creepage and clearance
According to IPC a 0.5mm wide copper trace and having a thickness of 1/2 Oz, running on the external surface of a PCB at room temperature is capable of carrying up to 1 Amp of current. A 10 mil trace might carry only 0.5Amp if the trace is on the external layers considering the IPC. However, if the copper thickness is increased the current carrying capacity of the same trace increases proportionally. That is, a 1 Oz copper trace with a trace width of 0.5mm can carry 1 Amp of current. The trace widths and copper thickness are two parameters that can be varied as per the end application and requirement.
Coming to the point of trace width and creepage distances...... Creepage distances vary based on the application and its category, class of the end equipment and the pollution degree. Also the creepage distances vary between insulated and un-insulated parts/ points/ conductors, between live and neutral parts/ points/ conductors, between points of the same polarity and between points of opposite polarity and between points of two different polarities.
The minimum creepage distances vary based on the standards to which we plan to certify the conformance of the product. For example, in case of industrial controls, 6.5mm will be the minimum spacing required to be maintained between the un-insulated conductors running on the external surfaces of the control assembly. In case of the conductors are insulated, then the spacing gets reduced.
As a matter of practice 6.4~6.5mm is the creepage distance required to be maintained between un-insulated copper conductors of dissimilar polarities.
In cases of traces which run parallel to one another, wherein one trace is a high voltage one compared to the other, the spacings greater than 2.54mm apply.
However, you need to first identify the end application and based on that identify the category of the equipment and then decide the creepage distance.