Electrical – Service Entry Wire/Ground Location (100 amp service)

I am assuming that you have 220V going to the barn? As far as wire size, industry standards say to aim for 1 - 2% voltage drop, but running 5000W, 200ft on #8 wire @ 220V results in just under 3% voltage drop. This will probably work just fine but it would work a little better if you were to upgrade it to a #6 or a #4. It is just a matter of weighing the options. During peak performance a system running at 5000W with a 3% drop would be losing around 150W. At 1% drop you would only lose 50W remember, this is only during peak performance; during the morning and evening, your losses are much less.

Here is the question to ask yourself... How much money is it going to cost to upgrade the wire? Is it worth it for a 100W increase?

Credits: I mostly use the Southwire Voltage Drop Calculator for my calculations... I can do it manually but the Vdrop calculator is much easier.

Thanks, Maxfield Solar

If you're running it through conduit, just buy single strand. It's cheaper. e.g. this
And obviously you'd be buying this at a proper electrical supply house, not the bigbox since they probably will not have it, and bigbox is really overpriced anyway. With wires of these sizes, it is not required to buy white or green wire for ground, you can designate that with tape.

Besides, it'd be murder to pull a 4-wire cable through conduit. Four single wires will be tough enough! Might want to bring in a commercial electrician simply for the pull because he has the gear on the truck to do the job easily.