First post and voltage drop calc help.

Cariboocoot wrote: »

Well I see a couple of things here. PV wire is usually 10 AWG, not 8 AWG. The drop from panel to combiner box usually doesn't make any real difference at all because it is short run at low current, and often high Voltage. So this part depends on the actual configuration of the array.

Now when you come out of the combiner box that's a different matter, as you will have the current of all the parallel connections and usually a longer run that 10 feet to the charge controller (or GTI).

Otherwise the two section constitute two separate circuits and need to be calculated in series if you want 'precise' numbers. Since PV allows its Voltage to go 'anywhere' depending on conditions the numbers are never that precise anyway. So first you'd calculate the drop on the initial run from PV to combiner, then take that Voltage as the base for the calculation from combiner to controller.

But there really isn't that much point in doing it that way. As I said the PV Voltage will run anywhere from O to Voc rating. Typically nominal Voltage is used for calculation as it is 'lowest practical potential' and will give the most conservative return (maximizing efficiency). Voltage below that probably means the system isn't producing anyway. Some may prefer to use Vmp, in order to get that OCD precision.

So ... Vmp @ Imp across 'X' feet of #AWG results in a Voltage at Combiner of Vmp - V-drop. Then Voltage at combiner @ combined Imp across 'Y' feet of #AWG results in Voltage at controller input of Voltage at Combiner - V-drop. Difference between Vmp and controller input Voltage determines % drop from array.

That's why the theoretical confuses and the practical makes sense.

Cariboocoot wrote: »

Do yourself a favour and use the system nominal Voltage for calculations.
The reason for this being that solar panels are current sources and their Voltage is not stable. With a PWM type controller the panel Voltage will be the same as battery Voltage through the Bulk stage, and that is when current is highest, Voltage is lowest, and V-drop at its maximum. If the wire size 'works' at those levels with minimal loss (less than 3% is usually the target) it will work at all other Voltage levels above that.

If you have to run a long distance between the array and charge controller you may have to bite the bullet and use an MPPT type controller which will allow the panels to be in series: much higher array Voltage thus lower V-drop over distance. The controller will take care of adjusting to battery Voltage.

As an example:

Two Grape 160 Watt panels with Vmp 18.5 and Imp 8.6 (17.2 Amps)
Both in parallel on a PWM controller, 40 feet from array you need 4 AWG to keep the V-drop below 3% (12 Volts nominal, 17 Amps current, 40 feet distance).

Put them in series on an MPPT type controller (24 Volts nominal, 8.5 Amps current, 40 feet distance) and you can use 10 AWG with the V-drop still below 3%.

The difference between wire cost difference and controller cost difference would be the main deciding factor. Also note that the 4 AWG will be more difficult to work with. The 30 Amp MPPT Kid controller would cost about $200 more than a 20 Amp SunSaver PWM.

Cariboocoot wrote: »

I have no actual experience with the Bogart equipment in question, so I can't speak to its quality or efficiency.

Usually the MPPT advantage is solely in the flexibility of array design; the 'extra' power is so minimal as to be not worthwhile considering. However in this case where the PV's have higher than normal Vmp there is a significant difference in output: 17.2 Amps is all you will see with PWM, whereas 20 Amps is more likely than not with MPPT. That's a 16% increase. what is more is that the 225 Amp hour batteries will recharge better with those extra 3 Amps; ideally you should have 22-23 Amps peak current. It is a difference between a 7.5% gross charge rate and an 8.8% gross charge rate. Subtract the load factor for net rate and you can have the difference between works/doesn't work. At the very least it will have an effect on the DOD that can be used with any given hours of sun.

What's more, if you want to add that third panel the 4 AWG & PWM suddenly goes out the window with another 8 Amps: the current at 25.8 puts the V-drop over 4%. Add the same third panel in series on the MPPT and you have a slight drop in controller efficiency, but no trouble with V-drop on the line because the Voltage goes up but the current remains the same.

How this fits with later redesign depends on how many changes are made.