i went back and reviewed how i drew my conclusions for wire size from my proposed rack mount from which I came up with the need for #6 wire at 250’. here is a link to the table i was using, or something similar. can you tell me where my assumption is wrong? for both your wiring and canem, this table (Electrical wire sizes & Diameters, Wire Size Tables for Normal & Long Wire Runs) would not allow a 12g wire at 120 or 170 feet for a 15 amp circuit.
That’s a recommendation based on voltage drop, not an NEC requirement (we’re under NEC 2017 here). Though I believe NEC may have some advice on voltage drop, I’m not sure it’s regulatory (just a “Hey, you should really consider this…” sort of thing).
Their wire gauge and conduit fill requirements are based on thermals, and at 10A, you dissipate so many watts per foot - which means, safety-wise, there’s no difference between a 10 foot and 100 foot run - 10x the heat loss over 10x the length means the same temperatures.
For my install, I’m DC to the inverters, on 600V nominal strings, so the short answer is I don’t care. Right now, I’m running 334V/7.86A on one string, 360V/3.39A on the other string - so they’re nowhere near the reasonable limits of 12AWG, and I almost never see 10A on them.
On the AC side, my designs also don’t peg out the inverters - I expect Canem’s 6kVA inverters to be pushing around 4000-4500W peak in practical use, so ~18A on wire sized for a good bit more. Using Voltage Drop Calculator | Southwire, 18A at 170’ on 10AWG is a 2.68% voltage (drop, rise) - so not enough to really matter. At 240V nominal, that’s 246V at the other end, well within spec.
I’m aware it’s rather at odds with how most solar installers work (a DC/AC ratio that leads to clipping in practice), but I just don’t like power electronics running at 100% rated power for long periods. I know the inverters are rated for it, I just don’t like doing it, and I was able to build my system around it. On paper my DC/AC ratio is over 1, but because the two strings face different directions, it’s never a problem in practice. I’ve only seen the inverters clip a few times, in some weird high haze cloud edge lensing conditions, but they almost never run past 4500W for any length of time.
i think i see what my issue is. i wanted to run from rack mount to the inverter which would be near the meter base at the house, about 250’. it seems my error was looking at AC wire run tables, not DC.
Welcome to the joys of high voltage runs! Voltage drop matters less!
A drop of 12V along a run on 120V is a 10% drop, which is a big deal.
That same 12V drop on a 600V run is a 2% drop, hardly worth worrying about.
That same 12V drop on a 12V system is a rather catastrophic 100% loss. 
In case anyone wonders why we don’t wire houses with 12V DC systems.
The issue isn’t AC or DC, it’s “Does the voltage drop matter?” For DC, especially coming off a string of panels, the answer (IMO) is “It doesn’t matter, and it’s not worth oversizing the wire.” Plus, in operation, I almost never see a full 10A rated current off my panels - they run at lower current.
The drop on 200’ of 12AWG copper at 400V/5A is 0.87% - hardly worth dropping a bunch more money on wire to reduce slightly.
This looks as good as a thread as any for general solar questions.
I have a non-optimal location (basically the only places I can put solar without trees would be an east-facing roof or a flat roof next to it). The little calculator thing I found online says I’d get maybe 1/3 to 1/2 of my total home usage average.
Given that, is it ever worthwhile to install “piddling” solar or is so much of the cost tied up in inverters and permits that you shouldn’t bother unless you can meet or exceed your usage?
For me a 7kw [nameplate] array would service an estimate 70% of house power use. If it was going to stay that way, it was right on the edge of making sense for me, basically keeping some small power bill but pretty minimal compared to the cost of doubling the array size.
However I plan to move primary heating and cooling to heat pumps soon, so I’m putting in a 14kw system which will will reduce that surplus estimate quite a bit.
There is another advantage that can be had with a solar system small than what will ‘zero out’ your power bill, and that is keeping you out of the kWh usage ranges that you get charged extra per kwh for.
Having said all that, its value there would depend on how much the price jumps into the next tier where you are at, and how much power you consume in that range. For an average house I’m not sure it would be worth the effort for <50% of power consumption.
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If you can do the work yourself, then I’d say it’s worth doing, even if it’s only offsetting the highest cost energy in a month (which will depend on your local rate schedules). If you’re going to pay $4/W, nah, don’t bother. The loan origination fees and salesman dial-a-commission alone will tank the value of it.
However, east facing isn’t a particularly high value spot for solar. Were I to do that sort of thing, I’d probably seriously overpanel the inverter.
Sadly (or good? I don’t really know), we apparently have a flat rate or I haven’t launched myself out of the base rate, so I’m at 2199 kWh @ 0.1004-0.0162 (some credit).
I have a suspicion that it’s just not worth it, that I’m better off preparing this property for sale at some point in the next five years, and buying something that has land more suitable.