Chris1957,
First, I will say that what follows is meant in a helpful way as my advice to you and should not be construed as anything else...
You should probably get a copy of the NEC or whatever standard is applicable for your installation and location and familiarize yourself with the appropriate standards before you get too far into this. I say this because of the impression that you are not familiar with doing this type of work. This is based on, for example, the fact that you have selected a conduit size (and quite large at that) before knowing the size and number of conductors it will contain and the fact that you are not familiar with sizing conductors. My advice is to develop enough understanding to be sure that whatever methods and materials are used for your application are safe, reliable, and economical since you are ultimately responsible for the outcome regardless of whose recommendations you follow in the design. The need for this will become very apparent when you realize that you may get a different recommendation from eachperson you ask...someone else may post here to disagree with me before it is all done, it wouldn't be the first time. Anyway, you need to know for yourself what is right or take recommendations from someone who you trust who also has the credentials to back it up. Sorry for the preaching but I felt it had to be said.
I will give you some things to get started. I will also tell you that I am not an expert on electrical construction nor the NEC so you should (always!) make sure you understand and agree with what is presented before acting on it.
- Voltage drop is the first big concern for your applications. The NEC allows 3% drop at the end of the line but your equipment may require tighter voltage regulation. There may be an easier way to address this, but the method I know is (1)select a wire size based on ampacity of the load using NEC ampacity tables for your conductor. Next, (2) calculate voltage drop for the distance using wire impedance from NEC tables. Finally, (3) select larger wire size and repeat if first result is unacceptable.
For an example, use a 60A service at 200ft. The NEC shows that a 90C conductor size AWG 6 is rated for 75A as stated by jbartos above. The NEC also shows that three AWG 6 conductors in a single steel conduit have an impedance of 0.45 ohms/1000ft. V=I*R, so 60A * .09 ohms = 5.4V or 2.6% voltage drop at 200ft for a 208V service. An acceptable answer, just make sure that it is 200ft 'as the conduit runs' and not 'as the crow flies'.
- For the distant locations, I would recommend using one large service to the center of the 1300ft location, then a subpanel to distribute the power to the four pads. This is based on the fact that 4 large conductors (3phase/4wire, 208V/120Y) will usually be cheaper than 8 small ones (single phase 120V). (Again, I am not an expert on NEC!) You may be able to feed the outlets at the pads directly from the single subpanel at the center, but I would check that. The one location at 700ft can be fed from either end or it may be possible to tap the center of the 1300ft feed. I will warn you on the center tap idea that you should definitely check the NEC before doing so as that practice may not be allowed for your application.
The decision on whether to use transformers at that distance will be based on the wire costs. For the 208V service, perform the calculations for voltage drop, select a wire size, and calculate the cost for the wire and conduit. (will be less than 3 in.). Next, for 480V determine the required transformer size (maybe 7.5kva) using a 480/208 delta-delta for step up and 480/208Y for step down, then the required wire size and conduit size. Calculate the cost of the transformers, wire, and conduit for 480V service and compare to the 208V service. Of course, a decision to feed the service at 480V may affect your plans for the 700ft service so you will need to calculate the cost changes, if any, for that as well.
- Finally, I do not think that the unbalanced load is an issue. I did address my thoughts on that in my previous post but will add that if you are concerned you should contact the manufacturer for the transformer you intend to use and ask for a specification for acceptable unbalance.
I hope this helps.