Actually 250 mcm sounds like it might be correct IF the load is covered by 430.
There is a strong tendency to over wire motor loads.
Under 430 a 50 Amp C/B can protect a #12 conductor.
If your calculations show that you're needing only 250A at the C/B -- AND you're dealing with a motor load, then your conductors seem fine.
It's looking like the prior EC may have been building to engineered specs.
