Feed wire size

[lockem]

My installer did not, but I did. Both my DC and AC feed loops are
at least 600 ft long. That resulted in an acceptable 1% loss for the
DC loop, operating at 400V. The AC loop loss was over 3%, which
besides losing a lot of energy and doing a warm up/cool down cycle
every sunny day, sometimes would trip inverter line voltage monitors.
I eventually reduced the AC lose to 1% using greatly oversized but
economical aluminum direct burial wire. That was the only large
expense, since I had my own trencher. Bruce Roe

But did you check the cost-benefit analysis, particularly for an even larger wire size?

In my case the run is 40feet, for 80 feet of wire. For 10ga wire that yields 0.08 ohms of resistance. For 8ga wire that yields 0.05 ohms. At 20.8A RMS that my system produces the power loss = I^2*R is 34.6 watts for 10ga and 21.6 watts for 8ga. The difference is 13watts. My system puts out 4400W, so that is only a 0.3% improvement.

However, 80 feet of copper 10ga THHN costs about $30 and 80 feet of 8ga TNNN costs about $45 (both prices from these guys: https://www.wireandcableyourway.com/...SABEgLcP_D_BwE) , a difference of only $15. Rounding down, my ground mount system cost $30,000, so the incremental cost of the wire upgrade is a meager 0.05% of the total system cost. It is bound to have a shorter pay off time.

In my case I have an average of about 5 sun/hours per day and electricity that costs $0.23/kwh. The value of the power is 13*5*365*0.23/1000 = $5.45.
$15/$5.45 => 2.75 years to break even, at least if you are on NEM1 or NEM2.

[bcroe]

My installer did not, but I did. Both my DC and AC feed loops are
at least 600 ft long. That resulted in an acceptable 1% loss for the
DC loop, operating at 400V. The AC loop loss was over 3%, which
besides losing a lot of energy and doing a warm up/cool down cycle
every sunny day, sometimes would trip inverter line voltage monitors.
I eventually reduced the AC lose to 1% using greatly oversized but
economical aluminum direct burial wire. That was the only large
expense, since I had my own trencher. Bruce Roe

[lockem]

As part of a study of my system I observed the 1.8V * 20A power loss on my 40 foot run of 10gauge wire doesn't sound like a lot of loss, but the incremental cost to have installed 8ga wire instead has a ROI of only 3 years. This ROI is constant for any length run; it is only dependent on the 20A current value and the local cost of electricity. AC or DC does not alter the calculation either.

I did the upgrade retroactively and indeed the Sunpower monitoring system consistently shows an increase in maximum power of 20W. Even as a retrofit the ROI on the materials is 5-6 years, although I already had a spool just lying around.

I suspect that going 1 size over size for the wires will have a similar benefit for any configuration, but did not bother to research the numbers.

So, do any installers out there bother making this calculation?