure 3 (see table at end of text), showing the current carrying capacity of several traces of differing cross-sectional areas with a 20°C temperature rise.
What constitutes an acceptable amount of heat rise will differ from project to project. Most circuit board dielectric materials can withstand temperatures of 100°C above ambient, although this amount of temperature change would be unacceptable in most situations.