voltage loss

I'm no electrician Tso so don't know whether that wire size is adequate but what's your 21-v estimate based upon and why can't you use bigger wire

LOl 250 mcm for a residential service, that is overkill, by a large margin. Standard homes generally dont have bigger than 4/0.

Then why do you suppose Tso expects such a voltage drop

Also not an electrician here but I'd guess he has a long run (very long for that kind of drop, or he's miscalculated the drop?) from the nearest transformer to his house, a rural setting maybe? I don't think 250MCM is that much bigger than 4/0, it is the next size up but as noted is not residential standard. I think he's asking about a step-up transformer and expect that is what he may need, unless he can get the power company to tap the transformer feed higher to compensate. Just a diy hobbiest's thoughts though.

P. S. Got curious and taking a break from work, quick calculation for that kind of voltage drop... I'm guessing a service run of about 2 miles from the transformer?

Tsolo, what is your load, what is the voltage and distance from the source? If you have a large plot of land with a home located a long distance from where your service panel will be located there are some options (buck-boost is not one) but more information would help.

You can boost using stepup but if the problem is a long, high-resistance lead-in you will still have a lot of voltage fluctuation dependent on changing load

I guess boost transformer would be of mighty help. Good luck

I would say a transformer should not considered as a first solution. There is a reason why you are getting voltage drop and it would be best to discover why before comitting to anything.

First off at which point on the connection did you take your measurement? Lugs in panels? At the top of your mast (if you have an overhead service)? What phase is showing the voltage drop? did you test both phases to ground than phase to phase? What were the results of that?

With that information, a good picture of the problem could start to be formed.