Step up transformers - confusion

[yllihp]

I know step up transformers decrease power loss by the electrical cables, but I'm not quite sure how this works...

From what I understand, step up transformers increase voltage, hence decrease current, which means less power is dissipated by the cables since P=(I^2)R.

But then couldn't you argue that since step up transformers increase voltage, then actually the power dissipated by the cables increases since P=(V^2)/R ??

Could someone please enlighten me??

 

[n.karthick]

It is not like that.
Power loss in a cable is given by
Ploss =I^2 R
The voltage drop V in the cable depends on current I passing through the cable.
This V is not the applied voltage Vs. Instead
Vs= V+Vl
where Vl is the load voltage (because resistance of cable is in series with load)

So your second equation, though it is correct, has to be understood properly. A numerical example may help you to understand.