Well, you would best consult a power engineer. The power companies have established REAL WORLD spacings to provide a factor of safety for arc over.
In air, you have to worry about the dielectric breakdown voltage of air. That voltage varies somewhat with altitude, humidity, etc.
If you are now trying to attach to this cable...other factors come in. You can take a ceramic cylindrical post and use it to support the cable. But what happens if the outside of the cylinder gets dirty (rain, dust, bird droppings). That dirt my start to burn (carbonize), and form a path for current to flow. More current flows, and then more carbonization occurs, and then booom, an arc over happens (even if the cylinder is theoretically long enough to not arc over). They solve problems like this by making the shape of the cylinder more bulbous.
Also, if there are any sharp metal objects, the elctric field gets concentrated at these points, possibly 10X higher than in free space. So anywhere there is a sharp or discontinuous metal object, you have to space things much further apart then you might think.