Cell voltage required for refining copper

If you put Zinc or Iron solids into aqueous copper (II) sulphate solutions, copper solids form spontaneously due to the fact, that Zinc/Iron oxides into ions more willingly than Copper does.


And if you put Silver bars into the same solution, hoping to get copper solids forming on them,
(yes, electroplating copper on silver)
you'd need a theoretical minimum of $\rm 0.46\,V$, as that is the difference between the standard reduction potential of $\ce{Ag}$ and $\ce{Cu}$ ions.


Our question is this.  

**While purifying/electrorefining copper, how much voltage is required to make it happen?**
If we simply think in standard potential dimensions, we wouldn't need to do anything. And yet, common sense tells me that surely wouldn't be right.


So, how much, and why?


