u/BlakeMW

I couldn't find this information through google, so I decided to test it and make the information. In short: 147 km is the highest completely safe altitude.

Circular 0 inclination equatorial orbits:

The inner radiation belt is a torus which is not equatorial, like Earth, Kerbalism's Kerbin's magnetic pole is about 10 degrees off from its rotation axis and the inclination of the radiation belt reflects this, as such a marginal orbit will dip into it for parts of each orbit.

But the highest low orbit that never touches the inner belt, is 147 km. The lowest high orbit that never touches it, is 630 km. The belt becomes much more intense as the orbit gets deeper into it, increasing from 0.014 rad/h to as high as 10.39 rad/h in the middle (about 400 km) so merely brushing the fringes of the belt isn't a serious threat (e.g. up to say 160 km it's not seriously threatening and an eccentric orbit with a low Pe might not hit it at all on a particular orbit).

Polar and highly inclined orbits:

There is no completely safe low altitude, as the outer radiation belt comes down to the atmosphere around the poles, although the radiation intensity is not very high relative to the inner belt and the orbit quickly passes through the radiation zones.

An altitude of around 3100 km is required to be completely outside the inner and outer belts, this altitude should be safe for any orbit regardless of inclination.

Magnetopause:

The magnetopause starts at around 5900 km, being outside it increases radiation only slightly, from 0.014 rad/h to 0.025 rad/h.