power loss in the cable

You can only use P=V^2/R to calculate the power loss in a transmission cable when the potential difference across the cable is know.
Notice that the [V] in the above equation stands for the voltage across the cable, NOT the voltage of the secondary coil of the transformer, which include voltages across the cable AND the load.
In most circumstances, the voltage across the cable is not known, but the current, which is the same for both the cable and the load, can be found easily using energy conservation (i.e. power supplied by primary coil equals to power delivered in secondary coil of a step-up trnasformer). Thus it is usually more convenient to use the equation P=I^2.R to work out the power loss.