Idealized breakdown in air is about 70 kV/inch (30 kV/cm), but you'll never get that much voltage across an inch. For ordinary, air insulated sorts of equipment, 1" per 10-15kV is a nice round figure to use.
For corona reasons, watch the radius of curvature. In air, at 20 kV, you want a ROC of at least 0.7 cm, or about 0.28 inches. Practically speaking, that's 7/16" to 1/2" diameter, which is pretty big. If the conductor is entirely encased in some insulator (no air bubbles or voids), then scale by the breakdown voltage of the insulator (this is why you can use small conductors insulated with polyethylene, for instance; or why the silicone putty or potting works (if properly applied))
Watch creepage distances. Breakdown voltage along a surface is typically 1/3 that of the same distance through free air. This is why HV components often have ridges and ribs. If you had, for instance, a pair of wires penetrating a piece of acrylic that were 1/2" apart, at 10 kV, you'll almost certainly get breakdown along the surface of the plastic, even if it is nice and clean.
Created on Monday, April 23, 2001 2:56 PM EDT by James Lux