Electricity Transmission

[Glenn Schuyler]

Hi,
I have an off grid cottage with 3.6kW on the roof. Unfortunately the surrounding forest is limiting my solar access. To remedy this, I plan to add 4kW on a 20 ft high rack about 130 m from the cottage where I get better site lines. I'm thinking that I need to run cables from collector pairs to my charge controllers in the cottage. This would keep the voltage at about 60 v and avoid excessive losses after the inverter. Does anyone have any suggestions or rules of thumb for acceptable line losses?

[Rob Beckers]

Hi Glen,

The trick is to keep the PV Voltage as high as you can, to avoid excessive wiring losses or thick wires. So, assuming regular 60-cell panels that get up to about 47 Volt at -35C in winter, you could have 4 of them in series connected (and then a number of parallel strings) going into a 200 Volt rated charge controller.

Wiring losses go with the square of the current through the wires. So, the lower the current, the lower the losses. That's why long strings of series-connected panels work best for long lines.

An example, we currently have 350 Watt rated panels that are 60-cell (hence 47V in winter in our area). For a 4kW system that would be 12 panels. With 4 in series that makes a total of 3 strings in parallel. Current at maximum output is right around 10 Amp, or 30 Amp for three strings.

Generally we use 5% peak-loss as 'acceptable' for off-grid systems. In practise panels rarely produce rated output, so actual losses are much less. Using that for wire size calculations, it works out to 4 AWG (copper) for 4.2% peak-loss, or 6 AGW for 6.6% peak-loss. If it was me I would just go with 6 AWG. As mentioned, actual overall losses will be much less than 6.6% since neither the panels nor the charge controller will be running at maximum current for much of the time.

For longer distances copper gets pricey, and you can use aluminum wiring as well. Wire size goes up two steps, so instead of 6 AWG copper it would be 4 AWG aluminum etc. Working with aluminum wire does have some drawbacks, and it needs special treatment (sanding it clear, using anti-corrosion compound, aluminum-rated terminals etc.).

Hope this helps!

-RoB-

[Glenn Schuyler]

Thanks Rob,
That is helpful. I have two concerns with going to more panels in series and the resulting higher voltage. One is the fact that I will be going over rock and unable to bury the conduit in places, so I am concerned about safety. The second concern is that my Midnite manual says that the charge controller is less efficient at higher voltages. I'm wondering if I would be trading one efficiency loss for another.

[Rob Beckers]

Hi Glen,

Electrical code would require either armoured cable such as Teck90, or conduit buried in concrete when putting electrical lines over rock. We've done the conduit encased in concrete (and that passed inspection).

Where did you see efficiency graphs for the MidNite Classic? The only graphs I've seen are maximum charge current vs. solar Voltage, but those don't say anything about conversion efficiency. In general, there MPPT charge controllers are pretty efficient in converting from higher to lower Voltages. I'm assuming your battery bank is at least 24V?

-RoB-

[Glenn Schuyler]

Thanks Rob,
I likely misspoke about the efficiency. There is a graph in my manual that shows the Classic 150 Power. It shows that with a 90v PV array on a 24v battery bank (my system) the classic puts out 2700W, while with a 120v array, the classic puts out 2353 W. I guess that's telling me about capacity and not efficiency. Would I have to connect all of my existing panels in groups of 4 as well? They are currently in pairs. Or could I run my existing controller at one voltage and a new one at another?

[Rob Beckers]

Hi Glenn,

Yes, that graph is about the maximum current it will do in various scenarios (and thus output power). It doesn't speak about efficiency.

There's no problem having multiple charging sources into the same set of batteries. Batteries don't care where the current is coming from. So, just leave your existing panels on their own controller and hook up the new panels to a new charge controller. What string lengths and such is what each controller can handle, it doesn't affect charging. To make both charge controllers do the same thing at the same time you set the bulk/absorb/float Voltages and absorb time the same (they'll still not be 100% in-sync but that's fine).

-RoB-

[Smith Leicester]

Absolutely! Running the cables from the additional solar panels to your existing charge controllers at the cottage sounds like a good plan.