Acceptable line loss

Voltage drop is not a simple x% answer... If you are using a MPPT (maximum power point tracking) charge controller (which is more expensive than a PWM pulse width modulation controller), The line loss from the solar array to the charge controller is just an operational question.

For example, if you have a 12 volt battery bank--You need ~15.0 volts minimum for charging (depending on battery type, temperature, etc.). And a minimum of 2 volt drop across the MPPT charge controller (reality, you probably want >10 volt drop--But that is another story).

So, for a typical higher end charge controller, you need ~17 volts minimum at the charge controller... And the maximum recommended input voltage Vmp-array is ~100 VDC (again, depends on local temperatures and specific controller model). But, roughly you are looking at an "allowable" drop of 100-17=83 volts from array to charge controller.

Realistically, there is almost never any reason to design for that high of drop. Typically 1% to 3% drop is a good design range. If you try to go below 1%, you are just throwing lots of money at copper for little gain (1% to 0% loss). 3% drop maximum is a typical number for functional loads without problems (i.e, 120 volt load * 3% = 3.6 volt drop for your AC loads).

Specifically, you have to look at your loads with solar... For example a 12 volt lead acid battery bank runs from ~10.5 volts dead to ~16.0 volts (equalization charge). Say you pick 11.5 volts as your minimum battery voltage (don't want to damage battery)... And you have a 12 volt AC inverter with 10.5 cutoff voltage... That gives you 1.0 allowable drop, but since a typical inverter can draw 2x current (for starting loads), you really only want a maximum voltage drop of 0.5 volts for wiring+breaker+fuses/etc. to load... 0.5 volts / 12 volts = 4% drop...

I recommend that you design two different systems (at least) on paper and see what works best for you... With the standard warning the solar PV panels only generate "useful" power in full sun, ANY SHADE will dramatically reduce output (by 50% to 100% typically). People do argue that parallel vs series connections of panels can reduce shading power reduction--Just try for no-shade between 9am and 3pm at least.

Let us try a couple very quick rule of thumb designs... One with a PWM controller and another with MPPT. Say you have 2x 6 volt @ 200 AH golf cart batteries (very nice for a starter/training/learning system--Decent power and cheap if you make any mistakes and murder the batteries--very common for first time systems).

Rule of thumb charging is 5% minimum (weekend/sunny season systems) and 10% to 13% rate of charge for full time off grid systems. For your needs, 5% minimum rate of charge should work:

• 200 AH battery bank * 14.5 volts charging * 1/0.77 solar panel+controller derating * 0.05 rate of charge = 188 Watt array

Lets pick two configurations. 2x 100 watt panels in parallel for PWM controller and 2x 100 watt panel in series for MPPT controller

Lets say you want a PWM controller and 125 feet of "one way" wire run (for the following voltage drop calculator)...

• 200 Watt array * 1/17.5 volts Vmp = 11.4 amps Imp-array
• 17.5 Vmp * 0.03 drop = 0.525 volt drop

Voltage drop calculator for 125 feet one way run:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.5127&voltage=17.5&phase=dc&noofconductor=1&distance=125&distanceunit=feet&amperes=11.4&x=56&y=20
>2 AWG:
Voltage drop: 0.45
Voltage drop percentage: 2.57%
Voltage at the end: 17.05

Now, same thing with MPPT charge controller but two panels in series:

• 200 Watts * 1/35 volts Vmp-array = 5.7 amps Imp-array
• 35 volts * 0.03 drop = 1.05 volt drop

Voltage drop calculator for 125 feet:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=35&phase=dc&noofconductor=1&distance=125&distanceunit=feet&amperes=5.7&x=67&y=21

>8 AWG
Voltage drop: 0.90
Voltage drop percentage: 2.57%
Voltage at the end: 34.1

And if you allowed >3% drop, you could even use 14 AWG cable:
Voltage drop: 3.60
Voltage drop percentage: 10.29%
Voltage at the end: 31.4

That would cost your ~20 Watts from your array (in full sun/maximum current)--But it would still work fine with MPPT controller and only a little bit less than the 5% rate of charge recommended for 2x golf cart batteries for charging current.

The higher voltage you go on the solar array (with MPPT controllers), the smaller copper wire you need from array to charge controller.

It all depends on how much energy you need (Watts*Hours or Amp*Hours*Volts defines size voltage & AH of battery bank).. And the battery bank plus hours of sun per day define the size of solar array... Then you start picking equipment and configurations that will meet your needs.

The above is not a complete design discussion--But trying to show your options for a 125 foot feed from array to point of use for a smallish 12 volt battery system.

Your thoughts?

-Bill

Yep, for a single panel that will be fine... Also, that is ~10% rate of charge for a 55 amp @ 12 volt battery.

If you are happy with the support of your loads (from battery)--It should be perfect with a PWM charge controller.

-Bill

MPPT controllers "need a voltage drop" across the charge controller to work efficiently. They really do not do quite as good with "low" Vmp input voltages (vs battery bank voltage).

Try it and see how well it works. If you see ~77% of rated panel output power on a cool/clear day near noon--It is doing fine. If you see less than 50%, then may be some issues.

Note that charge controllers only output "full array power" to a battery (bank) that is less than ~80% full (this is "bulk" charging)... Batteries that are near full, will naturally have reduced charging current (this is "absorb" charging).

Or, if you have a good size load on the battery (charger should output maximum power when there are heavy/steady loads).

-Bill