Unlike IPC requirements that deal with electrical clearance, IEC and UL additionally specify so-called creepage. The former is a distance between PCB conductors through air, and the latter by definition is shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface of the insulation. The diagram below shows examples of measurement of both quantities. The creepage requirements depend on the insulation material group, pollution degree and working voltage. For a given circuit and application, these requirements are always greater than or equal to the clearance. Our calculator provides the values from Table 2N of UL60950-1 2nd Edition. Note that these values apply to functional, basic and supplemental insulations. For reinforced insulation you need to double the results.

creepage measurement
The pollution degree depends on your intended application. Degree 1 means no pollution or only dry non-conductive one.
RMS working voltage
Pollution degree
Material group
Degree 2 applies where there is non-conductive contamination that might temporarily become conductive due to condensation. This is the most common type for general use equipment. Degree 3 applies to conductive pollution, or to dry non-conductive one which could become conductive due to condensation. If you don't know your PCB or insulator material group, assume worse case IIIb.

Creepage and clearance distances over slot and barrier Creepage distance can be increased by adding an insulated barrier or a slot wider than 1 mm (see diagram).

Note that this widget is for a preliminary estimation only. For a final design decision you need to work with actual standard which has many nuances. For example, if your result is less than the applicable clearance, you have to use the latter value. On the other hand, for glass, mica, or similar inorganic materials, if the minimum creepage is greater than the clearance, UL permits to apply that lower value. The spec also allows you to use linear interpolation, so you need to work with the UL table 2N. For circuits not covered by safety requirements see calculator of trace spacing between tracks in internal and external layers per IPC2221B and IPC9592B.

Legal: The information provided here reflects only a personal opinion of the author and does not constitute a professional or legal advice. It is not intended to substitute official standards-- consulted them for all final decisions. Also see our general Disclaimer linked below.