Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

jgh