PCB Clearances Table

For non-mains voltages, the IPC standard has a set of tables
that define the clearance required for various voltages.
A simplified table is shown here. The clearance will vary depending
on whether the tracks are on an internal layers or the external
surface. They also vary with the operational height of the board
above sea level, due to the thinning of the atmosphere at high
altitudes.
PCB Clearances for Electrical Conductors Table
Voltage (DC or Peak AC) - Internal - External (<3050m)
- External (>3050m)
0-15V - 0.05mm - 0.1mm - 0.1mm
16-30V - 0.05mm - 0.1mm - 0.1mm
31-50V - 0.1mm - 0.6mm - 0.6mm
51-100V - 0.1mm - 0.6mm - 1.5mm
101-150V - 0.2mm - 0.6mm - 3.2mm
151-170V - 0.2mm - 1.25mm - 3.2mm
171-250V - 0.2mm - 1.25mm - 6.4mm
251-300V - 0.2mm - 1.25mm - 12.5mm
301-500V - 0.25mm - 2.5mm - 12.5mm

PCB Design Tutorial by David L. Jones