AC Vs DC Voltage Drop - Is there a difference between V drop on the AC and DC side?

[Lenerd]

If I use #8 AWG for the 3 DC stings over the greater distance from the ground mounted Array to the inverer mounted at the house I get a voltage drop of 1.7V or .48% relative power loss
If I use #1 AWG for the AC side over the greater distance from the inverter mounted at the Array I get a voltage drop of 1.2V or .49% relative power loss
Considering only the Voltage drop, is one alternative better than the other?

Cost is just about the same either way
My approved line drawing has the inverter mounted outside with the AC extending 150ft in conduit in a ditch But my township will accept the last minute change. I recently liked the idea of mounting the inverter in the home since the SMA 7700 has a secure AC power supply that can be used to get power during the day in case of a grid power outage. But that just means extending a #10 AWG wire


Thanks, Leni

[inetdog]

If I use #8 AWG for the 3 DC stings over the greater distance from the ground mounted Array to the inverer mounted at the house I get a voltage drop of 1.7V or .48% relative power loss
If I use #1 AWG for the AC side over the greater distance from the inverter mounted at the Array I get a voltage drop of 1.2V or .49% relative power loss
Considering only the Voltage drop, is one alternative better than the other?

Cost is just about the same either way
My approved line drawing has the inverter mounted outside with the AC extending 150ft in conduit in a ditch But my township will accept the last minute change. I recently liked the idea of mounting the inverter in the home since the SMA 7700 has a secure AC power supply that can be used to get power during the day in case of a grid power outage. But that just means extending a #10 AWG wire


Thanks, Leni

If the cost is the same, there is nothing to recommend one over the other.
The general rule, when you have one long run and one short run is to make the longer run at the higher voltage. That will either mean placing the inverter at the array or at the grid end depending on your string voltage.
These days Rapid Shutdown puts an additional premium on having the inverter(s) within 10 feet of the edge of the array.

[sensij]

If the PV DC to Inverter AC ratio is large enough (maybe 1.2 or higher), there might be a slight difference in production (<1%) in having the long run on the DC side. In that scenario, in periods of peak power generation, you might avoid some clipping because of the power loss in the transmission to the inverter. If the long run is on the AC side, you would encounter clipping sooner, and still have the same transmission loss.

As inetdog suggested, safety considerations like rapid shutdown would probably be the deciding factor. You could have long DC runs that are powered only part of the time, or long AC runs that are powered 24/7. I'm not sure there is an objectively better choice.

[Amy@altE]

It's a ground mount, I don't think he needs rapid shutdown, just a manual shut down.

[Sunking]

For short runs of power you are referring to neither AC or Dc has any greater loss then the other. You most cost efficient and most power efficient is the one operating at the higher voltage for a given power level. From your calculations tells me your DC voltage is much higher then the AC voltage. No other way the math can work.

OTOH when you are talking about power transmission, DC has an advantage because the loss is pure resistance. In AC you have the exact same resistance loss, plus reactance that DC does not have. But we are talking many miles, not a few hundred feet.

I do question your loss calcs as something does not seem right. What voltages and power are you looking at. I find difficult to believe DC and AC wire size is so far apart. That would require a huge difference in voltage. I know voltage for the AC is 240. What did you use for the DC side. I must be missing something.

[Lenerd]

I don't mean to imply I understand the loss calcs. I just plugged in the numbers at the SMA web tool design app and then verified them with SMA support here is the site address http://www.sunnydesignweb.com/sdweb/#/Home

[Lenerd]

Thanks All,
I will be putting the inverter at the house not the array. (the sunnyboy 7700 really is deigned for it ) But if i do need a dc shut off at the array I will have to use a combi box with a disconnect rather than just a junction box Which is aggravating since I dont' need to combine strings since i only have 3 and i already have a DC and AC disconnect at the house, on each side of the inverter. I can't easily verify needing the shut-off at the array because My local inspector is only around 1 day a month