On 2013-02-05, wrote:
There are two wires so shouldn't that be 2.8kW and 400 kg of cable?

The calculation was for two wires (out and return), and to be pedantic
we're actually trying to dissipate 1.4kW of heat over 20m of cable, but
the assumption is that the two cables will be tied together thus reducing
the surface area enough that you can't really allow for the full 20m of
cable for heat sinking purposes.