Reg wrote:

"From LF to VHF it is ALWAYS power dissipated in conductor resistance
which limits the power handling capability of the line. Voltage has
nothing to do with it. Above VHF dielectric loss becomes be the
limitation. "

Always? Hardly. Transmission of pulses with low duty cycle will get
you to voltage-limited operation pretty quickly. Transmission of power
to a high-resistance load where the line is a very small fraction of a
wavelength long may get you into voltage-limited operation. Those
perhaps aren't typical ham applications, but they do happen in
practice. Also, though the cable itself may not have trouble with the
applied voltage, the connectors at the ends may. They're generally
rated for much lower voltage than the line itself.

Also, for the small-diameter (nom. RG-58 size) cables I've been using
lately, conductor loss exceeds dielectric loss out past 10GHz. YYMV.

Cheers,
Tom