Wiring a pole barn that is 750' from my main power box
Long-distance circuits are a huge opportunity to spend way too much on wire unnecessarily.  Given the amount of money that is at stake here, let's debunk the usual 3% knee-jerk.  

There are two huge mistakes when made when sizing circuits. 


sizing for 3% voltage drop.  3% is simply wrong.  There's no support in Code for it anywhere (it's a suggestion, one place). This myth is widely spread by the companies who sell wire, for obvious reasons -- the first place they do it is in their "free" voltage drop calculators.  They default to 3%, and by that they mean literally 3.00% - if a cheaper option would come in at 3.25%, they will hide it, and "blame the computer" for being too literal.  
Sizing for "breaker trip rating" instead of practical load.  Remember, loads are already supposed to be derated 20% (literally, breaker should be 125% of load), so sizing to breaker is always wrong.  Indeed, the simplest step of sizing to 80% of breaker usually results in a couple of wire size drops and thousand$ saved.  But you should go farther, and size to actual load. 


Voltage drop is proportional to current right now

Remember that voltage drop in a circuit is a function of current actually flowed.  Here's what's not true: "The voltage drop the calculator says will always apply to all loads". Actually it will never apply to any loads.  

Suppose someone puts in monster wire on their "50A" circuit and gets to 2.5% drop at 50A.  


You draw 10A @ 120V.  You actually get 1.0% voltage drop.  Not what we expected, eh? 
What happens if you have a "30A" dryer, which actually is 23A, and that actually is about 21A on the 230V side and 2A on the 120V side. So drops of 0.525%, 0.05%% and 0.575% per leg.  The 240V heating element sees 1.1% voltage drop, and the 120V mechanism sees a 0.575% voltage drop.  
A 40A (9600W) heater sees 2.0% voltage drop, which it doesn't need -- heaters will work on 30-40% voltage drop. 


Getting the idea how wasteful this is?    

For a welder, consult with your manufacturer, but voltage drop is pretty normal for a variety of reasons inside welders, so it's probably not going to bother them all that much. 

Transformers

Then there's using transformers to step up voltage.  All fixed-installation wiring is rated for 600V, and there is nothing wrong with stepping up power that high for the transition.  Transformers are expensive, but on a long haul, they're cheaper than wire.  Often, simply "stepping up" the circuit to 240V, and using a transformer at the far end to make 120V, is all you need.  I have plenty of postings about this. 

Let's run some numbers in your case.

And to be clear, we'll be running aluminum wire, because running so much copper isn't even stupid.  Use the goop and torque to spec. 

Scenario 1: Obedient Consumer.
Let's do exactly what the voltage drop calculator and wire companies say we should do. Compute on breaker trip and stick to 3%.  3/0 wire would have 3.05% drop and the computer says "no" to that and computers are smarter than us, so we are forced to 4/0 wire. 750' of 4-wire 4/0 URD will cost $2145, trench it at 24" and we're done. 

Scenario 2: Compromises.
On a 50A breaker, actual draw shouldn't be more than 80% or 40A.  We go for 4% voltage drop at 40A. This comes up as 1/0 wire.  750' of 4-wire 1/0 URD will cost $1297. 

Scenario 3: 480V Transformers.
In this case we use 15 KVA transformers and breaker for 60A (which is more power).  The transformer halves the current, which also halves the voltage drop, and voltage drop is only half as important anyway, since it's coming off 480V instead of 240V.  So now we can happily use much smaller wire.  240V at 40A becomes 480V@20A.   With #4 wire, voltage drop @ 20A draw happens at 2.45%.   Even if we max out the circuit to 60A@240V (30A@240V), voltage drop is only 3.67%.   Further, we only need 2 wires between the transformers, because neutral is created locally by the transformer.  


The two 15KVA transformers cost $2000. 
3-wire #4 SEU is only $555. 


Scenario 4: Mini-transformer.
In this case, we only aspire to provision 20A (16A) practical, but we use a (smaller) transformer so we can transmit at 240V on 2-wire, even though we are only using 120V.  Fortunately, these smaller 5 KVA transformers are readily available on Craigslist for about $100. If we go for only one 20A circuit, then we base voltage drop on 8A@240V and the calc says we're just fine with 6 AWG aluminum at 3.58% drop. 

However, instead, let's go for two 120V circuits at 20A breaker (that's table saw and dust collector). That will realistically be about 24A together, or about 12A@240V, but let's assume circuit continuous max of 16A x2, so 16A@240V. Voltage drop calc says that happens at 4.57% with 4 AWG Al.  


one used 5 KVA transformer for $100
3-wire #4 SEU for $555.


And hey, that's the same #4 wire we use on our 480 scenario, and we know we can pump that to 60A or even 80A with better transformers.