Wire Gauge Required Question

BFL13 wrote:
I wondered if that squaring factor, mentioned earlier in the thread, would mean there is a "loss multiplier" going from the 24v side to the 12v side of the controller.

But again it is the percentage of what? deal. I think I would rather have the high amps and a loss instead of low amps with little loss.

Thank goodness I have a portable set-up with lots of various wires lying around like fat jumper cables and several pieces of #8, so I can try out different things strung together and let the Trimetric tell me how it is.

If I fry my MPPT controller, I can't go to PWM with that new 24v 230w panel. A person with brains would have two 120w 12v panels and put them in series. Then if he fried his MPPT he could just change the panels to parallel and use an RJ special. :)

I've played around with my portable system with two 80W panels and MPPT controller (morningstar sunsaver 15A) and trimetric into a bank of 4 T-125's.
Panel Imp =4.25A, Vmp 18V.

case 1:
With a very short (<5ft) #10 wire run between the panels and controller, parallel or series yields basically the same battery current even with full panel output current. No real suprise, what I expected with adequate and short wiring.

case 2:
Longer cables between panel and controller, another story.

When length varied from 35' to 100ft of 30A RV cord with ground parallled to neutral to get the lowest resistance possible.

series panels with MPPT controller wins out every time over parallel panels when measuring current into the batteries when near full output current and batteries discharged enough to take near full current. The longer the cable the bigger the difference. How much? as much as 1A when ouput current is near 12A. parallel peaked at about 10.75A, series peaks at little about 11.7A at 100ft. For me 1A (10%) is signficant. If panel ouput current was higher, difference would be even greater. curent drop between parallel and series matched very closely with my calculated current drop. Again, no suprise.

And 100ft with series panels is only about 0.25A LESS than case 1!!

I can have my cake and eat it to!! And I get my excercise laying out the 100Ft of 30A rv cable.

And parallel MPPT wins out over parallel PWM as well under same conditions. No suprise here either.
Case 3
Once I get to battery acceptance down below about 11A, then all of them converge to basically the same current into the batteries. And again, what would be expected with adequate wire sizing. The panel even with the cable power loss is able to provide more power to the controller than the controller can deliver to the batteries.

But you say, I don't have a portable, and ONLY have a 30ft run from my trailer roof to the controller, so with "ONLY" 4.2A, #14 should do JUST FINE. Well, guess what, a 30 ft run of #14 gets you the SAME peak current loss and I get with 100ft of my roughly #8 wire.!!!

But with 160W of solar and 500AH of battery I seldom get to case 3, case 2 is the "standard" charging mode, and with lots of trees, being able to have 100ft of cable to work with is sure nice. Many a time has let me have full sun all day instead of only for a few hours.

So, IMHO, no reason not to go with series connections IF you have a MPPT controller and can stay below controller max input, and reasons NOT to go with a parallel conection with MPPT.