What voltage will blow a fuse with given load resistance?
"Ohms" law Jesse !

U = I * R -> (2.5 * 20) = 50V

That's for an average value, the fuse might be able to handle a VERY short burst of currents greater than 2.5A

So for your application, under the presumption that the load doesn't change any voltage &lt;= 50V will not blow the fuse.

If you want to get fancy, you include the tolerance in the load (i.e. +/- 5%)
So a 20ohm load might have a real value of 21 or 19 ohm, always take the worst case scenario!

** Edit - The load might not be the only thing of interest, you say a "circuit" - So what kind of components are in this circuit ? Can they handle 2.5 A? **