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quinn77:
You have to look at the kinds of loads that exist on the circuit.
First, if the wire from the main panel to the load area is sufficiently large for the steady-state load, then from the point of view of said wire voltage drop is of no concern.
This means you only need to study the needs or limitations of the actual loads.
Incandescent light bulbs do not care what the input voltage is so long as it is below their rating. Radios may not care much about voltage so long as it is reasonable. Electric heaters with no fan won't care.
An air conditioner, or other motor device, may be something to check on.
Humans may not like the lights momentarily flickering when an air conditioner turns on.
With 122 nominal I would not boost more than possibly 3 V.
If you use step up and down transformers, then you need to evaluate how their internal impedance, sum of the two transformers, trades off relative to the reduced drop on the transmission line. You could be worse off if you do not use oversized transformers.
#10 copper for a 250 ft distance has about 0.5 ohms loop resistance. 24 A provides the 12 V drop you mentioned. 12 V is about 10%. Double the voltage on the distribution line and the current is 12 A, and the voltage drop is 6 V, or 2.5%.
Next suppose each transformer has a 5% internal impedance. Then the voltage drop will be about 1 - (0.95*0.95*0.975) = 0.12 or 12%.
You would need lower impedance transformers, or oversized transformers.
Instead of changing the distribution line voltage you could at the destination end use a boost transformer, may be 5 V, and have a control with slight hysteresis that at the destination end senses the incoming voltage and switches the boost transformer in and out as needed. Maybe it switches on at 115 V, and off at 120 V.
Another technique is to use a motor driven Variac at the destination end feeding the boost transformer and using appropriate electronics to provide voltage regulation. To adjust for about 15% at 20 A would require a Variac of about 500 VA.
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