MPPT controller and voltage drop?

[Almot]

Warning - long and technical. Trying to figure out the simplest way to get power from panels to MPPT Rogue, and brains are overheating already.

Didn't want to continue this in my thread on roof top box. Have 2 panels in series, so there are only 2 cables from the array. Narrowed down the options to

1) AWG 10 to combiner box, and AWG 6 to controller. Imp = 8.1A. Voltage drop to controller = 1.25%.

or 2) AWG 10 all the way to controller, no junction box. Imp = 8.1A. Voltage drop 2%.

I cross-checked the results, Southwire and Rogue manual give the same numbers.

Then there is ~1% loss in battery cable at 30A max, but this is a different subject.

Correct me if I'm wrong, voltage drop = AH loss, in case of MPPT. With Option #2 - AWG 10 all the way - I am losing about 0.3 AH daily in winter and 0.6 AH in summer compared to Option #1 (box+ AWG 6), assuming controller works at peak 30A 30% of time in winter and 50% time in summer (fat chance), and the bulk stage goes up to 100% SOC (nonsense, I know).


Sounds like AWG 10 is the ticket. BUT... there is Rogue controller manual, written so well that I'm reading is like a text book, and "the Book" says 😉

The wire used to connect the PV array and battery bank to the MPT-3048 should be of a heavy enough gauge so as not to introduce excessive voltage drops and/or current limitations. At higher currents, light gauge wire may introduce a large voltage drop from the PV array, causing an extreme loss of power and erratic operation of the MPT-3048.

Then they have nice tables (that I used to get my numbers from), so the manual is not just a bla-bla. I am not sure about "erratic operation" though. To me, with MPPT all that matter is "how much AH I can afford to lose", doesn't matter if voltage drop is 2% or 10%, as long as the wire ampacity is sufficient.
Am I missing something?

[Almot]

SMK, 100' AWG 8 is $88, or $28 more than 100' AWG 10, but the difference is small indeed.

AWG 8 could be useful if in future I expand beyond 500W, though this would require replacing 30A Rogue that I have just bought, so those plans might never materialize. And then again - you have 750W with AWG 10 and it works, so what the heck...

[time2roll]

I am not changing mine but for $17 (100') I would have bought 8 vs 10.

[mena661]

Almot wrote:

smkettner wrote:
Have not seen MC4 #8 but that would be a temptation.

Indulge yourself 😉 AWG 8 MC4 cable

Nice find!

[Almot]

Thinking to buy AWG 8?

[time2roll]

Nice selection of MC4 at good prices.
http://www.unlimited-solar.com/8_AWG_MC4_EXTENSIONS_s/2191.htm
Same company as the ebay listing.
Panel prices are too high for 12v and no 24v listed 😞

[Almot]

smkettner wrote:
Have not seen MC4 #8 but that would be a temptation.

Indulge yourself 😉 AWG 8 MC4 cable

[time2roll]

Also keep in mind the losses assume full max power all day which is probably unrealistic. Have not seen MC4 #8 but that would be a temptation.

[KJINTF]

My installation uses 15ft of #10 wires from a series parallel array of 2 each K135 and 2 each K140 panels. Array voltage stays in the high 30's close to the 40 volt mark with array current of as much as 15amps. The controller has about 6ft of #4 cables to the house batteries which are two CG batteries.

Not sure what you expect from your system a "Battery Charger" or a "Power Source" as I do. I regularly draw 30 or more amps from 12 Volt loads.

I will post a few graphs later today. Nice that the MS View application easily enables this stuff

[Almot]

I forgot to tell 60V total array voltage - but does it matter? Any 2 panels in series will have more than enough voltage after MPPT controller to get 14.8V output. RLMiller - it's 70ft total 2-way run to controller, so the voltage/wattage drop is quite measurable though unlikely to disturb my comfort.

RLMiller wrote:
The output from the MPPT converter to the battery {with 500W array} can run as high as 45 amps

Hopefully I won't get that much with 480W array. Not because of controller 30A limit (too late, have bought it already), but because I won't have a load higher than 10-11 A at any given moment. Mostly it will be "pure charging" in 60-100% SOC range, so with 2% wire loss + 4-5% controller loss the battery+load won't have a chance to pull more than 31-32A.

0.6 Ah a day was an estimate. 150 AH from 480W array on a good summer day (if I get lucky), so with 0.75% difference in watts output between Option #1 and #2, it means 0.0075*150/2 (if half a day it hits 30A) = 0.6 AH difference in daily harvest.

I guess there is nothing to worry. Marc the Rogue 😉 just put in standard warnings in case if somebody would try and run 100V*8A over AWG 12 or other crazy things.

Btw, have found AWG 8 with MC4 in 75ft and 100ft length. Temptation...

[mena661]

Even I wouldn't sweat .3 or .6 Ah a day.

[time2roll]

You want to go #10 direct to the controller and not worry.
This is just how mine is wired... ~8a & 90v through #10 wire direct to controller.
If you want more power just add a third like panel in series with the others.

Don't lose any sleep for losing 2 Ah per day.

[RLMiller]

First of all there is some information missing? What is the voltage output of the two panels in series in full sunlight with an 8 amp load? Evidently the two panels in series produce 8 amps.

Assuming 30 ft (15’ x 2) #10 AWG from the panels to the controller the resistance in the wiring would be about .03 ohms (#10 AWG is .99 ohms/1000’). Discounting any voltage drop in connections, the IIR (current * resistance) loss in the wire would be about .2 volts, a negligible amount.

To put that in perspective, for my MH I have two 250 watt panels in series that produce 60 volts at 8 amps in full sunlight. The voltage drop with #10 AWG wire is negligible. The output from the MPPT converter to the battery can run as high as 45 amps so I use a short run of #8 THWN rated at 50 amps. Again the voltage drop is negligible. One concern is the maximum voltage that can be presented to the converter under a light, or no load condition. In my case the panels will produce about 80 volts with no load. Since my MPPT converter is rated at 100 volts so I’m well within specs.

While at a solar presentation in Quartzite earlier this year, the presenter was advocating very large wire sizes, with panels in series. I see no reason for that, unless you own copper stock, and my performance has demonstrated the wisdom of the lighter wire gauge and higher voltage.

To put things in perspective, I have two solar arrays that produce 2400 watts in each array for a total full sunlight output of 4800 watts. Since the two arrays with panels in series, produce 8 amps each, but at 300 volts, I can still use the #10 AWG wire for each array and stay within ampacity and voltage rating for THWN wire, with negligible voltage drop. The entire system is to code, and inspected by Building and Safety to permit Net Metering by the utility.

With my two panels delivering 500 watts, I have no problem running the satellite receiver, TV and some ham radios all day and into the evening. I use the generator for the coffee pot, or the stove, but the inverter will run the coffee pot, but the demand on the 12 volt battery is brutal.

Bottom line is you’re OK with the #10 wire from the panels to the converter, and in any case, the MPPT converters are quite tolerant of changes in voltage. You may now shift your brain to 'cool down' mode.